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, benzonitrile-(H2O)(1-3), and benzonitrile-(CH3OH)(1-3), has been carried out by using stimulated Raman-UV (ultraviolet) and IR-UV (infrared-ultraviolet) double resonance methods. CC stretching (nu(12)), CN stretching (nu(CN)), CH stretching (nu(CH)), and OH stretching (nu(OH)) vibrations have been examined for each species, and characteristic frequency shifts were observed for nu(CN) and nu(OH) upon the cluster formation. The cluster structures were determined by comparing the observed spectra with those obtained by ab initio 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 group 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 benzonitrile was clearly identified for those clusters. In the small size cluster, the OH group of H2O or CH3OH is hydrogen-bonded perpendicularly to the 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 group (sigma-type hydrogen bond). The lifetimes and the relaxation of the vibrationally 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-H2O and benzonitrile-CH3OH, on the other hand, the lifetime of the nu(12) level 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 intracluster vibrational redistribution (IVR), while the nCN excitation is followed by the vibrational predissociation (VP). (C) 1999 American Institute of Physics. [S0021-9606(99)00918-6].