Infrared spectroscopy of CH stretching vibrations of jet-cooled alkylbenzene cations by using the "messenger" technique

The CH stretching vibrations of the benzene-Ar, toluene-Ar, and ethylbenzen e-Ar clusters prepared in jet expansion were observed in both the neutral a nd cationic ground states by using infrared-ultraviolet double resonance an d infrared photodissociation spectroscopy, respectively. Vibrational freque ncies for the in-plane modes of the clusters have been found to be practica lly the same as those of the corresponding bare molecules. The aromatic CH stretching vibrations showed high frequency shifts upon ionization, and the ir infrared absorption intensities remarkably decreased. The alkyl CH stret ching vibrations were also significantly changed in both frequency and inte nsity upon ionization. Density functional calculations well reproduced the observed infrared spectra of the neutral and cationic states, and enhanceme nt of hyperconjugation in the cationic state was pointed out. (C) 2000 Amer ican Institute of Physics. [S0021-9606(00)01114-4].