307 results on '"Leutwyler, Samuel"'
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2. Accurate computations of the structures and binding energies of the imidazole[formula omitted]benzene and pyrrole[formula omitted]benzene complexes
- Author
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Ahnen, Sandra, Hehn, Anna-Sophia, Vogiatzis, Konstantinos D., Trachsel, Maria A., Leutwyler, Samuel, and Klopper, Wim
- Published
- 2014
- Full Text
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3. Intermolecular dissociation energies of 1-naphthol complexes with large dispersion-energy donors: Decalins and adamantane.
- Author
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Knochenmuss, Richard, Sinha, Rajeev K., Balmer, Franziska A., Ottiger, Philipp, and Leutwyler, Samuel
- Subjects
TWO-photon-spectroscopy ,DENSITY functionals ,ELECTRONIC excitation ,ADAMANTANE derivatives ,ADAMANTANE ,DENSITY functional theory ,BINDING energy - Abstract
The ground-state intermolecular dissociation energies D
0 (S0 ) of supersonic-jet cooled intermolecular complexes of 1-naphthol (1NpOH) with the bi- and tricycloalkanes trans-decalin, cis-decalin, and adamantane were measured using the stimulated-emission-pumping/resonant two-photon ionization (SEP-R2PI) method. Using UV/UV holeburning, we identified two isomers (A and B) of the adamantane and trans-decalin complexes and four isomers (A–D) of the cis-decalin complex. For 1NpOH·adamantane A and B, the D0 (S0 ) values are 21.6 ± 0.15 kJ/mol and 21.2 ± 0.32 kJ/mol, those of 1NpOH·trans-decalin A and B are 28.7 ± 0.3 kJ/mol and 28.1 ± 0.9 kJ/mol, and those of 1NpOH·cis-decalin A and B are 28.9 ± 0.15 kJ/mol and 28.7 ± 0.3 kJ/mol. Upon S0 → S1 electronic excitation of the 1NpOH moiety, the dissociation energies of adamantane, trans-decalin, and the cis-decalin isomer C change by <1% and those of cis-decalin isomers A, B, and D increase only slightly (1%–3%). This implies that the hydrocarbons are dispersively adsorbed to a naphthalene "face." Calculations using the dispersion-corrected density functional theory methods B97-D3 and B3LYP-D3 indeed predict that the stable structures have face geometries. The B97-D3 calculated D0 (S0 ) values are within 1 kJ/mol of the experiment, while B3LYP-D3 predicts D0 values that are 1.4–3.3 kJ/mol larger. Although adamantane has been recommended as a "dispersion-energy donor," the binding energies of the trans- and cis-decalin adducts to 1NpOH are 30% larger than that of adamantane. In fact, the D0 value of 1NpOH·adamantane is close to that of 1NpOH·cyclohexane, reflecting the nearly identical contact layer between the two molecules. [ABSTRACT FROM AUTHOR]- Published
- 2020
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4. Excited-state vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-ethylcytosine.
- Author
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Trachsel, Maria A., Blaser, Susan, Siffert, Luca, Wiedmer, Timo, and Leutwyler, Samuel
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FLUORESCENCE yield ,IONIZATION energy ,DYNAMICS ,CYTOSINE - Abstract
The S
1 excited-state lifetime of jet-cooled 1-ethylcytosine (1ECyt) is ∼1 ns, one of the longest lifetimes for cytosine derivatives to date. Here, we analyze its S0 → S1 vibronic spectrum using two-color resonant two-photon ionization and UV/UV holeburning spectroscopy. Compared to cytosine and 1-methylcytosine, the S0 → S1 spectrum of 1ECyt shows a progression in the out-of-plane "butterfly" mode ν 1 ′ , identified by spin-component scaled-second-order coupled-cluster method ab initio calculations. We also report time-resolved S1 state nonradiative dynamics at ∼20 ps resolution by the pump/delayed ionization technique. The S1 lifetime increases with the number of ν 1 ′ quanta from τ = 930 ps at v 1 ′ = 0 to 1030 ps at v 1 ′ = 2 , decreasing to 14 ps at 710 cm−1 vibrational energy. We measured the rate constants for S1 ⇝ S0 internal conversion and S1 ⇝ T1 intersystem crossing (ISC): At the v′ = 0 level, kIC is 8 × 108 s−1 or three times smaller than 1-methylcytosine. The ISC rate constant from v′ = 0 to the T1 (3 ππ* ) state is kISC = 2.4 × 108 s−1 , 10 times smaller than the ISC rate constants of cytosine, but similar to that of 1-methylcytosine. Based on the calculated S1 (1 ππ* ) state radiative lifetime τrad = 12 ns, the fluorescence quantum yield of 1ECyt is Φfl ∼ 7% and the intersystem crossing yield is ΦISC ∼ 20%. We measured the adiabatic ionization energy of 1-ethylcytosine via excitation of the S1 state as 8.353 ± 0.008 eV, which is 0.38 eV lower than that of amino-keto cytosine. Measurement of the ionization energy of the long-lived T1 (ππ* ) state formed via ISC reveals that it lies 3.2–3.4 eV above the S0 ground state. [ABSTRACT FROM AUTHOR]- Published
- 2019
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5. Face, Notch, or Edge? Intermolecular dissociation energies of 1-naphthol complexes with linear molecules.
- Author
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Knochenmuss, Richard, Sinha, Rajeev K., and Leutwyler, Samuel
- Subjects
TWO-photon-spectroscopy ,DENSITY functionals ,GROUND state energy ,EDGES (Geometry) ,DENSITY functional theory ,QUADRUPOLE moments - Abstract
The stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method was used to determine the intermolecular dissociation energies D
0 of jet-cooled 1-naphthol(1NpOH)·S complexes, where S is a linear molecule (N2 , CO, CO2 , OCS, N2 O, and ethyne) or symmetric-top molecule (2-butyne) that contains double or triple bonds. The dissociation energies D0 (S0 ) are bracketed as follows: 6.68 ± 0.08 kJ/mol for S=N2 , 7.7 ± 0.8 kJ/mol for CO, 12.07 ± 0.10 kJ/mol for CO2 , 13.03 ± 0.01 kJ/mol for N2 O, 14.34 ± 0.08 kJ/mol for ethyne, 15.0 ± 1.35 kJ/mol for OCS, and 29.6 ± 2.4 kJ/mol for 2-butyne. The minimum-energy structures, vibrational wavenumbers, and zero-point vibrational energies were calculated using the dispersion-corrected density functional theory methods such as B97-D3 and B3LYP-D3 with the def2-QZVPP basis set. These predict that N2 and CO are dispersively bound Face complexes (S bound to a naphthalene Face), while CO2 , N2 O, and OCS adsorb into the "Notch" between the naphthyl and OH groups; these are denoted as Notch complexes. Ethyne and 2-butyne form Edge complexes involving H-bonds from the —OH group of 1NpOH to the center of the molecule. The presence of a double or triple bond or an aromatic C=C bond within S does not lead to a specific calculated geometry (Face, Notch or Edge). However, a correlation exists between the structure and the sign of the quadrupole moment component Θzz of S: negative Θzz correlates with Face or Notch, while positive Θzz correlates with Edge geometries. [ABSTRACT FROM AUTHOR]- Published
- 2019
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6. Intermolecular dissociation energies of hydrogen-bonded 1-naphthol complexes.
- Author
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Knochenmuss, Richard, Sinha, Rajeev K., Poblotzki, Anja, Den, Takuya, and Leutwyler, Samuel
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DISSOCIATION (Chemistry) ,NAPHTHOL ,FURAN derivatives ,FURANS ,DENSITY functional theory - Abstract
We have measured the intermolecular dissociation energies D
0 of supersonically cooled 1-naphthol (1NpOH) complexes with solvents S = furan, thiophene, 2,5-dimethylfuran, and tetrahydrofuran. The naphthol OH forms non-classical H-bonds with the aromatic π-electrons of furan, thiophene, and 2,5-dimethylfuran and a classical H-bond with the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization method, the ground-state D0 (S0 ) values were bracketed as 21.8 ± 0.3 kJ/mol for furan, 26.6 ± 0.6 kJ/mol for thiophene, 36.5 ± 2.3 kJ/mol for 2,5-dimethylfuran, and 37.6 ± 1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set), and ωB97X-D [using the 6-311++G(d,p) basis set] predict that the H-bonded (edge) isomers are more stable than the face isomers bound by dispersion; experimentally, we only observe edge isomers. We compare the calculated and experimental D0 values and extend the comparison to the previously measured 1NpOH complexes with cyclopropane, benzene, water, alcohols, and cyclic ethers. The dissociation energies of the nonclassically H-bonded complexes increase roughly linearly with the average polarizability of the solvent, α ¯ (S). By contrast, the D0 values of the classically H-bonded complexes are larger, increase more rapidly at low α ¯ (S), but saturate for large α ¯ (S). The calculated D0 (S0 ) values for the cyclopropane, benzene, furan, and tetrahydrofuran complexes agree with experiment to within 1 kJ/mol and those of thiophene and 2,5-dimethylfuran are ∼3 kJ/mol smaller than experiment. The B3LYP-D3 calculated D0 values exhibit the lowest mean absolute deviation (MAD) relative to experiment (MAD = 1.7 kJ/mol), and the B97-D3 and ωB97X-D MADs are 2.2 and 2.6 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]- Published
- 2018
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7. Accurate determination of the structure of 1,3,5-trifluorobenzene by femtosecond rotational Raman coherence spectroscopy and ab initio calculations
- Author
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Kummli, Dominique S., Frey, Hans-Martin, and Leutwyler, Samuel
- Published
- 2010
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8. Intermolecular dissociation energies of 1-naphthol·n-alkane complexes.
