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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]

Structures and order–disorder transitions of the rare-gas solvent clusters carbazole·Arn were studied for n=1–7 by molecular dynamics and Monte Carlo simulation methods. This study was motivated by the investigation of these clusters by size-specific electronic spectroscopy. Cluster structures corresponding to absolute and local energy minima were obtained by molecular-dynamics slow cooling and annealing. For n=1–3, single minimum-energy structures of Cs symmetry were found, with all n atoms on the same side of the molecular substrate. For n=4–7, the number of energetically low-lying monolayer isomers rises very rapidly with n. Orientational ordering of the adcluster by the substrate is unimportant for n≥4. The isomers for n≥4 differ by (i) rotational/translational displacements of the cluster relative to the substrate, (ii) promotion of atoms to the second solvent layer, and (iii) adsorption of atoms on the second side of the substrate. Order–disorder transitions of the monolayer solvent clusters were studied by MC methods as a function of temperature. Four different quasi-two-dimensional order–disorder transitions were found: the racemization and surface-decoupling transitions reflect the loss of cluster–substrate orientational correlation, while the cluster rigid–fluxional and melting transitions derive from the loss of intracluster bond-orientational order, and increase in bond-length fluctuations. Analogies to order–disorder phase transitions in two-dimensional rare-gas films are discussed. [ABSTRACT FROM AUTHOR]

Excited-state proton transfer was studied in supersonically cooled neutral acid-base clusters of 2-naphthol·(NH3)n with n=1-10, using size-selective two-color resonant two-photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited-state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic-vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited-state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≊8000 cm-1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2-naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for clusters with n≥4, but are not shifted to the red, as is the case for the analogous 1-naphthol·(NH3)n clusters. The cluster-size dependence of the proton transfer reaction is mainly controlled by the cluster proton affinity. [ABSTRACT FROM AUTHOR]

Carbazole·B complexes (B=H2O, D2O, NH3) were synthesized and cooled in pulsed supersonic nozzle beams. The intermolecular hydrogen-bond vibrations and dissociation energies were studied by several laser-spectroscopic techniques (fluorescence excitation and emission, resonance-two-photonionization with mass-specific detection). The following results were obtained for both S0 and S1 electronic states: (1) determination of the structural symmetry of the complexes, (2) measurement of the intermolecular stretching (νσ) and bending (νβ) frequencies, (3) determination of stretching force constants, (4) hydrogen-bond dissociation energies D0 for carbazole·H2O/D2O, and (5) electronic spectral shifts δν relative to the bare carbazole molecule. The latter are large (500–710 cm-1) and reflect an increase of the hydrogen-bond energy by approx. 40% upon electronic excitation. Fermi resonance couplings between the intermolecular νσ and an intramolecular b1 vibration of carbazole are observed and partially analyzed. To complement the experimental work, extensive ab initio quantum chemical calculations of the same complexes were performed at the Hartree–Fock level. The calculated complex structures are consistent with the experimental information. Intermolecular potential-energy curves for the stretching vibrational coordinate were calculated for a number of increasingly flexible basis sets (STO-3G, 4-31G, 6-31G, 4-31G*); anharmonic vibrational frequencies were then obtained numerically. Excellent agreement with the experimental intermolecular stretching frequencies was found for all three complexes using the 4-31G* basis set. Good agreement with experiment was also found for the calculated 4-31G* hydrogen-bond dissociation energy. [ABSTRACT FROM AUTHOR]

Mass-selective ground-state vibronic spectra of molecular van der Waals complexes carbazole·S, S=N2, CO, and CH4, were measured by stimulated emission pumping followed by resonant two-photon ionization of the vibrationally hot complexes. S0-state vibrational modes were accessed from ≊200 cm-1 up to the ground-state 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 cm-1 for N2, 716.5±29.8 cm-1 for CO, and 668.6±15.1 cm-1 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 cm-1, 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. © 1995 American Institute of Physics. [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]

