Vibrational spectra and relaxation of benzonitrile and its clusters using time‐resolved stimulated Raman–UV double resonance spectroscopy

Raman spectra of ring-breathing (ν12) and CN stretching vibrations (νCN) of jet-cooled benzonitrile and its clusters with Ar, H2O, CH3OH, CH3CN, CCl4, CHCl3 and CHF3 were observed by stimulated Raman–UV double resonance spectroscopy. Cluster structures were determined on the basis of the observed vibrational spectra, rotational contour analyses of the high resolution S1–S0 electronic spectra and ab initio molecular orbital calculations with the HF/6–31G(d,p) basis set. It was found that the types of the cluster structures are classified into three groups. (1) The clusters with H2O, CH3OH and CH3CN represent the ‘side’-type structure, in which the intermolecular bond is formed in the phenyl plane of benzonitrile, and the benzonitrile dimer also belongs to this structure. (2) The clusters with Ar and CCl4 form the ‘on-top’-type structure, where Ar or CCl4 is located above the phenyl plane. (3) The clusters with CHCl3 and CHF3 have the ‘linear’-type structure, in which these molecules are located at the N end of the CN group of benzonitrile. The rate constants of the vibrational relaxation for these clusters occurring after the Raman excitation to their ν12 vibrations were also observed. It was found that different rate constants are associated with the different types of cluster structures. That is, the 121 levels of the ‘side’-type clusters relax much faster than those of other types of the clusters. The results represent a clear indication that the anharmonic coupling between ν12 and intermolecular vibrations are much larger in the ‘side’-type clusters than others. Copyright © 2000 John Wiley & Sons, Ltd.