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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 D0 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. Despite 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]

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 000+ 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 000rotational 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]

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]

The S1↔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 000+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 000 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 000 and 000+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]

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]

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]

A spectroscopic study of supersonic jet-cooled catechol (1,2-dihydroxybenzene) and its d1- and d2-isotopomers, deuterated at the hydroxy groups, was performed by resonant two-photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular-beam hole-burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S0→S1 spectrum from the 000 band to 400 cm-1 above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S1 and S0 state frequencies and the Franck–Condon factors in absorption and emission, using a double-minimum potential for the S1 state and a harmonic potential for the S0 state. In the S1 state one of the two –O–H torsional mode frequencies is lowered from τ2≊250 to ≊50 cm-1, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates. [ABSTRACT FROM AUTHOR]

The effect of rare gas complexation on the electronically excited S1(E’) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two-color resonant two-photon ionization (2C-R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene·Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high-symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1+E)xe coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T·Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V-shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T·Ne and T·Ne2 were fitted with a considerable uniaxial strain of -19.73 and 19.07 cm-1, respectively, confirming both the predicted C2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V-shaped compartments by ∼0.8°. The spectra of T·Ne and T·Ne2 are much more complex compared to triptycene and T·Ne3 due to the splitting of the E vibronic levels in C2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties. [ABSTRACT FROM AUTHOR]

The vibronic structure in the S1(E)←S0(A1) resonant two-photon ionization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—Exe, (A+E)xe, and (A+E)x(a2+e). The basic Exe interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron-withdrawing effect of the F atom. For the same reason, the exciton splitting between the S1 and the S2 states, as calculated in the (A+E)xe model, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm-1 are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S0 frequencies, as well as those of triptycene. A 273 cm-1 degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher-energy (200–360 cm-1) part of the R2PI spectrum, and may represent an example of e+e multimode coupling within a degenerate electronic state. [ABSTRACT FROM AUTHOR]

The irregular vibronic structure resolved in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10-dihydro-9,10[1’2’]benzenoanthracene) is assigned in terms of a single-mode E’xe’ Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe’ =47.83 cm-1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≊98% of the observed absorption band intensities over the observable range 0–350 cm-1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry-equivalent minima. Emission spectra from 13 vibronic levels in the excited E’ state show extended vibrational progressions with up to 25 members in the analogous e’ ground state vibration, which is highly harmonic in the electronic ground state. The Franck–Condon factors of the fluorescence emission spectra calculated with the E’ state Jahn–Teller parameters fitted to the absorption spectrum also yield a quantitative fit to observed emission intensities. The eigenvectors of the E’ state vibronic levels are hence determined to great precision; the lowest five can be classified as radial oscillator and/or hindered rotor states, while higher levels have mixed character. Several eigenvectors are strongly localized in the upper sheet of the adiabatic Jahn–Teller surface, corresponding to ‘‘cone’’ states. [ABSTRACT FROM AUTHOR]

