Intermolecular vibrations of phenol(H2O)2-5 and phenol(D2O)2-5-d1 studied by UV double-resonance spectroscopy and ab initio theory

The intermolecular vibrations of jet-cooled phenol(H2O)(2-5) and phenol(D2O)(2-5)-d(1) were investigated in the S-0 and S-1 electronic states by using mass-selective UV spectral hole burning (SHB) and single vibronic level dispersed fluorescence (DF) spectroscopy. Phenol(H2O)(2) shows broad bands with congested structure. We succeeded in obtaining its intermolecular vibrations via double-resonance spectroscopy. Previous studies of phenol(H2O)(3) were completed. By employing soft two-color ionization and spectral hole burning, the vibronic spectra of phenol(H2O)(4) and phenol(H2O)(5) were unambiguously assigned according to cluster size and discriminated for possible isomers. An essentially complete picture of the vibronically active intermolecular vibrations was obtained. This was possible because SHB proves to be sensitive to the higher frequency intermolecular vibrations which tend to fast intramolecular Si vibrational relaxation in the larger clusters and therefore are of low intensity or absent in the two-color ionization spectra. The experimental results are compared to normal mode calculations based on fully optimized cluster structures obtained from ab initio studies at the Hartree-Fock level. Phenol(H2O)(2-4) exhibit cyclic structures of the water moiety, while in case of phenol(H2O)(5) the cyclic and a bridged "double-donor" structure are of comparable energy. The 6n - 6 intermolecular vibrations of the cyclic clusters with n greater than or equal to 3 monomers can be classified into three small amplitude mutual rotations of the phenyl ring and the oxygen moiety, 2n - 6 oxygen ring deformation vibrations, n intermolecular stretch vibrations, and 3n - 3 hindered rotations of the water molecules in the cluster. The "double-donor" clusters exhibit a strong coupling of some of these modes. Many of the intermolecular vibrations, especially the mutual ring motions and the stretch vibrations, are optically active and can be assigned in the S-0 state by comparison with the calculated vibrational frequencies and deuteration shifts. The propensity rule helps to assign the corresponding vibrations in the S-1 state.