Charge transfer interaction in the acetic acid–benzene cation complex

Abstract

Geometrical and electronic structures of the acetic acid–benzene cation complex, (CH3COOH)⋅(C6H6)+, are studied experimentally and theoretically. Experimentally, a vibrational spectrum of (CH3COOH)⋅(C6H6)+ in the supersonic jet is measured in the 3000–3680 cm−1 region using an ion-trap photodissociation spectrometer. An electronic spectrum is also observed with this spectrometer in the 12 000–29 600 cm−1 region. Theoretically, ab initio molecular orbital calculations are performed for geometry optimization and evaluation of vibrational frequencies and electronic transition energies. The vibrational spectrum shows two distinct bands in the O–H stretching vibrational region. The frequency of the strong band (3577 cm−1) is close to that of the O–H stretching vibration of acetic acid and the weak one is located at 3617 cm−1. On the basis of geometry optimizations and frequency calculations, the strong band is assigned to the O–H stretching vibration of the cis-isomer of acetic acid in the hydrogen-bonded complex (horizontal cis-isomer). The weak one is assigned to the vertical trans-isomer where the trans-isomer of acetic acid interacts with the π-electron system of the benzene cation. The weakness of the high frequency band in the photodissociation spectrum is attributed to the binding energy larger than the photon energy injected. Only hot vertical trans-isomers can be dissociated by the IR excitation. The electronic spectrum exhibits two bands with intensity maxima at 17 500 cm−1 and 24 500 cm−1. The calculations of electronic excitation energies and oscillator strengths suggest that charge transfer bands of the vertical trans-isomer can be observed in this region in addition to a local excitation band of the horizontal cis-isomer. We assign the 17 500 cm−1 band to the charge transfer transition of the vertical trans-isomer and the 24 500 cm−1 band to the π–π transition of the horizontal cis-isomer. The calculations also suggest that the charge transfer is induced through the intermolecular C⋯O=C bond formed between a carbon atom of benzene and the carbonyl oxygen atom of acetic acid.