- Author
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Knochenmuss, Richard, Maity, Surajit, Balmer, Franziska, Müller, Charlotte, and Leutwyler, Samuel
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DISSOCIATION (Chemistry) ,INTERMOLECULAR forces ,NAPHTHOL ,BUTANE ,ISOMERS ,MOLECULAR vibration ,DENSITY functional theory - Abstract
Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we have determined accurate intermolecular dissociation energies D
0 of supersonic jet-cooled intermolecular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane, propane, and n-butane. Experimentally, the smaller alkanes form a single minimum-energy structure, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation energies D0 (S0 ) for the complexes with propane and n-butane (isomers A and B) were bracketed within ±0.5%, being 16.71 ± 0.08 kJ/mol for S = propane and 20.5 ± 0.1 kJ/mol for isomer A and 20.2 ± 0.1 kJ/mol for isomer B of n-butane. All 1NpOH·S complexes measured previously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weak SEP-R2PI bands are observed above the putative dissociation onset for the methane and ethane complexes. We attribute these bands to long-lived complexes that retain energy in rotation-type intermolecular vibrations, which couple only weakly to the dissociation coordinates. Accounting for this, we find dissociation energies of D0 (S0 ) = 7.98 ± 0.55 kJ/mol (±7%) for S = methane and 14.5 ± 0.28 kJ/mol (±2%) for S = ethane. The D0 values increase by only 1% upon S0 → S1 excitation of 1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, and ωB97X-D predict that the n-alkanes bind dispersively to the naphthalene “Face.” The assignment of the complexes to Face structures is supported by the small spectral shifts of the S0 → S1 electronic origins, which range from +0.5 to −15 cm−1 . Agreement with the calculated dissociation energies D0 (S0 ) is quite uneven, the B97-D3 values agree within 5% for propane and n-butane, but differ by up to 20% for methane and ethane. The ωB97X-D method shows good agreement for methane and ethane but overestimates the D0 (S0 ) values for the larger n-alkanes by up to 20%. The agreement of the B3LYP-D3 D0 values is intermediate between the other two methods. [ABSTRACT FROM AUTHOR]- Published
- 2018
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9. An efficient one-pot synthesis of strongly fluorescent (hetero)arenes polysubstituted with amino and cyano groups
- Author
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Yi, Chenyi, Blum, Carmen, Liu, Shi-Xia, Frei, Gabriela, Neels, Antonia, Stoeckli-Evans, Helen, Leutwyler, Samuel, and Decurtins, Silvio
- Published
- 2008
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10. High-accuracy structure of cyclobutane by femtosecond rotational Raman four-wave mixing
- Author
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Kummli, Dominique S., Frey, Hans M., and Leutwyler, Samuel
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Raman spectroscopy -- Usage ,Butane -- Research ,Butane -- Chemical properties ,Cyclic hydrocarbons -- Research ,Chemicals, plastics and rubber industries - Abstract
The rotational and centrifugal constants of cyclobutanes, [C.sub.4][H.sub.8] which was measured accurately by employing femtosecond rotational Raman coherence, detected by degenerate four-wave mixing is presented. The vibrational levels of cyclobutane exhibit tunneling splitting due to the ring-puckering interconversion between the symmetry-equivalent minima via a planar barrier.
- Published
- 2007
11. An investigation of electronic structure and properties of new chromophore: 3,3′-bithiazolo[3,4- a]pyridinium perchlorate
- Author
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Liu, Shi-Xia, Tanner, Christian, Leutwyler, Samuel, Bigler, Peter, and Decurtins, Silvio
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- 2007
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12. Hydrogen bonding of the nucleobase mimic 2-pyridone to fluorobenzenes: An ab initio investigation
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Frey, Jann A., Leist, Roman, and Leutwyler, Samuel
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Fluorine compounds -- Chemical properties ,Benzene -- Chemical properties ,Hydrogen bonding -- Structure ,Nucleotides -- Chemical properties ,DNA -- Research ,Chemicals, plastics and rubber industries - Abstract
An investigation is conducted of the relative importance of hydrogen bonding versus pie stacking interactions in DNA where the hydrogen-bonded complexes of the nucleobase mimic 2-pyridone (2PY) with seven different fluorinated benzenes. Ab initio studies are performed on the individual hydrogen bonds occurring between nucleobases and fluronated isomers.
- Published
- 2006
13. Fluorobenzene-nucleobase interactions: Hydrogen bonding or pie-stacking?
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Leist, Roman, Frey, Jann A., and Leutwyler, Samuel
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Hydrogen bonding -- Structure ,Benzene -- Chemical properties ,Fluorine compounds -- Chemical properties ,Polymerase chain reaction -- Research ,Nucleotides -- Chemical properties ,Chemicals, plastics and rubber industries - Abstract
A study conducted on modified DNA oligomers and polymerase reactions have demonstrated that canonical nucleobases can exhibit stable and even selective pairing with shape-complementary fluorobenzene nucleotides. Hydrogen bonds are believed to be absent because of the fluorination of the pairing edges and the local DNA stability is attributed pie-stacking and shape complementarity.
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- 2006
14. Accurate gas-phase structure of para-dioxane by fs Raman rotational coherence spectroscopy and ab initio calculations.
- Author
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Takuya Den, Menzi, Samuel, Frey, Hans-Martin, and Leutwyler, Samuel
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DIOXANE ,GAS phase reactions ,RAMAN spectroscopy ,MICROWAVE spectroscopy ,MOLECULAR structure - Abstract
p-Dioxane is non-polar, hence its rotational constants cannot be determined by microwave rotational coherence spectroscopy (RCS). We perform high-resolution gas-phase rotational spectroscopy of para-dioxane-h
8 and -d8 using femtosecond time-resolved Raman RCS in a gas cell at T = 293 K and in a pulsed supersonic jet at T ~ 130 K. The inertial tensor of p-dioxane-h8 is strongly asymmetric, leading to a large number of asymmetry transients in its RCS spectrum. In contrast, the d8 -isotopomer is a near-oblate symmetric top that exhibits a much more regular RCS spectrum with few asymmetry transients. Fitting the fs Raman RCS transients of p-dioxane-h8 to an asymmetric-top model yields the ground-state rotational constants A0 = 5084.4(5) MHz, B0 = 4684(1) MHz, C0 = 2744.7(8) MHz, and (A0 + B0 )/2 = 4884.5(7) MHz (±1σ±1σ). The analogous values for p-dioxane-d8 are A0 = 4083(2) MHz, B0 = 3925(4) MHz, C0 = 2347.1(6) MHz, and (A0 + B0 )/2 = 4002.4(6) MHz. We determine the molecular structure with a semi-experimental approach involving the highly correlated coupled-cluster singles, doubles and iterated triples method and the cc-pCVXZ basis set series from double- to quadruple-zeta (X = D, T, Q). Combining the calculated vibrationally averaged rotational constants A0 calC (X),B0 calC (X),C0 calC (X)A0 calc (X),B0 calc (X),C0 calc (X) for increasing basis-set size X with non-linear extrapolation to the experimental constants Aexp0,Bexp0,Cexp0A0 exp,B0 exp,C0 exp allows to determine the equilibrium ground state structure of p-dioxane. For instance, the equilibrium C-C and C-O bond lengths are re(CC) = 1.5135(3) Å and re(CO) = 1.4168(4) Å, and the four axial C-H bond lengths are 0.008 Å longer than the four equatorial ones. The latter is ascribed to the trans-effect (anomeric effect), i.e., the partial delocalization of the electron lone-pairs on the O atoms that are oriented trans, relative to the axial CH bonds. [ABSTRACT FROM AUTHOR]- Published
- 2017
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15. Planarizing cytosine: The S1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine.