The global and local minima, stationary points, and torsional rearrangement processes of cyclic homodromic (H2O)4 were studied on its four-dimensional torsional intermolecular potential energy surface. Eight different energetically low-lying torsional stationary point structures were found by ab initio theory, and fully structure-optimized at the second-order Mo\ller–Plesset level, using large basis sets. Second-order energies close to the one-particle basis set limit were obtained at these geometries using the explicitly correlated Mo\ller–Plesset method. The effects of higher-order correlation energy terms were investigated by coupled cluster theory, and terms beyond second order were found to cancel in good approximation. The S4 symmetric global minimum has a square and almost planar O...O...O...O arrangement with free O–H bonds alternating ‘‘up’’ and ‘‘down’’ relative to this plane, with two isometric versions of this structure. Another torsional conformer with two neighboring up O–H bonds followed by two neighboring down O–H bonds is a local minimum, 0.93 kcal/mol above the global minimum. The four versions of this structure are connected to the global minima via two distinct but almost degenerate first-order torsional saddle points, which occur as two sets of eight isometric versions each, both about 1.24 kcal/mol above the global minimum. Yet another set of eight second-order saddle points lies at 1.38 kcal/mol. The structure with three O–H bonds up and one down is not a stationary point, while the structure with all four O–H bonds on the same side of the plane is a first-order saddle point.The fully planar C4h symmetric structure is a fourth-order stationary point 2.8 kcal/mol above the minimum. The torsional interconversion paths between this multitude of points are complex, and are discussed in three-dimensional spaces of symmetry-adapted... [ABSTRACT FROM AUTHOR]

Mass-selective ground-state vibrational spectra of jet-cooled carbazole·R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground-state intra- and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two-photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from ≊200 cm-1 up to the dissociation energy D0. Upon dumping to ground-state 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 cm-1 (R=Ne), 530.4±1.5 cm-1 (R=Ar), 687.9±4.0 cm-1 (R=Kr), and 890.8±1.6 cm-1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm-1, 576.3±1.6 cm-1, 756.4±4.5 cm-1, and 995.8±2.5 cm-1, respectively. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An intermolecular potential energy surface was derived for the hydrogen-bonded water trimer as a function of the three torsional angles ω1, ω2, ω3, for energies up to 1300 cm-1 (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 ≊11 cm-1. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and -values and other properties of this cluster. Also given is a classification of the low-lying torsional wave functions according to nodal properties. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A torsional potential energy surface for the cyclic water trimer was calculated at the level of second-order Mo\ller–Plesset perturbation theory. For the construction of this ab initio surface, the first-order wave function was expanded in a many-electron basis which linearly depends on the interelectronic coordinates r12. The one-electron basis of Gaussian orbitals was calibrated on the water monomer and dimer to ensure that the ab initio surface computed represents the (near- ) basis set limit for the level of theory applied. The positions of the free O—H bonds are described by three torsional angles. The respective three-dimensional torsional space was investigated by 70 counterpoise corrected single-point calculations for various values of these angles, providing a grid to fit an analytical representation of the potential energy surface. The four symmetry unique stationary points previously found at the Hartree–Fock and conventional Mo\ller–Plesset levels [Schütz et al., J. Chem. Phys. 99, 5228 (1993)] were studied in detail: Relative energies of the structures were calculated by applying second-order Mo\ller–Plesset and coupled cluster methods; harmonic vibrational frequencies were calculated at the second-order Mo\ller–Plesset level with a 6-311++G(d,p) basis set at these stationary points. It is expected that the present torsional potential energy surface for the water trimer will play an important role in the understanding of the vibrational transitions observed by far-infrared vibration–rotation–tunneling spectroscopy in terms of a nearly free pseudorotational interconversion on a cyclic vibrational–tunneling path. © 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 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 Hartree–Fock and second-order Mo\ller–Plesset levels, using the 6-311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen-bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational-tunneling 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 low-energy saddle points on the surface are of second and third order; they correspond to crown-type 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 symmetry-adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed. [ABSTRACT FROM AUTHOR]

Studies effects of intermolecular torsional and symmetric stretching exciations of rotational constants water trimer. Decrease in the rotational constant with the increase of torsional energy; Calculated rotational constant and the root-mean-square torsional angles of water.

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.