Vibrational Structure of a High-Resolution UV Absorption Spectrum of Benzaldehyde in the Gas Phase

Abstract

A high-resolution UV spectrum of benzaldehyde vapor is obtained in the region of 26 600–28 500 cm−1 at a high temperature (140°C). An analysis is performed of the permitted vibrational structure of the bands of n–π* transmission in the UV spectrum. It is found that the UV spectrum of benzaldehyde vapors consists of 60 absorption bands. The reliability of the frequencies of 0– $${v}$$ transitions of torsional vibration in the ground (S0) and excited (S1) electronic states is confirmed by their multiple repetition in the Deslandres tables (DTs) on the 0–0 band and the frequencies of vibration of the S1 state (cm–1): $$\nu {\kern 1pt} '$$ = 728, 1091, and 1315. The 0–0 band for benzaldehyde corresponds to 26 912.8 cm−1. The frequency of the torsional vibration of this molecule (111.2 ± 0.4 cm−1) in the S0 state is determined, and the frequencies of transition of 0– $${v}$$ torsional vibrations in the S0 state are determined up to a high value of vibrational quantum number: $${v}$$ = 7. The frequency of the torsional vibrations of benzaldehyde (138.0 ± 0.4 cm−1) and that of the 0– $${v}$$ transitions of torsional vibration up to $${v}$$ = 6 are also determined from the constructed DTs in analyzing the vibrational structure of the UV spectrum for the S1 state. Harmonic frequencies ωe and coefficients x11 of torsional vibration anharmonicity are found for both electronic states. A total assignment of 60 bands of vibrational structure was carried out.