- Author
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Trachsel, Maria A., Lobsiger, Simon, Schär, Tobias, Blancafort, Lluís, and Leutwyler, Samuel
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VIBRATION (Mechanics) ,CYTOSINE ,METHYLCYTOSINE ,FLUORESCENCE yield ,TAUTOMERISM ,ADIABATIC ionization ,ENERGY bands - Abstract
We measure the S
0 →S1 spectrum and time-resolved S1 state nonradiative dynamics of the 'clamped' cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonant two-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization. The experiments are complemented with spin-component scaled second-order approximate coupled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-order perturbation-theory (MS-CASPT2) ab initio calculations. While the R2PI spectrum of cytosine breaks off ~500 cm1 above its 00 0 band, that of TMCyt extends up to +4400 cm-1 higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C5 -C6 bond allows us to explore the S1 state vibrations and S0 →S1 geometry changes in detail. The TMCyt S1 state out-of-plane vibrations v'1 , v'3 , and v'5 lie below 420 cm-1 , and the in-plane v'11 , v'12 , and v'23 vibrational fundamentals apper at 450, 470, and 944 cm-1 . S0 → S1 vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated v'1 , v'3 , and v'5 frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S1 S0 conical intersection (CI) increases from +366 cm-1 in cytosine to >6000 cm-1 in TMCyt, explaining the long lifetime and extended S0 →S1 spectrum. The lowest-energy S1 S0 CI of TMCyt is the "amino out-of-plane" (OPX ) intersection, calculated at +4190 cm-1 . The experimental S1 S0 internal conversion rate constant at the S1 (v' = 0) level is kIC = 0.98-2.2 108 s-1 , which is ~10 times smaller than in 1-methylcytosine and cytosine. The S1 (v' = 0) level relaxes into the T1 (3 ππ*) state by intersystem crossing with kISC = 0.41-1.6 108 s-1 . The T1 state energy is measured to lie 24 580 ± 560 cm-1 above the S0 state. The S1 (v' = 0) lifetime is = 2.9 ns, resulting in an estimated fluorescence quantum yield of ϕfl = 24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm-1 . A sharp S1 ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of 8.114 ± 0.002 eV. [ABSTRACT FROM AUTHOR]- Published
- 2017
- Full Text
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16. Accurate dissociation energies of two isomers of the 1-naphtho·cyclopropane complex.
- Author
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Maity, Surajit, Knochenmuss, Richard, Holzer, Christof, Féraud, Géraldine, Frey, Jann, Klopper, Wim, and Leutwyler, Samuel
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CYCLOPROPANE derivatives ,IONIZATION (Atomic physics) ,CYCLOPROPANE synthesis ,NAPHTHOL derivatives ,BINDING energy - Abstract
The 1-naphthol·cyclopropane intermolecular complex is formed in a supersonic jet and investigated by resonant two-photon ionization (R2PI) spectroscopy, UV holeburning, and stimulated emission pumping (SEP)-R2PI spectroscopy. Two very different structure types are inferred from the vibronic spectra and calculations. In the "edge" isomer, the OH group of 1-naphthol is directed towards a C--C bond of cyclopropane, the two ring planes are perpendicular. In the "face" isomer, the cyclopropane is adsorbed on one of the π-aromatic faces of the 1-naphthol moiety, the ring planes are nearly parallel. Accurate ground-state intermolecular dissociation energies D
0 were measured with the SEP-R2PI technique. The D0 (S0 ) of the edge isomer is bracketed as 15.35 « 0.03 kJ/mol, while that of the face isomer is 16.96 « 0.12 kJ/mol. The corresponding excited-state dissociation energies D0 (S1 ) were evaluated using the respective electronic spectral shifts. De spite the D0 (S0 ) difference of 1.6 kJ/mol, both isomers are observed in the jet in similar concentrations, so they must be separated by substantial potential energy barriers. Intermolecular binding energies, De , and dissociation energies, D0 , calculated with correlated wave function methods and two dispersion-corrected density-functional methods are evaluated in the context of these results. The density functional calculations suggest that the face isomer is bound solely by dispersion interactions. Binding of the edge isomer is also dominated by dispersion, which makes up two thirds of the total binding energy. [ABSTRACT FROM AUTHOR]- Published
- 2016
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17. The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine.
- Author
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Trachsel, Maria A., Wiedmer, Timo, Blaser, Susan, Frey, Hans-Martin, Quansong Li, Ruiz-Barragan, Sergi, Blancafort, Lluís, and Leutwyler, Samuel
- Subjects
METHYLCYTOSINE ,EXCITED states ,CRYSTAL structure ,JETS (Fluid dynamics) ,VIBRATIONAL spectra - Abstract
We have investigated the S
0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics of jet-cooled amino-keto 1-methylcytosine (1MCyt) using two-color resonant two-photon ionization, UV/UV holeburning and depletion spectroscopies, as well as nanosecond and picosecond timeresolved pump/delayed ionization measurements. The experimental study is complemented with spin-component-scaled second-order coupled-cluster and multistate complete active space second order perturbation ab initio calculations. Above the weak electronic origin of 1MCyt at 31 852 cm-1 about 20 intense vibronic bands are observed. These are interpreted as methyl group torsional transitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotation and out-of-plane distortions upon 1ππ* excitation predicted by the calculations. The methyl torsion and ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrum breaks off at a vibrational excess energy Eexc ~ 500 cm-1 , indicating that a barrier in front of the ethylene-type S1 S0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm-1 . The S1 ⇝ S0 internal conversion rate constant increases from kIC = 2 · 109 s-1 near the S1 (ν = 0) level to 1 · 1011 s-1 at Eexc = 516 cm-1 . The 1ππ* state of 1MCyt also relaxes into the lower-lying triplet T1 (3ππ*) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) value is 2.4 cm-1 . The ISC rate constant is 10-100 times lower than kIC ; it increases from kISC = 2 · 108 s-1 near S1 (ν = 0) to kISC = 2 · 109 s-1 at Eexc = 516 cm-1 . The T1 state energy is determined from the onset of the time-delayed photoionization efficiency curve as 25 600 ± 500 cm-1 . The T2 (3nπ*) state lies >1500 cm-1 above S1 (ν = 0), so S1 ⇝ T2 ISC cannot occur, despite the large SOC parameter of 10.6 cm-1 . An upper limit to the adiabatic ionization energy of 1MCyt is determined as 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionization energy by ≥0.32 eV and leads to a much higher density of vibronic bands in the S0 → S1 spectrum. The effect of methylation on the radiationless decay to S0 and ISC to T1 is small, as shown by the similar break-off of the spectrum and the similar computed mechanisms. [ABSTRACT FROM AUTHOR]- Published
- 2016
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18. Nucleobase pair analogues 2-pyridone.uracil, 2-pyridone.thymine, and 2-pyridone.5-fluorouracil: hydrogen-bond strengths and intermolecular
- Author
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Muller, Andreas and Leutwyler, Samuel
- Subjects
Chemistry, Physical and theoretical -- Analysis ,Isomerism -- Analysis ,Isomerism -- Structure ,Intermolecular forces -- Analysis ,Hydrogen bonding -- Analysis ,Chemicals, plastics and rubber industries - Abstract
The mixed dimers 2-pyridone.uracil(2PY.U), 2-pyridone.thymine(2PY.T), and 2-pyridone.5-florouracil(2PY.5-FU) are investigated, in which 2PY acts as the UV chromophore, avoiding the photophysical drawbacks of U, T, and 5-FU as shown by theoretical investigations, 2-pyridone forms only two H-bonds, which restricts the number of isomers to only three. The results revealed that the comparison of the spectra of 2PY.U, 2PY.T, and 2PY.5-FU allows determining the effects of methylation and fluorination on the ground- and excited-state binding energies, vibrational frequencies, and intermolecular forces.
- Published
- 2004
19. Hydrogen bond vibrations of 2-aminopyridine.2-pyridone, a Watson-Crick analogue of adenine.uracil
- Author
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Muller, Andreas, Talbot, Francis, and Leutwyler, Samuel
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Adenine -- Chemical properties ,Pyridine -- Chemical properties ,Hydrogen bonding -- Research ,Chemistry - Abstract
The 2-aminopuridine.2-pyridone complex (2AP.2PY) is linked by antiparallel N-H...O=C and N-H...N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine-thymine and adenine-uracil nucleobase pairs. The hydrogen bond vibrations and the three in-plane vibrations v3 (opening), v5 (shear), and v6 (stretch) were observed in the S(sub 0) and S(sub 1) states.
- Published
- 2002
20. Grotthus-type and diffusive proton transfer in 7-hydroxyquinoline.(NH3)(sub n) clusters
- Author
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Meuwly, Markus, Bach, Andreas, and Leutwyler, Samuel
- Subjects
Ammonia -- Chemical properties ,Protons -- Atomic properties ,Quinoline -- Chemical properties ,Chemistry - Abstract
The proton translocation along ammonia wires is investigated in 7-hydroxyquinoline.(NH3)(sub n) clusters, both experimentally by laser spectroscopy and theoretically by Hartree-Fock and density functional theory (DFT) calculations. It was seen that the step with the highest barrier involves a complex proton transfer mechanism, involving structural reorganization and large-scale diffusive motions of the cluster.
- Published
- 2001
21. Isotopically resolved {ie30-01} electronic spectrum of Ag3 and calculation of its Jahn-Teller effects
- Author
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Wallimann, Franz, Frey, Hans-Martin, Leutwyler, Samuel, and Riley, Mark
- Published
- 1997
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22. Intersystem crossing rates of S1 state keto-amino cytosine at low excess energy.
