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 ethylbenzene-Ar clusters prepared in jet expansion were observed in both the neutral and cationic ground states by using infrared-ultraviolet double resonance and infrared photodissociation spectroscopy, respectively. Vibrational frequencies for the in-plane modes of the clusters have been found to be practically the same as those of the corresponding bare molecules. The aromatic CH stretching vibrations showed high frequency shifts upon ionization, and their infrared absorption intensities remarkably decreased. The alkyl CH stretching vibrations were also significantly changed in both frequency and intensity upon ionization. Density functional calculations well reproduced the observed infrared spectra of the neutral and cationic states, and enhancement of hyperconjugation in the cationic state was pointed out.