We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet-cooled phenol·(H2O)3 and d1-phenol·(D2O)3 in the S0 and S1 electronic states. Two-color resonant two-photon ionization combined with time-of-flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass-selective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven low-frequency intermolecular modes were observed for phenol·(H2O)3, and seven for the D isotopomer. For the S1 state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol·(H2O)3 were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H-bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal-mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S0 state vibrational frequencies and isotope shifts. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the intermolecular vibrations of 2,3-dimethylnaphthalene·Ar (2,3-DMN·Ar), for the first excited electronic state (S1), is reported. Methyl groups at C2 and C3 positions of naphthalene lower the symmetry of the complex, so that transitions involving excitation of the intermolecular long-axis in-plane x mode become allowed in electronic spectra, in addition to the out-of-plane z mode. Two-color resonant two-photon ionization (2C-R2PI) spectrum of the van der Waals (vdW)-mode region (000+70 cm-1) of 2,3-DMN·Ar exhibits six bands to the high-frequency side of the electronic origin 000, which arise from excitation of low-frequency intermolecular vibrations of the complex in the S1 state. Accurate quantum three-dimensional (3D) calculations of vdW vibrational (J=0) levels of S1 2,3-DMN·Ar have been performed, using a recently developed quantum method based on the 3D discrete variable representation. Since no approximation is made in the treatment of coupled, very anharmonic vdW vibrations, the calculated eigenstates are essentially exact for the intermolecular potential energy surface (PES) employed, thus enabling direct comparison between theory and experiment. The intermolecular PES was modeled as a sum of atom–atom Lennard-Jones (LJ) pair potentials. Some of the initial LJ parameters were modified until very good agreement was achieved between the calculated and measured vdW frequencies of S1 2,3-DMN·Ar. This allowed assignment of the vdW bands to the blue side of 000, and resulted in an improved intermolecular PES of the complex. In addition, the quantum 3D calculations provided a quantitative description of the vdW vibrational level structure and floppiness of S1 2,3-DMN·Ar up to ∼60–70 cm-1 above the ground vdW state. The wave functions of all vdW states below ∼49 cm-1, relative to the ground state, are sufficiently regular to allow assignment... [ABSTRACT FROM AUTHOR]

The supersonically cooled 7-hydroxyquinoline·(H[sub 2]O)[sub 3] cluster was investigated by mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectroscopy. UV(ultraviolet)/UV-holeburning measurements prove that a single cluster isomer is formed, although different tautomers (7-keto- and 7-hydroxyquinoline), rotamers (cis and trans) and isomers (cyclic and water-wire) are possible. Ab initio calculations of structures and vibrations of different tautomers and isomers of this cluster predict that the cis-enol-7-hydroxyquinoline·(H[sub 2]O)[sub 3] "water-wire" cluster is the most stable species. The experimental S[sub 0] and S[sub 1] inter- and intramolecular vibrational frequencies are in good agreement with the calculated harmonic frequencies. S[sub 1]←S[sub 0] excitation leads to contraction of all four hydrogen bonds along the hydrogen bonded water wire, inducing intense intermolecular stretching vibrations of the O-H...O and HO-H...N(quinoline) hydrogen bonds which terminate the water-wire. There are no spectroscopic signs of proton transfer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline·(H[sub 2]O)[sub 2] cluster. UV/UV-holeburning measurements show that >98% of the spectrum is due to a single "water-chain" cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline·(H[sub 2]O)[sub 2] "up/down" water-chain form is the most stable cluster. The experimentally observed S[sub 0] and S[sub 1] state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S[sub 1]←S[sub 0] excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecular stretching vibrations, but no proton transfer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the analysis of the S1 ← S0 rotational band contours of jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. Unlike thymine, which exhibits a structureless spectrum, the vibronic spectrum of 5M2HP is well structured, allowing us to determine the rotational constants and the methyl group torsional barriers in the S0 and S1 states. The Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. band contours were measured at 900 MHz (0.03 cm-1) resolution using mass-specific two-color resonant two-photon ionization (2C-R2PI) spectroscopy. All four bands are polarized perpendicular to the pyrimidine plane (>90% c type), identifying the S1 ← S0 excitation of 5M2HP as a ¹nπ* transition. All contours exhibit two methyl rotor subbands that arise from the lowest 5-methyl torsional states 0A" and 1E". The S0 and S1 state torsional barriers were extracted from fits to the torsional subbands. The 3-fold barriers are V3" = 13 cm-1 and V3' = 51 cm-1; the 6-fold barrier contributions V6" and V6' are in the range of 2-3 cm-1 and are positive in both states. The changes of A, B, and C rotational constants upon S1 ← S0 excitation were extracted from the contours and reflect an "anti-quinoidal" distortion. The Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. contour can only be simulated if a 3 GHz Lorentzian line shape is included, which implies that the S1(¹nπ*) lifetime is 55 ps. For the Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. bands, the Lorentzian component increases to 5.5 GHz, reflecting a lifetime decrease to 30 ps. The short lifetimes are consistent with the absence of fluorescence from the ¹nπ* state. Combining these measurements with the previous observation of efficient intersystem crossing (ISC) from the S1 state to a long-lived T1 (³nπ*) state that lies 2200 cm-1 below [S. Lobsiger, S. et al. Phys. Chem. Chem. Phys. 2010, 12, 5032] implies that the broadening arises from fast intersystem crossing with kISC ≈ 2 x 1010 s-1. In comparison to 5-methylpyrimidine, the ISC rate is enhanced by at least 10 000 by the additional hydroxy group in position 2. [ABSTRACT FROM AUTHOR]