- Author
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Lobsiger, Simon, Etinski, Mihajlo, Blaser, Susan, Frey, Hans-Martin, Marian, Christel, and Leutwyler, Samuel
- Subjects
INTERSYSTEM crossing (Chemistry) ,AMINO ketones ,CYTOSINE ,TAUTOMERISM ,ULTRAVIOLET lasers ,IONIZATION cross sections ,INTERNAL conversion (Nuclear physics) - Abstract
The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S
1 (¹ππ*) state. We investigate these ISC rates experimentally and theoretically as a function of S1 state vibrational excess energy Eexc . The S1 vibronic levels are pumped with a ~5 ns UV laser, the S1 and triplet state ion signals are separated by prompt or delayed ionization with a second UV laser pulse. After correcting the raw ISC yields for the relative S1 and T1 ionization cross sections, we obtain energy dependent ISC quantum yields QISC corr =1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates kISC = 0.4-1.5 · 109 s-1 , the corresponding S1 ⇝S0 internal conversion (IC) rates are 30-100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S1 ; v = 0 state with kISC = 3. 109 s-1 to the T1 (3ππ*) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of ¹nO π* character into the S1 (¹ππ*) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below Eexc = 550 cm-1 in the S1 state, S1 ⇝S0 internal conversion dominates the nonradiative decay with kIC ≥ 2 . 1010 s-1 , (2) the calculated S1 ⇝T1 (¹π#960;*³ππ*) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S1 ⇝ T1 ISC is moderately fast (kISC = 3. 109 s-1 ), and not ultrafast, as claimed by other calculations, and (4) at Eexc ~ 550 cm-1 the IC rate increases by ~50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels. [ABSTRACT FROM AUTHOR]- Published
- 2015
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23. Rotational constants and structure of para-difluorobenzene determined by femtosecond Raman coherence spectroscopy: A new transient type.
- Author
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Takuya Den, Frey, Hans-Martin, Felker, Peter M., and Leutwyler, Samuel
- Subjects
FEMTOSECOND pulses ,FLUOROBENZENE ,RAMAN spectroscopy ,GAS phase reactions ,CRYSTAL structure ,GROUND state (Quantum mechanics) - Abstract
Femtosecond Raman rotational coherence spectroscopy (RCS) detected by degenerate four-wave mixing is a background-free method that allows to determine accurate gas-phase rotational constants of non-polar molecules. Raman RCS has so far mostly been applied to the regular coherence patterns of symmetric-top molecules, while its application to nonpolar asymmetric tops has been hampered by the large number of RCS transient types, the resulting variability of the RCS patterns, and the 10
3 –104 times larger computational effort to simulate and fit rotational Raman RCS transients. We present the rotational Raman RCS spectra of the nonpolar asymmetric top 1,4-difluorobenzene (para-difluorobenzene, p-DFB) measured in a pulsed Ar supersonic jet and in a gas cell over delay times up to ∼2.5 ns. p-DFB exhibits rotational Raman transitions with ΔJ = 0, 1, 2 and ΔK = 0, 2, leading to the observation of J −, K −, A −, and C–type transients, as well as a novel transient (S–type) that has not been characterized so far. The jet and gas cell RCS measurements were fully analyzed and yield the ground-state (v = 0) rotational constants A0 = 5637.68(20) MHz, B0 = 1428.23(37) MHz, and C0 = 1138.90(48) MHz (1σ uncertainties). Combining the A0 , B0 , and C0 constants with coupled-cluster with single-, double- and perturbatively corrected triple-excitation calculations using large basis sets allows to determine the semi-experimental equilibrium bond lengths re (C1 –C2 ) = 1.3849(4) Å, re (C2 –C3 ) = 1.3917(4) Å, re (C–F) = 1.3422(3) Å, and re (C2 –H2 ) = 1.0791(5) Å. [ABSTRACT FROM AUTHOR]- Published
- 2015
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24. Physical chemistry: Acids caught in the act
- Author
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Leutwyler, Samuel
- Published
- 2002
25. The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer.
- Author
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Balmer, Franziska A., Trachsel, Maria A., van der Avoird, Ad, and Leutwyler, Samuel
- Subjects
EXCIMERS ,DIMERS ,TWO-photon-spectroscopy ,BENZENE compounds ,CHARGE transfer ,POTENTIAL energy surfaces - Abstract
We observe the weak S
0 → S2 transitions of the T-shaped benzene dimers (Bz)2 and (Bz-d6 )2 about 250 cm−1 and 220 cm−1 above their respective S0 → S 1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S0 → S2 electronic oscillator strength fel (S2 ) is ~10 times smaller than fel (S1 ) and the S2 state lies ~240 cm−1 above S1 , in excellent agreement with experiment. The S0 → S1 (ππ *) transition is mainly localized on the “stem” benzene, with a minor stem → cap charge-transfer contribution; the S0 → S2 transition is mainly localized on the “cap” benzene. The orbitals, electronic oscillator strengths fel (S1 ) and fel (S2 ), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S1 and S2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz)2 ground-state surface allow to construct approximate S1 and S2 potential energy surfaces and reveal their relation to the “excimer” states at the stacked-parallel geometry. The fel (S1 ) and fel (S2 ) transition dipole moments at the C2v -symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ~10 times smaller than at the tipped T-shaped geometry. This unusual property of the S0 → S1 and S0 → S2 transition-dipole moment surfaces of (Bz)2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S0 → S1 /S2 spectra of (Bz)2 are compared to those of imidazole ⋅ (Bz)2 , which has a rigid triangular structure with a tilted (Bz)2 subunit. The S0 → S1 / S2 transitions of imidazole-(benzene)2 lie at similar energies as those of (Bz)2 , confirming our assignment of the (Bz)2 S0 → S2 transition. [ABSTRACT FROM AUTHOR]- Published
- 2015
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26. Analysis of the S2 ← S0 vibronic spectrum of the ortho-cyanophenol dimer using a multimode vibronic coupling approach.
- Author
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Kopec, Sabine, Ottiger, Philipp, Leutwyler, Samuel, and Köppel, Horst
- Subjects
DIMERS ,POTENTIAL energy surfaces ,SULFUR ,VIBRONIC coupling ,TWO-photon-spectroscopy ,PHENOLS - Abstract
The S
2 S1 vibronic spectrum of the ortho-cyanophenol dimer (oCP)2 is analyzed in a joint experimental and theoretical investigation. Vibronic excitation energies up to 750 cm-1 are covered, which extends our previous analysis of the quenching of the excitonic splitting in this and related species [Kopec et al., J. Chem. Phys. 137, 184312 (2012)]. As we demonstrate, this necessitates an extension of the coupling model. Accordingly, we compute the potential energy surfaces of the orthocyanophenol dimer (oCP)2 along all relevant normal modes using the approximate second-order coupled cluster method RI-CC2 and extract the corresponding coupling constants using the linear and quadratic vibronic coupling scheme. These serve as the basis to calculate the vibronic spectrum. The theoretical results are found to be in good agreement with the experimental highly resolved resonant two-photon ionization spectrum. This allows to interpret key features of the excitonic and vibronic interactions in terms of nodal patterns of the underlying vibronic wave functions. [ABSTRACT FROM AUTHOR]- Published
- 2015
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- View/download PDF
27. Accurate rotational constant and bond lengths of hexafluorobenzene by femtosecond rotational Raman coherence spectroscopy and ab initio calculations.
- Author
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Den, Takuya S., Frey, Hans-Martin, and Leutwyler, Samuel
- Subjects
HEXAFLUOROBENZENE ,RAMAN spectroscopy ,MOLECULES ,EQUILIBRIUM ,CHEMICAL bond lengths ,MICROWAVES - Abstract
The gas-phase rotational motion of hexafluorobenzene has been measured in real time using femtosecond (fs) time-resolved rotational Raman coherence spectroscopy (RR-RCS) at T = 100 and 295 K. This four-wave mixing method allows to probe the rotation of non-polar gasphase molecules with fs time resolution over times up to ~5 ns. The ground state rotational constant of hexafluorobenzene is determined as B
0 = 1029.740(28) MHz (2σ uncertainty) from RR-RCS transients measured in a pulsed seeded supersonic jet, where essentially only the v = 0 state is populated. Using this B0 value, RR-RCS measurements in a room temperature gas cell give the rotational constants Bv of the five lowest-lying thermally populated vibrationally excited states v7/8 , v9 , v11/12 , v13 , and v14/15 . Their Bv constants differ from B0 by between -1.02 MHz and +2.23 MHz. Combining the B0 with the results of all-electron coupled-cluster CCSD(T) calculations of Demaison et al. [Mol. Phys. 111, 1539 (2013)] and of our own allow to determine the C-C and C-F semi-experimental equilibrium bond lengths re (C-C) = 1.3866(3) Å and re (C-F) = 1.3244(4) Å. These agree with the CCSD(T)/wCVQZ re bond lengths calculated by Demaison et al. within ±0.0005 Å. We also calculate the semi-experimental thermally averaged bond lengths rg (C-C)=1.3907(3) Å and rg (C-F)=1.3250(4) Å. These are at least ten times more accurate than two sets of experimental gas-phase electron diffraction rg bond lengths measured in the 1960s. [ABSTRACT FROM AUTHOR]- Published
- 2014
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28. Hydrogen bonding and tunneling in the 2-pyridone·2-hydroxypyridine dimer. Effect of electronic excitation
- Author
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Borst, David R., Roscioli, Joseph R., Pratt, David W., Florio, Gina M., Zwier, Timothy S., Müller, Andreas, and Leutwyler, Samuel
- Published
- 2002
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29. Low-lying excited states and nonradiative processes of 9-methyl-2-aminopurine.
