Structures and the vibrational relaxations of size-selected benzonitrile-(H2O)(n=1-3) and -(CH3OH)(n=1-3) clusters studied by fluorescence detected Raman and infrared spectroscopies

Vibrational spectroscopy of jet-cooled benzonitrile and its clusters, benzo nitrile-(H2O)(1-3), and benzonitrile-(CH3OH)(1-3), has been carried out by using stimulated Raman-UV (ultraviolet) and IR-UV (infrared-ultraviolet) do uble resonance methods. CC stretching (nu(12)), CN stretching (nu(CN)), CH stretching (nu(CH)), and OH stretching (nu(OH)) vibrations have been examin ed for each species, and characteristic frequency shifts were observed for nu(CN) and nu(OH) upon the cluster formation. The cluster structures were d etermined by comparing the observed spectra with those obtained by ab initi o calculations with HF/SCF (Hartree-Fock self-consistent field) 6-31G(d,p) basis set. It was found that the stable structures of benzonitrile-(H2O)(n) and benzonitrile-(CH3OH)(n) are of the ring form, in which a linear chain of (H2O)(n) cluster or (CH3OH)(n) cluster is hydrogen bonded to the CN grou p and to the ortho hydrogen of benzonitrile. A size dependent transmutation of the hydrogen-bond structure between H2O or CH3OH and the CN group of be nzonitrile was clearly identified for those clusters. In the small size clu ster, the OH group of H2O or CH3OH is hydrogen-bonded perpendicularly to th e CN triple bond (pi-type hydrogen bond), while in the larger size clusters the OH group forms hydrogen bond linearly to the nitrogen atom of the CN g roup (sigma-type hydrogen bond). The lifetimes and the relaxation of the vi brationally excited benzonitrile and its clusters were also investigated by the Raman pump and UV probe technique. The lifetimes of nu(12) and nu(CN) vibrations of bare benzonitrile were longer than 1 mu s. For benzonitrile-H 2O and benzonitrile-CH3OH, on the other hand, the lifetime of the nu(12) le vel is less than 1 ns. The relaxation channels for benzonitrile-CH3OH have been characterized by observing the electronic transition from the relaxed levels or the transition of the dissociation product. It was concluded that the main relaxation channel after the nu(12) excitation is the intracluste r vibrational redistribution (IVR), while the nCN excitation is followed by the vibrational predissociation (VP). (C) 1999 American Institute of Physi cs. [S0021-9606(99)00918-6].