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Accurate dissociation energies were determined for gas-phase complexes between 1-naphthol and benzene, d6-benzene and cyclohexane, using the stimulated emission pumping resonant two-photon ionization spectroscopy technique in supersonic jets. The dissociation energies obtained for the electronic ground state are surprisingly large being D0 = 5.07±0.07 kcal/mol for 1-naphthol · benzene, 5.08±0.06 kcal/mol for 1-naphthol · d6-benzene, and 6.92±0.03 kcal/mol for 1-naphthol · cyclohexane, respectively. The dissociation energies scale well with the parallel molecular polarizabilities. |
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An intermolecular potential energy surface was derived for the hydrogenâbonded water trimer as a function of the three torsional angles Ï1, Ï2, Ï3, for energies up to 1300 cmâ1 (3.7 kcal/mol) above the global minimum. The O...O distances and the intramolecular geometry of the H2O molecules are held fixed. This surface is based on the ab initio calculations presented in a companion paper [W. Klopper et al., J. Chem. Phys. 103, 1085 (1995)], which involve very large basis sets and the most extensive treatment of correlation energy for calculations of (H2O)3 so far. The 70 ab initio interaction energies, multiplied by six due to the S6 symmetry of the surface, were fitted using an analytical potential function, with an average error of â11 cmâ1. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and âvalues and other properties of this cluster. Also given is a classification of the lowâlying torsional wave functions according to nodal properties. |
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We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jetâcooled phenolâ
(H2O)3 and d1âphenolâ
(D2O)3 in the S0 and S1 electronic states. Twoâcolor resonant twoâphoton ionization combined with timeâofâflight mass spectrometry and dispersed fluorescence emission spectroscopy provided massâselective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven lowâfrequency intermolecular modes were observed for phenolâ
(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 phenolâ
(H2O)3 were performed at the HartreeâFock level. Calculations for the eight possible conformers differing in the position of the ââfreeââ OâH bonds with respect to the almost planar Hâbonded ring predict that the ââupâdownâupâdownââ conformer is differentially most stable. The calculated structure, rotational constants, normalâmode 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. |
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Massâselective groundâstate vibrational spectra of jetâcooled carbazoleâ
R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating groundâstate intraâ and intermolecular levels via stimulated emission pumping, followed by time delayed resonant twoâphoton ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from â200 cmâ1 up to the dissociation energy D0. Upon dumping to groundâstate levels above D0, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S0 state van der Waals binding energies very accurately. The D0(S0) values are <214.5±0.5 cmâ1 (R=Ne), 530.4±1.5 cmâ1 (R=Ar), 687.9±4.0 cmâ1 (R=Kr), and 890.8±1.6 cmâ1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cmâ1, 576.3±1.6 cmâ1, 756.4±4.5 cmâ1, and 995.8±2.5 cmâ1, respectively. |
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Massâselective groundâstate vibronic spectra of molecular van der Waals complexes carbazoleâ
S, S=N2, CO, and CH4, were measured by stimulated emission pumping followed by resonant twoâphoton ionization of the vibrationally hot complexes. S0âstate vibrational modes were accessed from â200 cmâ1 up to the groundâstate dissociation limit D0(S0) of the van der Waals bond. Above D0, efficient vibrational predissociation of the complexes occurs, allowing accurate determination of the van der Waals dissociation energies as 627.2±7.9 cmâ1 for N2, 716.5±29.8 cmâ1 for CO, and 668.6±15.1 cmâ1 for CH4. In the S1 excited state, the van der Waals binding energies increase to 678.5±8.0, 879.2±29.9, and 753.8±15.2 cmâ1, respectively. The relative increases upon electronic excitation are about 8% and 13% for N2 and CH4, similar to the analogous rare gases Ar and Kr. For CO, the relative increase of van der Waals binding energy is 23%. The differences are primarily due to electrostatic interactions. |
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Accurate hydrogen-bond dissociation energies were determined for gas-phase hydrogen-bonded complexes between 1-naphthol or 1-naphthol-d3 and H2O, CH3OH, NH3 and ND3, using the stimulated emission pumping-resonant two-photon ionization spectroscopy technique in supersonic jets. The hydrogen-bond dissociation energies obtained for the electronic ground state are D0 = 2035 ± 69 cmâ1 for 1-naphthol · H2O, 2645 ± 136 cmâ1 for 1-naphthol · CH3OH, 2680 ± 5cm â1 for 1-naphthol · NH3 and 2801 ± 14 cmâ1 for 1-naphthol-d3 · ND3, respectively. Upon electronic excitation to the S1 state the binding energies increase by approximately 8%. |
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A coupled three-dimensional model calculation of the low-frequency large-amplitude intermolecular torsional states in (H2O)3 and (D2O)3 is presented, based on the analytical modEPEN intermolecular potential surface and a three-dimensional discrete variable representation approach. The lowest torsional levels of both (H2O)3 and (D2O)3 lie above the sixfold (upd) torsional barrier. The first eight (eleven) torsions of (H2O)3 ((D2O)3) are pseudorotational states. The âradialâ and âpolarâ torsional fundamental frequencies are predicted at 151 and 160 cmâ1 for (D2O)3, and for (H2O)3 at 185.0 and 185.3 cmâ1, respectively. Each of these in turn support a ladder of pseudorotational levels. |
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A spectroscopic study of supersonic jetâcooled catechol (1,2âdihydroxybenzene) and its d1â and d2âisotopomers, deuterated at the hydroxy groups, was performed by resonant twoâphoton ionization (R2PI) and fluorescence emission techniques, and supplemented by molecularâbeam holeâburning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S0âS1 spectrum from the 000 band to 400 cmâ1 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 FranckâCondon factors in absorption and emission, using a doubleâminimum potential for the S1 state and a harmonic potential for the S0 state. In the S1 state one of the two âOâH torsional mode frequencies is lowered from Ï2â250 to â50 cmâ1, and the molecule is only quasiplanar with respect to the âOâH torsional coordinates. |