- Author
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Trachsel, Maria A., Lobsiger, Simon, Schär, Tobias, and Leutwyler, Samuel
- Subjects
ULTRAVIOLET radiation ,RESONANCE ,IONIZATION (Atomic physics) ,AMINO acids ,VIBRATION measurements - Abstract
The UV spectrum of the adenine analogue 9-methyl-2-aminopurine (9M-2AP) is investigated with one- and two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm
-1 resolution in a supersonic jet. The electronic origin at 32 252 cm-1 exhibits methyl torsional subbands that originate from the 0A1 ″ (l = 0) and 1E″ (l = ±1) torsional levels. These and further torsional bands that appear up to 00 0 + 230 cm-1 allow to fit the threefold (V3 ) barriers of the torsional potentials as ∣V3 ′∣ = 50 cm-1 in the S0 and ∣V3 ′∣ = 126 cm-1 in the S1 state. Using the B3LYP density functional and correlated approximate second-order coupled cluster CC2 methods, the methyl orientation is calculated to be symmetric relative to the 2AP plane in both states, with barriers of V3 ″= 20 cm-1 and V3 ′ = 115 cm-1 . The 00 0 rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis ¹ππ* excitation. The residual 25% c-axis polarization may indicate coupling of the ¹ππ* to the close-lying ¹nπ* state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated ¹nπ oscillator strength is only 6% of that of the ¹ππ* transition. The ¹ππ* vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm-1 . The methyl torsion and the low-frequency out-of-plane ν1 ′ and ν2 ′ vibrations are strongly coupled in the ¹ππ* state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the ¹ππ* spectrum of 9H-2-aminopurine [S. Lobsiger, R. K. Sinha, M. Trachsel, and S. Leutwyler, J. Chem. Phys. 134, 114307 (2011)]. From the Lorentzian broadening needed to fit the 000 contour of 9M-2AP, the ¹ππ* lifetime is τ ≥ 120 ps, reflecting a rapid nonradiative transition. [ABSTRACT FROM AUTHOR]- Published
- 2014
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30. Excitonic splitting and coherent electronic energy transfer in the gas-phase benzoic acid dimer.
- Author
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Ottiger, Philipp and Leutwyler, Samuel
- Subjects
- *
ENERGY transfer , *GAS phase reactions , *BENZOIC acid , *HYDROGEN bonding , *SYMMETRY (Physics) , *DIPOLE moments , *BAND gaps - Abstract
The benzoic acid dimer, (BZA)2, is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S1/S2 state splitting and coherent electronic energy transfer within supersonically cooled (BZA)2 and its 13C-, d1 -, d2 -, and 13C/d1 - isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)2-(h - h) and (BZA)2-(d - d) dimers are C2h symmetric, hence only the S2 ← S0 transition can be observed, the S1 ← S0 transition being strictly electric-dipole forbidden. A single 12C/13C or H/D isotopic substitution reduces the symmetry of the dimer to Cs, so that the isotopic heterodimers (BZA)2 - 13C, (BZA)2 -(h - d), (BZA)2 -(h13C-d), and (BZA)2 -(h - d13C) show both S1 ← S0 and S2 ← S0 bands. The S1/S2 exciton splitting inferred is Δexc = 0.94 ± 0.1 cm-1. This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, Δiso, arise from the change of the zero-point vibrational energy upon electronic excitation and range from Δiso = 3.3 cm-1 upon 12C/13C substitution to 14.8 cm-1 for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S1 ← S0 and S2 ← S0 origin bands; near-complete localization is observed even for a single 12C/13C substitution. The S1/ S2 energy gap of (BZA)2 is Δcalcexc=11 cm-1 when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to Δvibronexc=2.1 cm-1 [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed Δexc = 0.94 cm-1. The observed excitonic splittings can be converted to exciton hopping times τexc. For the (BZA)2-(h - h) homodimer τexc = 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)2 in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]. Thus, the electronic energy transfer is much faster than the proton-transfer in (BZA)2*. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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31. Vibrational quenching of excitonic splittings in H-bonded molecular dimers: Adiabatic description and effective mode approximation.
- Author
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Kopec, Sabine, Ottiger, Philipp, Leutwyler, Samuel, and Köppel, Horst
- Subjects
EXCITON theory ,HYDROGEN bonding ,DIMERS ,APPROXIMATION theory ,QUENCHING (Chemistry) ,QUANTUM perturbations ,ELECTRIC distortion - Abstract
The quenching of the excitonic splitting in hydrogen-bonded molecular dimers has been explained recently in terms of exciton coupling theory, involving Förster's degenerate perturbation theoretical approach [P. Ottiger, S. Leutwyler, and H. Köppel, J. Chem. Phys. 136, 174308 (2012)]. Here we provide an alternative explanation based on the properties of the adiabatic potential energy surfaces. In the proper limit, the lower of these surfaces exhibits a double-minimum shape, with an asymmetric distortion that destroys the geometric equivalence of the excitonically coupled monomers. An effective mode is introduced that exactly reproduces the energy gain and amount of distortion that occurs in a multi-dimensional normal coordinate space. This allows to describe the quenched exciton splitting as the energy difference of the two (S1 and S2) vibronic band origins in a one-dimensional (rather than multi-dimensional) vibronic calculation. The agreement with the earlier result (based on Förster theory) is excellent for all five relevant cases studied. A simple rationale for the quenched exciton splitting as nonadiabatic tunneling splitting on the lower double-minimum potential energy surface is given. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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32. Benchmark Experimental Gas-Phase Intermolecular Dissociation Energies by the SEP-R2PI Method.
- Author
-
Knochenmuss, Richard, Sinha, Rajeev K., and Leutwyler, Samuel
- Abstract
The gas-phase ground-state dissociation energy D0(So) of an isolated and cold bimolecular complex is a fundamental measure of the intermolecular interaction strength between its constituents. Accurate D0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parameterization of dispersion-corrected density functionals or force-field models that are used in fields ranging from crystallography to biochemistry. We review experimental measurements of the gas-phase D
0 (S0 ) and Do(Si) values of 55 different M-S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experiments employ the triply resonant SEP-R2PI laser method, which involves M-centered (S0 -> S1 ) electronic excitation, followed by S1 -> S0 stimulated emission spanning a range of So state vibrational levels. At sufficiently high vibrational energy, vibrational predissociation of the M-S complex occurs. A total of 49 dissociation energies were bracketed to within ≤1.0 kj/mol, providing a large experimental database of accurate noncovalent interactions. [ABSTRACT FROM AUTHOR]- Published
- 2020
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- View/download PDF
33. The S1/S2 exciton interaction in 2-pyridone·6-methyl-2-pyridone: Davydov splitting, vibronic coupling, and vibronic quenching.
- Author
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Heid, Cornelia G., Ottiger, Philipp, Leist, Roman, and Leutwyler, Samuel
- Subjects
PYRIDONE ,ELECTRONIC excitation ,IONIZATION (Atomic physics) ,ULTRAVIOLET radiation ,FLUORESCENCE spectroscopy ,METHYL groups ,HARTREE-Fock approximation - Abstract
The excitonic splitting between the S1 and S2 electronic states of the doubly hydrogen-bonded dimer 2-pyridone·6-methyl-2-pyridone (2PY·6M2PY) is studied in a supersonic jet, applying two-color resonant two-photon ionization (2C-R2PI), UV-UV depletion, and dispersed fluorescence spectroscopies. In contrast to the C2h symmetric (2-pyridone)2 homodimer, in which the S1 ← S0 transition is symmetry-forbidden but the S2 ← S0 transition is allowed, the symmetry-breaking by the additional methyl group in 2PY·6M2PY leads to the appearance of both the S1 and S2 origins, which are separated by Δexp = 154 cm-1. When combined with the separation of the S1 ← S0 excitations of 6M2PY and 2PY, which is δ = 102 cm-1, one obtains an S1/S2 exciton coupling matrix element of VAB, el = 57 cm-1 in a Frenkel-Davydov exciton model. The vibronic couplings in the S1/S2 ← S0 spectrum of 2PY·6M2PY are treated by the Fulton-Gouterman single-mode model. We consider independent couplings to the intramolecular 6a′ vibration and to the intermolecular σ′ stretch, and obtain a semi-quantitative fit to the observed spectrum. The dimensionless excitonic couplings are C(6a′) = 0.15 and C(σ′) = 0.05, which places this dimer in the weak-coupling limit. However, the S1/S2 state exciton splittings Δcalc calculated by the configuration interaction singles method (CIS), time-dependent Hartree-Fock (TD-HF), and approximate second-order coupled-cluster method (CC2) are between 1100 and 1450 cm-1, or seven to nine times larger than observed. These huge errors result from the neglect of the coupling to the optically active intra- and intermolecular vibrations of the dimer, which lead to vibronic quenching of the purely electronic excitonic splitting. For 2PY·6M2PY the electronic splitting is quenched by a factor of ∼30 (i.e., the vibronic quenching factor is Γexp = 0.035), which brings the calculated splittings into close agreement with the experimentally observed value. The 2C-R2PI and fluorescence spectra of the tautomeric species 2-hydroxypyridine·6-methyl-2-pyridone (2HP·6M2PY) are also observed and assigned. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
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34. Low-lying excited states and nonradiative processes of the adenine analogues 7H- and 9H-2-aminopurine.
