Search

The S2←S0 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 ortho-cyanophenol 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.

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 2Π1/2 and D 2Π3/2 spin-orbit states. In addition, we report the results of ab initio calculations, which consider electronic states arising from the 6 2S, 5 2D, and 6 2P 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]

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]

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 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]

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 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]

Molecular dynamics simulations were performed for van der Waals clusters naphthalene·Arn, n=1 to 4. For all isomers and conformers of these clusters, dynamical quantities such as velocity autocorrelation functions, vibrational power spectra, and semiclassical electronic absorption spectra were calculated over a wide energy range, and averaged over a canonical distribution at temperatures in the range T=5 to 30 K. Electronic absorption spectra were calculated for the origin bands according to the semiclassical method [L. E. Fried and S. Mukamel, J. Chem. Phys. 96, 116 (1992)] and are compared with the corresponding experimental naphthalene·Arn R2PI spectra [T. Troxler and S. Leutwyler, J. Chem. Phys. 95, 4010 (1991)]. The appearance of distinct absorption bands due to specific isomers for a given cluster size, as observed experimentally, is well reproduced by the simulations. Comparison of calculated electronic shifts for different isomers allows clear assignments in the experimental spectra. Increasing the simulation temperature to T=15–25 K is accompanied by band broadening and the appearance of sidebands towards the blue. Especially strong sidebands appear for naph·Ar2 and all clusters containing the Ar2 subunit, due to large-amplitude surface rotation/translation of the argon dimer on the naphthalene surface, in agreement with experiment. For clusters containing the n=3 and n=4 subunits the spectral broadening is smaller. For the n=4 (4+0)-isomer, the calculated band shape increases less than for the other n=4 isomers, mainly due to the motional narrowing effect of cluster fluxionality. Above 25 K, isomerization between different possible topological structures also occurs by side-crossing motion of one or several argon atoms. [ABSTRACT FROM AUTHOR]

In this paper we report on the electronic spectroscopy of mass-resolved tetracene·Arn (n=1–26) and tetracene·Krn (n=1–14) heteroclusters, utilizing two-photon, two-color near-threshold ionization in conjunction with mass-spectrometric detection. The spectra of the S0 → S1 transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S0 → S1 transition of small- and medium-sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S0 → S1 spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer-specific inhomogeneous line broadening was interrogated by the observation of isomer-specific ionization potentials for medium-sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion.The ionization thresholds of the tetracene·An (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S0 and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo (MC) constant temperature simulations, in conjunction with a two-parameter fit of the excited-state tetracene–rare-gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified... [ABSTRACT FROM AUTHOR]

Reports coherent anti-Stokes Raman (CARS) spectra of water dimers and higher clusters in seeded supersonic jets. Determination of the gas-phase structure of water clusters from microwave/radio frequency spectra; spectral position of the CARS bands; Results of ab initio calculations.

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]

We combine the technique of femtosecond degenerate four-wave mixing (fs-DFWM) with a high repetition-rate pulsed supersonic jet source to obtain the rotational coherence spectrum (RCS) of cold cyclohexane (C6H12) with high signal/noise ratio. In the jet expansion, the near-parallel flow pattern combined with rapid translational cooling effectively eliminate dephasing collisions, giving near-constant RCS signal intensities over time delays up to 5 ns. The vibrational cooling in the jet eliminates the thermally populated vibrations that complicate the RCS coherences of cyclohexane at room temperature [Brügger, G.; et al. J. Phys. Chem. A 2011, 115, 9567]. The rotational cooling reduces the high-J rotational-state population, yielding the most accurate ground-state rotational constant to date, B0 = 4305.859(9) MHz. Based on this B0, a reanalysis of previous room-temperature gas-cell RCS measurements of cyclohexane gives improved vibration-rotation interaction constants for the v32, v6, v16, and v24 vibrational states. Combining the experimental B0(C6H12) with CCSD(T) calculations yields a very accurate semiexperimental equilibrium structure of the chair isomer of cyclohexane. [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]

Abstract: An efficient one-pot synthesis simultaneously results in three types of densely substituted mono-, di- and tetracyclic π-systems, which can easily be isolated. Each chromophore presents a strong fluorescence emission, either in the red, green or blue part of the spectrum. [Copyright &y& Elsevier]

The 2-pyridone·2-hydroxypyridine (2PY·2HP) mixed dimer has been studied using high resolution ultraviolet spectroscopy in the region of the 2PY S1–S0 origin, and fluorescence-dip infrared spectroscopy in the region of the hydride stretch fundamentals. The dense rotational structure of the electronic spectrum is characteristic of a b-type transition with a transition moment at 8°±3° to the b-axis, consistent with excitation of the 2PY half of the dimer. A tunneling splitting of 520±10 MHz appears in the spectrum, due to a double proton transfer in 2PY·2HP. The double proton transfer exchanges the chemical identity of the two monomer units, thereby leading to a double tautomerization. Theoretical calculations suggest that the barrier to such motion is about 8 kcal/mol in the ground state; hence, the observed tautomerization apparently occurs in the excited state. An approximate fit of the high resolution spectrum gives rotational constants that are consistent with an excited state structure in which only the OH⋯O hydrogen bond in the dimer is lengthened substantially. The infrared spectrum out of the pair of ground state zero-point tunneling levels in the XH stretch region is reminiscent of that in the pure (2PY)2 dimer. Its peak absorption frequency is at 2700 cm−1, but the infrared band is spread over about 500 cm−1, with reproducible sub-structure due to strong, anharmonic coupling. The excited state spectrum, in contrast, is dominated by a transition at 3135 cm−1. This band is assigned to the OH fundamental, which is shifted to higher frequency by the weakening of the OH⋯O hydrogen bond upon electronic excitation. [Copyright &y& Elsevier]