Wehave measured the mass- and tautomer-specific S0→S1vibronic spectra and S1state lifetimes ofthe keto–amino tautomer of cytosine cooled in supersonic jets,using two-color resonant two-photon ionization (R2PI) spectroscopyat 0.05 cm–1resolution. The rotational contoursof the 000bandand nine vibronic bands up to +437 cm–1are polarizedin the pyrimidinone plane, proving that the electronic excitationis to a 1ππ* state. All vibronic excitationsup to +437 cm–1are overtone and combination bandsof the low-frequency out-of-plane ν1′(butterfly), ν2′(boat),and ν3′(H–N–C6–H twist) vibrations. UVvibronic spectrum simulations based on approximate second-order coupled-cluster(CC2) calculations of the ground and 1ππ* statesare in good agreement with the experimental R2PI spectrum, but onlyif the calculated ν1′and ν2′frequencies are reduced by afactor of 4 and anharmonicity is included. Together with the highintensity of the ν1′and ν2′overtone vibronic excitations, thisimplies that the 1ππ* potential energy surfaceis much softer and much more anharmonic in the out-of-plane directionsthan predicted by the CC2 method. The 1ππ*state lifetime is determined from the Lorentzian line broadening necessaryto reproduce the rotational band contours: at the 000band it is τ = 44 ps, remainsat τ = 35–45 ps up to +205 cm–1, anddecreases to 25–30 ps up to +437 cm–1. Theselifetimes are 20–40 times longer than the 0.5–1.5 pslifetimes previously measured with femtosecond pump–probe techniquesat higher vibrational energies (1500–3800 cm–1). Thus, the nonradiative relaxation rate of keto–amino cytosineclose to the 1ππ* state minimum is knr∼ 2.5 × 1010s–1, much smaller than at higher energies. An additionalnonradiative decay channel opens at +500 cm–1excessenergy. Since high overtone bands of ν1′and ν2′are observed in the R2PIspectrum but only a single weak 2ν3′band, we propose that ν3′is a promotingmode for nonradiative decay, consistent with the observation thatthe ν3′normal-mode eigenvector points toward the “C6-puckered”conical intersection geometry. [ABSTRACT FROM AUTHOR]

Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H2O)n clusters with n = 1–4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n = 1, 2 clusters are characteristic of water wires stretching from the NH to the CO group of 2PY. We identify two isomers (3A and 3B) of the n = 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n = 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H2O donating to two waters that form H-bonds to the CO group. This increases the H-bond strength between the NH group of 2PY and the proximal H2O molecule, lowering the NH stretch to ∼2800 cm–1. The n = 4 cluster is also bifurcated with two water wires between the bifurcating H2O and the CO group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n = 1–4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size. [ABSTRACT FROM AUTHOR]

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine-H2O monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift δv of the S1←S0 transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm-1), while that of isomer B is much larger (δv = -889 cm-1). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H2O acting as an acceptor to a N-H or -NH2 group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy De = 56.4 kJ/mol. Isomers B and C are H-bonded between the -NH2 group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest 1ππ* states of A and B in excellent agreement with the observed 000 bands; also, the relative intensities of the A and B origin bands agree well with the calculated S0 state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited 1ππ* state to the lower-lying 1nπ* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the 1ππ* state of B is planar and decoupled from the 1nπ* state. These observations agree with the calculations, which predict the 1nπ* above the 1ππ* state for isomer B but below the 1ππ* for both 9H-2AP and isomer A. [ABSTRACT FROM AUTHOR]