- Author
-
Lobsiger, Simon, Sinha, Rajeev K., Trachsel, Maria, and Leutwyler, Samuel
- Subjects
ADENINE ,EXCITON theory ,ULTRAVIOLET spectra ,DIPOLE moments ,DEFORMATIONS (Mechanics) ,CLUSTER analysis (Statistics) ,VIBRATION (Mechanics) - Abstract
We have investigated the UV vibronic spectra and excited-state nonradiative processes of the 7H- and 9H-tautomers of jet-cooled 2-aminopurine (2AP) and of the 9H-2AP-d4 and -d5 isotopomers, using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.045 cm-1 resolution. The S1 ← S0 transition of 7H-2AP was observed for the first time. It lies ∼ 1600 cm-1 below that of 9H-2AP, is ∼1000 times weaker and exhibits only in-plane vibronic excitations. In contrast, the S1 ← S0 spectra of 9H-2AP, 9H-2AP-d4, and 9H-2AP-d5 show numerous low-frequency bands that can be systematically assigned to overtone and combinations of the out-of-plane vibrations ν1′, ν2′, and ν3′. The intensity of these out-of-plane bands reflects an out-of-plane deformation in the 1ππ*(La) state. Approximate second-order coupled-cluster theory also predicts that 2-aminopurine undergoes a 'butterfly' deformation in its lowest 1ππ* state. The rotational contours of the 9H-2AP, 9H-2AP-d4, and 9H-2AP-d5 0
0 0 bands and of eight vibronic bands of 9H-2AP up to 00 0 +600 cm-1 exhibit 75%-80% in-plane (a/b) polarization, which is characteristic for a 1ππ* excitation. A 20%-25% c-axis (perpendicular) transition dipole moment component may indicate coupling of the 1ππ* bright state to the close-lying 1nπ* dark state. However, no 1nπ* vibronic bands were detected below or up to 500 cm-1 above the 1ππ* 00 0 band. Following 1ππ* excitation, 9H-2AP undergoes a rapid nonradiative transition to a lower-lying long-lived state with a lifetime >=5μs. The ionization potential of 9H-2AP was measured via the 1ππ* state (IP = 8.020 eV) and the long-lived state (IP > 9.10 eV). The difference shows that the long-lived state lies >=1.08 eV below the 1ππ* state. Time-dependent B3LYP calculations predict the 3ππ* (T1) state 1.12 eV below the 1ππ* state, but place the 1nπ* (S1) state close to the 1ππ* state, implying that the long-lived state is the lowest triplet (T1) and not the 1nπ* state. [ABSTRACT FROM AUTHOR]- Published
- 2011
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35. S1/S2 excitonic splittings and vibronic coupling in the excited state of the jet-cooled 2-aminopyridine dimer.
- Author
-
Ottiger, Philipp, Leutwyler, Samuel, and Köppel, Horst
- Subjects
- *
EXCITED state chemistry , *PHYSICAL & theoretical chemistry , *DIMERS , *OLIGOMERS , *VIBRATION (Mechanics) - Abstract
We analyze the vibronic band structure of the excitonically coupled S1←S0/S2←S0 excitations of the 2-aminopyridine (2AP) self-dimer (2AP)2, using a linear vibronic coupling model [R. Fulton and M. Gouterman, J. Chem. Phys. 41, 2280 (1964)]. The vibronic spectra of supersonically cooled (2AP)2 and its 13C-isotopomer were measured by two-color resonant two-photon ionization and UV/UV-depletion spectroscopies. In the C2-symmetric form of (2AP)2, the S1←S0 (1A←1A) transition is very weak, while the close-lying S2←S0 (1B←1A) transition is fully allowed. A single 12C/13C isotopic substitution breaks the symmetry of the dimer so that the (2AP)2-13C isotopologue exhibits both S1 and S2 electronic origins, which are split by 11 cm-1. In Fulton–Gouterman-type treatments, the linear vibronic coupling is mediated by intramolecular vibrational modes and couplings to intermolecular vibrations are not considered. For (2AP)2, a major vibronic coupling contribution arises from the intramolecular 6a′ vibration. However, the low-energy part of the spectrum is dominated by intermolecular shear (χ′) and stretching (σ′) vibrational excitations that also exhibit excitonic splittings; we apply a linear vibronic coupling analysis for these also. The respective excitation transfer integrals VAB are 50%–80% of that of the intramolecular 6a′ vibration, highlighting the role of intermolecular vibrations in mediating electronic energy exchange. The S1/S2 electronic energy gap calculated by the approximate second-order coupled-cluster method is ∼340 cm-1. This purely electronic exciton splitting is quenched by a factor of 40 by the vibronic couplings to the Franck–Condon active intramolecular vibrations. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
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36. Electronic spectroscopy of the Au(6p)–Kr complex.
- Author
-
Plowright, Richard J., Watkins, Mark J., Gardner, Adrian M., Wright, Timothy G., Breckenridge, W. H., Wallimann, Franz, and Leutwyler, Samuel
- Subjects
ABSORPTION spectra ,RESONANCE ,MULTIPHOTON processes ,SPECTRUM analysis ,ATOMS ,NUCLEAR excitation ,ENERGY levels (Quantum mechanics) - Abstract
We report electronic absorption spectra, recorded using one- and two-color resonance-enhanced multiphoton ionization spectroscopy, of the Au–Kr complex. The transition is localized on the gold atom, and corresponds to a 6p←6s atomic excitation; we observe transitions to the D
2 Π1/2 and D2 Π3/2 spin-orbit states. In addition, we report the results of ab initio calculations, which consider electronic states arising from the 62 S, 52 D, and 62 P atomic energy levels of Au. Further, we also report an accurate value for the dissociation energy of the ground state of Au–Kr, based on basis set extrapolated RCCSD(T) calculations. The experimental results are discussed in the light of the theoretical ones. [ABSTRACT FROM AUTHOR]- Published
- 2008
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- View/download PDF
37. Spectral tuning by switching C–H...O hydrogen bonds: Rotation-induced spectral shifts of 7-hydroxyquinoline·HCOOH isomers.
- Author
-
Thut, Markus, Manca, Carine, Tanner, Christian, and Leutwyler, Samuel
- Subjects
SPECTRUM analysis ,HYDROGEN bonding ,PHYSICAL & theoretical chemistry ,DIHYDROGEN bonding ,HYDROXYQUINOLINE - Abstract
Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline·HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH...N hydrogen bond but with different (quinoline)C–H...O==C hydrogen bonds. A 180° rotation of the HCOOH moiety around the O–H...N hydrogen bond exchanges the C–H...O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an ∼850 cm
-1 shift of the electronic spectrum. Mass-selected S1 ←S0 resonant two-photon ionization, UV-UV holeburning, S1 →S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline·HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration χ in the S1 ←S0 spectra indicate that the weak C–H...O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O–H...N hydrogen bond undergoes little change upon S1 ←S0 excitation. [ABSTRACT FROM AUTHOR]- Published
- 2008
- Full Text
- View/download PDF
38. 2-pyridone: The role of out-of-plane vibrations on the S1↔S0 spectra and S1 state reactivity.
- Author
-
Frey, Jann A., Leist, Roman, Tanner, Christian, Frey, Hans-Martin, and Leutwyler, Samuel
- Subjects
PYRIDONE ,VIBRATIONAL spectra ,FLUORESCENCE spectroscopy ,IONIZATION (Atomic physics) ,SPECTRUM analysis - Abstract
The S
1 ↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N–H deuterated isotopomer (d-2PY) have been recorded by two-color resonant two-photon ionization, laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combining these methods, the B origin of 2PY at 00 0 +98 cm-1 and the bands at +218 and +252 cm-1 are identified as overtones of the S1 state out-of-plane vibrations ν1 ′ and ν2 ′ , as are the analogous bands of d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-plane coordinates that predict further ν1 ′ and ν2 ′ overtones and combinations, reproducing ∼80% of the vibronic bands up to 600 cm-1 above the 00 0 band. The fluorescence spectra excited at the electronic origins and the ν1 ′ and ν2 ′ out-of-plane overtone levels confirm these assignments. The S1 nonplanar minima and S1 ←S0 out-of-plane progressions are in agreement with the determination of nonplanar vibrationally averaged geometries for the 00 0 and 00 0 +98 cm-1 upper states by Held et al. [J. Chem. Phys. 95, 8732 (1991)]. The fluorescence lifetimes of the S1 state vibrations show strong mode dependence: Those of the out-of-plane levels decrease rapidly above 200 cm-1 excess vibrational energy, while the in-plane vibrations ν5 ′ , ν8 ′ , and ν9 ′ have longer lifetimes, although they are above or interspersed with the “dark” out-of-plane states. This is interpreted in terms of an S1 ′ state reaction with a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-plane vibrational states can directly surmount this barrier, whereas in-plane vibrations are much less efficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S0 ′ state fundamentals, overtones of the S0 state ν1 ″ and ν2 ″ out-of-plane vibrations, and >30 other overtones and combination bands. The B3LYP/6-311++G(d,p) calculated anharmonic wave numbers are in very good agreement with the observed fundamentals, overtones, and combinations, with a deviation Δrms =1.3%. [ABSTRACT FROM AUTHOR]- Published
- 2006
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39. Femtosecond degenerate four-wave mixing of carbon disulfide: High-accuracy rotational constants.