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Mass-selective ground state vibrational spectroscopy of the jet-cooled carbazole·Ar complex was performed by populating ground-state levels via a pump/dump laser pulse sequence, followed by selective resonant two-photon ionization of the vibrationally relaxed complexes. Intra- and inter-molecular van der Waals modes in the S0 state are measurable with good signal/noise. The ground-state binding energy can be determined by detecting the negative signals resulting from loss of ground-state population via vibrational predissociation of the complex. |
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The cyclic water trimer shows a fascinating complexity of its intermolecular potential-energy surface as a function of the three intermolecular torsional coordinates: there are six isometric but permutationally distinct minimum-energy structures of C1 symmetry, which can interconvert by torsional motions via six isometric transition states, also of C1 symmetry. A second type of interconversion can occur through different torsional motions via two C3 symmetric transition structures, and a third interconversion type via a planar C3h symmetric transition structure. The equivalence of the six minima is broken if the âfreeâ H atom of one H2O molecule in the cluster is chemically substituted, yielding three distinct conformers, which occur in enantiomeric pairs. Not all three conformers are necessarily locally stable minima; this depends on the substituent. The phenolâ(H2O)2, p-cyanophenolâ(H2O)2, 1-naphtholâ(H2O)2 and 2-naphtholâ(H2O)2 clusters, which are the phenyl, p-cyanophenyl and naphthyl derivatives of (H2O)3, were examined by resonant two-photon ionization spectroscopy in supersonic beams. These clusters exhibit S0â S1 vibronic spectra with very different characteristics. These reflect the number of cluster structures formed, their low-frequency intermolecular vibrations and indirectly give information about the cluster fluxionality. |
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Extensive ab initio calculations of the phenolâ
H2O complex were performed at the HartreeâFock level, using the 6â31G(d,p) and 6â311++G(d,p) basis sets. Fully energyâminimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowestâenergy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6â311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intraâ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenolâ
H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the ÎČ2 intermolecular wag. Furthermore, the H2O torsion Ï around the Hâbond axis, and the ÎČ2 mode are strongly anharmonically coupled, and a twoâdimensional Ï/ÎČ2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of hâphenolâ
H2O and dâphenolâ
D2O employing twoâcolor resonanceâtwoâphoton ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The ÎČ1 and ÎČ2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular Ï and ÎČ2 modes. |
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A combined experimental and theoretical study of the 2ânaphtholâ
H2O/D2O system was performed. Two different rotamers of 2ânaphthol (2âhydroxynaphthalene, 2HN) exist with the OâH bond in cisâ and transâposition relative to the naphthalene frame. Using HartreeâFock (HF) calculations with the 6â31G(d,p) basis set, fully energyâminimized geometries were computed for both cisâ and transâ2HNâ
H2O of (a) the equilibrium structures with transâlinear Hâbond arrangement and Cs symmetry and (b) the lowestâenergy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenolâ
H2O geometries. The Hâbond stabilization energies with zero point energy corrections included are â5.7 kcal/mol for both rotamers, â2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenolâ
H2O at the same level of theory. Extension of the aromatic Ïâsystem therefore hardly affects the Hâbonding conditions. The barrier height to internal rotation around the Hâbond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6â31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and Mâmatrices reflect considerable mode scrambling for the deuterated isotopomers. For the aâČ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The ÎČ2 wag mode shows the largest anharmonic contributions. For the torsional mode Ï (H2O Hâatom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonantâtwoâphoton ionization and dispersed fluorescence emission spectra of 2HNâ
H2O and dâ2HNâ
D2O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed. |
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The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the HartreeâFock and secondâorder MĂžllerâPlesset levels, using the 6â311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free OâH bonds on one and the third OâH bond on the other side of an approximately equilateral hydrogenâbonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free OâH bonds which are on the same side of the plane across this plane via a transition structure with this OâH bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrationalâtunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C6 symmetry and a very low barrier V6=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further lowâenergy saddle points on the surface are of second and third order; they correspond to crownâtype and planar geometries with C3 and C3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetryâadapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the OâH bonds. Implications of these results for the interpretation of spectroscopic data are discussed. |
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Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol · H2O and d-phenol · D2O were performed at the Hartree-Fock 4-31G and 6-31G** levels. Both phenol and phenol · H2O were fully structure optimized. Based on the minimumenergy structures so obtained, full normal coordinate analyses were carried out. The resulting harmonic frequencies were scaled and compared to available experimental data. The agreement is satisfactory and allows for an assignment of a majority of the bands observed in the experimental spectra. Comparison with previous calculations on (H2O)2 reveals a considerable increase in the strength of the hydrogen bond on going from (H2O)2 to phenol · H2O. |