We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jetâ€�cooled phenolâ‹…(H₂O)₃ and d₁â€�phenolâ‹…(D₂O)₃ in the S₀ and S₁ electronic states. Twoâ€�color resonant twoâ€�photon ionization combined with timeâ€�ofâ€�flight mass spectrometry and dispersed fluorescence emission spectroscopy provided massâ€�selective vibronic spectra of both isotopomers in both electronic states. In the S₀ state, eleven lowâ€�frequency 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 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 Hâ€�bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normalâ€�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.

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 C_3h symmetric transition structure. The equivalence of the six minima is broken if the ‘free’ 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 phenol–(H₂O)₂, p-cyanophenol–(H₂O)₂, 1-naphthol–(H₂O)₂ and 2-naphthol–(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 S₀→ 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.

Extensive ab initio calculations of the phenolâ‹…H₂O complex were performed at the Hartree–Fock level, using the 6â€�31G(d,p) and 6â€�311++G(d,p) basis sets. Fully energyâ€�minimized 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 lowestâ€�energy transition state structure, which is nonplanar (C₁ symmetry) and has the H₂O moiety rotated by ±90°. The calculated MP2/6â€�311G++(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 β₂ intermolecular wag. Furthermore, the H₂O torsion Ï„ around the Hâ€�bond axis, and the β₂ mode are strongly anharmonically coupled, and a twoâ€�dimensional Ï„/β₂ 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 hâ€�phenolâ‹…H₂O and dâ€�phenolâ‹…D₂O employing twoâ€�color resonanceâ€�twoâ€�photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β₁ and β₂ 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 σ and β₂ modes.

A combined experimental and theoretical study of the 2â€�naphtholâ‹…H₂O/D₂O system was performed. Two different rotamers of 2â€�naphthol (2â€�hydroxynaphthalene, 2HN) exist with the O–H bond in cisâ€� and transâ€�position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6â€�31G(d,p) basis set, fully energyâ€�minimized geometries were computed for both cisâ€� and transâ€�2HNâ‹…H₂O of (a) the equilibrium structures with transâ€�linear Hâ€�bond arrangement and C_s symmetry and (b) the lowestâ€�energy 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 Hâ€�bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 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 Ï€â€�system therefore hardly affects the Hâ€�bonding conditions. The barrier height to internal rotation around the Hâ€�bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6â€�31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and Mâ€�matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β₂ wag mode shows the largest anharmonic contributions. For the torsional mode Ï„ (H₂O Hâ€�atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonantâ€�twoâ€�photon ionization and dispersed fluorescence emission spectra of 2HNâ‹…H₂O and dâ€�2HNâ‹…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.

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 secondâ€�order Møller–Plesset levels, using the 6â€�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 hydrogenâ€�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 vibrationalâ€�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 lowâ€�energy saddle points on the surface are of second and third order; they correspond to crownâ€�type and planar geometries with C₃ and C_3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetryâ€�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.

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.