- Author
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Kummli, Dominique S., Frey, Hans M., and Leutwyler, Samuel
- Subjects
CARBON disulfide ,FOURIER transform infrared spectroscopy ,CARBON dioxide lasers ,CHEMICAL equilibrium ,EXTRAPOLATION ,PHYSICAL & theoretical chemistry - Abstract
Femtosecond degenerate four-wave mixing (fs-DFWM) rotational coherence spectroscopy (RCS) has been used to determine the rotational and centrifugal distortion constants of the 00
0 0 ground and 011 0 vibrationally excited states of gas-phase CS2 . RCS transients were recorded over the 0–3300 ps optical delay range, allowing the observation of 87 recurrences. The fits yield rotational constants B00 =3.271 549 2(18) GHz for0 012 C32 S2 and B00 =3.175 06(21) GHz for the0 012 C32 S34 S isotopomer. The rotational constants of the degenerate 011 0 bending level of12 C32 S2 are B01 =3.276 72(40) and 3.279 03(40) GHz for the e and f substrates, respectively. These fs-DFWM rotational constants are ten times more accurate than those obtained by CO1 02 laser/microwave heterodyne measurements and are comparable to those obtained by high-resolution Fourier transform infrared spectroscopy. Ab initio calculations were performed at two levels, second-order Mo\ller-Plesset theory and coupled-cluster singles, doubles, and iterative triples [CCSD(T)]. The equilibrium and vibrationally averaged C==S distances were calculated using large Dunning basis sets. An extrapolation procedure combining the ab initio rotational constants with the experiment yields an equilibrium C==S bond length of 155.448 pm to an accuracy of ±20 fm. The theoretical C==S bond length obtained by a complete basis set extrapolation at the CCSD(T) level is re (C==S)=155.579 pm, or 0.13 pm longer than that in the experiment. [ABSTRACT FROM AUTHOR]- Published
- 2006
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40. Femtosecond degenerate four-wave mixing of cyclopropane.
- Author
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Kummli, Dominique S., Frey, Hans M., Keller, Michael, and Leutwyler, Samuel
- Subjects
FEMTOCHEMISTRY ,CYCLOPROPANE ,INFRARED spectroscopy ,MICROWAVE spectroscopy ,ATOMIC orbitals ,MOLECULAR orbitals - Abstract
Femtosecond degenerate four-wave mixing (fs-DFWM) is applied for the measurement of rotational constants of cyclopropane (C
3 H6 ). The rotational coherence method yields a very accurate B0 =20 093.322(12) MHz and centrifugal distortion constants DJ and DJK . To exploit the full resolution of the fs-DFWM method, the accuracy of the optical delay measurement was increased by nearly two orders of magnitude, including elimination of effects from the refractive index of air. The fs-DFWM molecular constants are comparable in accuracy to those from high-resolution infrared spectroscopy and are only surpassed by those of dipole distortion microwave spectroscopy. In parallel, the equilibrium structure, vibrationally averaged structure parameters and rotational constants were calculated using high-level ab initio methods and large basis sets. Combining these with the results of previous calculations and the measured rotational constants yields re (C–C)=1.5034(3) Å, re (C–H)=1.0775(5) Å, and αe (H–C–H)=115.09(10)°. [ABSTRACT FROM AUTHOR]- Published
- 2005
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41. Exploring excited-state hydrogen atom transfer along an ammonia wire cluster: Competitive reaction paths and vibrational mode selectivity.
- Author
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Tanner, Christian, Manca, Carine, and Leutwyler, Samuel
- Subjects
PHYSICAL & theoretical chemistry ,NITROGEN compounds ,DISSOCIATION (Chemistry) ,HYDROGEN ,CHARGE exchange ,ELECTRONIC excitation - Abstract
The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline·(NH
3 )3 cluster involves a crossing from the initially excited1 ππ* to a1 πσ* state. The nonadiabatic coupling between these states induces homolytic dissociation of the O–H bond and H-atom transfer to the closest NH3 molecule, forming a biradical structure denoted HT1, followed by two more Grotthus-type translocation steps along the ammonia wire. We investigate this reaction at the configuration interaction singles level, using a basis set with diffuse orbitals. Intrinsic reaction coordinate calculations of the enol→HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire, as well as large O–H...NH3 hydrogen bond contraction and expansion. The calculations also predict an excited-state proton transfer path involving synchronous proton motions; however, it lies 20–25 kcal/mol above the ESHAT path. Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate: Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these Tn states branches the reaction back to the enol reactant side, decreasing the ESHAT yield. In fact, a product yield of ≈40% 7-ketoquinoline·(NH3 )3 is experimentally observed. The vibrational mode selectivity of the enol→HT1 reaction step [C. Manca, C. Tanner, S. Coussan, A. Bach, and S. Leutwyler, J. Chem. Phys. 121, 2578 (2004)] is shown to be due to the large sensitivity of the diffuse πσ* state to vibrational displacements along the intermolecular coordinates. [ABSTRACT FROM AUTHOR]- Published
- 2005
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42. Infrared depletion spectra of 2-aminopyridine·2-pyridone, a Watson–Crick mimic of adenine·uracil.
- Author
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Frey, Jann A., Müller, Andreas, Frey, Hans-Martin, and Leutwyler, Samuel
- Subjects
AMINOPYRIDINES ,SPECTRUM analysis ,INFRARED spectra ,DENSITY functionals ,HYDROGEN bonding ,MOLECULAR association - Abstract
The 2-aminopyridine·2-pyridone (2AP·2PY) dimer is linked by N–H...O==C and N–H...N hydrogen bonds, providing a model for the Watson–Crick hydrogen bond configuration of the adenine·thymine and adenine·uracil nucleobase pairs. Mass-specific infrared spectra of 2AP·2PY and its seven N–H deuterated isotopomers have been measured between 2550 and 3650 cm
-1 by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N–H stretch is a very intense band spread over the range 2700–3000 cm-1 due to large anharmonic couplings. It is shifted to lower frequency by 710 cm-1 or ≈20% upon H bonding to 2AP. On the 2AP moiety, the “bound” amino N–H stretch gives rise to a sharp band at 3140 cm-1 , which is downshifted by 354 cm-1 or ≈10% upon H bonding to 2PY. The amino group “free” N–H stretch and the H–N–H bend overtone are sharp bands at ≈3530 cm-1 and 3320 cm-1 . Ab initio structures and harmonic vibrations were calculated at the Hartree–Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]- Published
- 2004
- Full Text
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43. H atom transfer along an ammonia chain: Tunneling and mode selectivity in 7-hydroxyquinoline·(NH3)3.
- Author
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Manca, Carine, Tanner, Christian, Coussan, Stephane, Bach, Andreas, and Leutwyler, Samuel
- Subjects
HYDROGEN ,ATOMS ,EXCITON theory ,AMMONIA ,HYDROXYQUINOLINE - Abstract
Excitation of the 7-hydroxyquinoline·(NH
3 )3 [7HQ·(NH3 )3 ] cluster to the S1 1 ππ* state results in an O-H→NH3 hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S1 ↔S0 spectra of the 7HQ·(NH3 )3 and the d2 -7DQ·(ND3 )3 clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S1 ←S0 spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200–250 cm-1 for 7HQ·(NH3 )3 accelerates the HAT reaction by an order of magnitude or more. The d2 -7DQ·(ND3 )3 cluster exhibits a more gradual threshold from 300 to 650 cm-1. For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H→NH3 coordinate. The barrier results from a conical intersection of the optically excited1 ππ* state with an optically dark1 πσ* state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH3 distance and the energetic separation between the1 ππ* and1 πσ* states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the1 ππ* ↔1 πσ* interaction to a much smaller extent. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]- Published
- 2004
- Full Text
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44. Ground- and excited state proton transfer and tautomerization in 7-hydroxyquinoline·(NH[sub 3])[sub n] clusters: Spectroscopic and time resolved investigations.
- Author
-
Bach, Andreas, Tanner, Christian, Manca, Carine, Frey, Hans-Martin, and Leutwyler, Samuel
- Subjects
TAUTOMERISM ,PROTON transfer reactions ,HYDROXYQUINOLINE - Abstract
Mass-selected S[sub 1]↔S[sub 0] two color resonant two photon ionization (2C-R2PI) spectra, fluorescence spectra and fluorescence decay times are measured for supersonically cooled 7-hydroxyquinoline (7HQ)·(NH[sub 3])[sub n] clusters with n=4–10. For n=4, the S[sub 1]←S[sub 0] 2C-R2PI spectrum shows a 20 cm[sup -1] broad electronic origin at 27 746 cm[sup -1], followed by an intermolecular vibrational progression with band widths that increase up to ≈45 cm[sup -1]. In contrast, the 2C-R2PI spectra of the mixed 7HQ·(NH[sub 3])[sub 3]H[sub 2]O and 7HQ·(NH[sub 3])[sub 2](H[sub 2]O)[sub 2] clusters exhibit narrow bands of 1–2 cm[sup -1] width. The large band widths of 7HQ·(NH[sub 3])[sub 4] are due to a fast (k>10[sup 12] s[sup -1]) excited state process which is blocked when replacing one or more NH[sub 3] molecules by H[sub 2]O in the cluster. For the n=5–10 clusters, the 2C-R2PI spectra display two broad absorption bands peaking at 25 000 and 27 000 cm[sup -1]. The latter is characteristic of the 7-quinolinate (7Q[sup -]) anion, implying that ground state proton transfer from 7HQ to the ammonia cluster occurs for n>=5. Excitation at 27 000 cm[sup -1] leads to fluorescence from the 7Q[sup -*] anion clusters at 22 500 cm[sup -1] with τ[sub fl]=14–27 ns, and also to fluorescence emission from 7KQ[sup *] clusters at 18 370 cm[sup -1] with τ[sub fl]≈7 ns, signaling excited state anion→keto proton transfer. The 25 000 cm[sup -1] absorption band is characteristic of ground state 7KQ, implying that the n>=5 clusters also undergo ground state enol→keto tautomerization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
