Comparative Analysis of the Capabilities of Spectral Methods in Studying the Internal Rotation of Compounds of the Benzoic Series

In this review the methods used to study internal rotation (IR) in the ground (S₀) and excited (S₁) electronic states for compounds of the benzoic series C₆H₅–COR, where R = H, F, and CI, are compared. In the (S₀) electronic state, differences in the values of (0–v) transitions of the torsional vibration for the studied compounds are revealed in the methods of analysis of the vibrational structure of the n–π* transition of high-resolution UV absorption spectra and Fourier-transform IR spectra. The reasons for such differences are established. In the excited (S₁) state for benzaldehyde, the method of analyzing the vibrational structure of the n–π* transition of high-resolution UV absorption spectra and the method of analyzing the excitation spectra of the sensitized phosphorescence of this compound in a cooled jet are compared. It is concluded that the method of analyzing the vibrational structure of the n–π* transition of the high-resolution UV absorption spectra of vapors of the investigated compounds is more reliable and accurate when studying the IR in both electronic states.