45. Ammonia-chain clusters: Vibronic spectra of 7-hydroxyquinoline·(NH[sub 3])[sub 2].
- Author
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Coussan, Stéphane, Manca, Carine, Tanner, Christian, Bach, Andreas, and Leutwyler, Samuel
- Subjects
NUCLEAR isomers ,FLUORESCENCE ,HYDROXYQUINOLINE ,AMMONIA - Abstract
Mass- and isomer-selected S[SUB1]←S[SUB0] resonant two-photon ionization and S[SUB1]→S[SUB0] fluorescence spectra were measured for the 7-hydroxyquinoline·(NH[SUB3])[SUB2] [7HQ·(NH[SUB3])[SUB2]] and d[SUB2]-7-hydroxyquinoline ·(ND[SUB3])[SUB2] clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an ''ammonia wire'' hydrogen bonded to the -OH and N groups of the cis-7HQ rotamer. The experimental S[SUB0] and S[SUB1] frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S[SUB1]←S[SUB0] excitation leads to contractions of the -O-H...N and NH[SUB3]...NH[SUB3] hydrogen bonds, as well as smaller displacements for the NH[SUB3]...N(quinoline) stretch and the in plane rotation (or bend) of the ammonia dimer relative to 7HQ. The coupling of these modes to the S[SUB1]←S[SUB0] electronic excitation indicates that hydrogen bond contractions in the excited state are important and may be prerequisite for the S[SUB1] state proton transfer processes that occur in the larger 7HQ· (NH[SUB3])[SUBn] (n≥4) clusters. The calculated electron density differences upon S[SUB1]←S[SUB0] excitation show large π-electron flows on the 7HQ moiety. However, the σ-electronic rearrangements that directly drive the hydrogen bond rearrangements are one to two orders of magnitude smaller. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
46. O–H flipping vibrations of the Cage water hexamer: An ab initio study.
- Author
-
Losada, Martin and Leutwyler, Samuel
- Subjects
- *
BONDS (Finance) , *HYDROGEN , *OXYGEN , *MOLECULES , *MOLECULAR orbitals - Abstract
In the Cage[1] isomer of the water hexamer, the free O-H bonds of the two end-cap water molecules can flip between "up" (u) and "down" (d) orientations, giving four conformers denoted uu, ud, du, and dd. Using the Møller-Plesset second order perturbation method and large basis sets, we calculate fully relaxed potential energy curves as a function of both u↔d torsional angles, denoted φ[SUB1], φ[SUB2]. These predict du as the lowest conformer, with uu nearly degenerate and ud and dd at 30-40 and 50- 70 cm[SUP-1] higher energy, respectively. Along φ[SUB1] the torsional barriers are about 200 cm[SUP-1], along φSUB1]. The torsional zero-point energies are high, the φ[SUB2] between 80 and 110 cm vibrational ground states are strongly delocalized and averaging of the cluster properties is important along both φ[SUB1] and φ[SUB2]. The dipole moment components vary strongly along both φ[SUB1] and φ[SUB2]: μ[SUBa] changes from +0.8 to +2.2 D, μ[SUBb] from +0.5 to +1.2 D, and μ[SUBc] from +1.4 to -0.9 D. The φ[SUB2] torsional fundamental of H[SUB2]O[SUB6] is predicted in the range 65- 72 cm[SUP1] with an intensity of ≈0.5 D[SUP2],theφ[SUP1] fundamental is in the 22- 32 cm[SUP-1] range, with an intensity of ≈0.3 D[SUP2]. Both excitations are b/c hybrid perpendicular bands with a dominant b component. The torsional overtones should be very weak. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
47. Structural study of the hydrogen-bonded 1-naphthol·(NH[sub 3])[sub 2] cluster.
- Author
-
Tanner, Christian, Henseler, Debora, Leutwyler, Samuel, Connell, Leslie L., and Felker, Peter M.
- Subjects
HYDROGEN bonding ,SPECTRUM analysis ,IONIZATION (Atomic physics) - Abstract
The structure of the 1-naphthol (NH[SUB3])[SUB2] cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol (NH[SUB3])[SUB2] as A = 1197, B = 500, and C = 413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Mφller-Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol-OH… NH[SUB3]… (NH[SUB3] chains, with the second NH[SUB3] (bent above the proximal aromatic ring and pointing towards the p-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster fragmentation, the S[SUB1]←S[SUB0] electronic origin region of the 1-naphthol (NH[SUB3])] [SUB2 - 4] cluster series as reassigned, in agreement with the work of Dedonder-Lardeux et al. [Phys. Chem. Chem. Phys. 3, 4316 (2001)]. In one-color experiments, the 1-naphthol (NH[SUB3])[SUB3] cluster fragments with nearly 100% efficiency into the 1-naphthol (NH[SUB3])[SUB2SUP+] mass channel. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
48. Water hexamer clusters: Structures, energies, and predicted mid-infrared spectra.
- Author
-
Losada, Martin and Leutwyler, Samuel
- Subjects
- *
WATER , *NUCLEAR isomers , *MOLECULAR dynamics , *PHYSICS - Abstract
We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D[sub 0]. Møller-Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and DO levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O-H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H[sub 2]O)[sub 6] gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8-26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair+-+Book ↔ Cage isomerization pathways. [ABSTRACT FROM AUTHOR]
- Published
- 2002
- Full Text
- View/download PDF
49. S[sub 1]/S[sub 2] exciton splitting in the (2-pyridone)[sub 2] dimer.
- Author
-
Mu¨ller, Andreas, Talbot, Francis, and Leutwyler, Samuel
- Subjects
DIMERS ,IONIZATION (Atomic physics) ,FLUORESCENCE spectroscopy - Abstract
The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)[sub 2] and its [sup 13]C-, d[sub 1]-, and d[sub 2]-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C[sub 2h] symmetric (2PY)[sub 2]–h[sub 2] and (2PY)[sub 2]–d[sub 2] complexes, the 2PY moieties are equivalent and the S[sub 1]↔S[sub 0] ([sup 1]A[sub g]↔[sup 1]A[sub g]) transition is forbidden. A single H/D or [sup 12]C/[sup 13]C isotopic substitution reduces the symmetry to C[sub s], so that (2PY)[sub 2]–[sup 13]C and (2PY)[sub 2]–d[sub 1] both exhibit S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] transitions. The S[sub 1]/S[sub 2] state exciton splittings are 43.6 cm[sup -1] and 52.4 cm[sup -1], respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)[sub 2]–d[sub 1], whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] spectra are analyzed and assigned. Several b[sub u] intermolecular vibrations of S[sub 1] appear via vibronic coupling to the S[sub 2](B[sub u]) state. Combination of the fluorescence data from excitation of the S[sub 1] and S[sub 2] origins and vibrational excitations of (2PY)[sub 2]–h[sub 2], (2PY)[sub 2]–d[sub 1], and (2PY)[sub 2]–d[sub 2] allows the determination of the six S[sub 0] state intermolecular vibrational frequencies.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]
- Published
- 2002
- Full Text
- View/download PDF
50. Accurate dissociation energies of O–H...O hydrogen-bonded 1-naphthol·solvent complexes.
- Author
-
Wickleder, Claudia, Henseler, Debora, and Leutwyler, Samuel
- Subjects
DISSOCIATION (Chemistry) ,COMPLEX compounds - Abstract
Accurate O–H...O hydrogen-bond dissociation energies were measured for the supersonic-jet-cooled complexes 1-naphthol·S with S=D[sub 2]O, ethanol, oxirane, and oxetane. A mass-selective pump–dump–probe method was used, combining stimulated emission pumping with resonant two-photon ionization and ion-dip techniques. The ground-state dissociation energies D[sub 0](S[sub 0]) are 5.83±0.13 kcal/mol for d[sub 1]-1-naphthol·D[sub 2]O, 7.94±0.02 kcal/mol for 1-naphthol·ethanol, 7.71±0.14 kcal/mol for 1-naphthol·oxirane and >8.17 kcal/mol for 1-naphthol·oxetane. The D[sub 0]’s increase by 5%–7% upon excitation of 1-naphthol to the S[sub 1] state. These dissociation energies are compared to those of the analogous complexes with S=H[sub 2]O, methanol, NH[sub 3], and ND[sub 3] [Chem. Phys. Lett. 246, 291 (1996)]. The trends in D[sub 0] are compared to the electric dipole moments μ, molecular polarizabilities α¯, and gas-phase proton affinities of the H bond acceptor molecules. For the O-containing acceptors, the D[sub 0]’s correlate well with α¯, but the only good overall correlation for both O- and N-containing acceptors was found between the dissociation energies and proton affinities. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]
- Published
- 2002
- Full Text
- View/download PDF
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