Comparative Analysis of the Capabilities of Molecular Spectroscopy Methods in Studying Internal Rotation

In this review, the methods used to study internal rotation in the ground (S-0) and excited (S-1) electronic states in the following series of alpha,beta-unsaturated carbonyl compounds: R4R3C = CR2-COR1, where R-1 = H, R-1 = F, R-1 = C-I; R-2 = H, R-2 = CH3, R-2 = F; and R-3 = R-4 = H = CH3, are compared. Differences in the values of (0-v)-transitions of torsional vibration for s-trans- and s-cis-isomers for some studied compounds are revealed in the methods of analysis of the vibrational structure of the n-pi*-transition of high-resolution UV absorption spectra and Fourier-transform IR spectra used in the study of the internal rotation in the (S-0) electronic state. The reasons for such differences are established. It is shown that in the (S-0) state, a more reliable determination of the values of (0-v)-transitions of the torsional vibration of both isomeric forms of molecules is obtained in the method of analysis of the vibrational structure of n-pi*-transition of UV absorption spectra. A new assignment for transitions of the torsional vibration of the s-trans isomer of acrolein in the spectrum of the cavity ringdown spectroscopy (CRDS) method in the excited (S-1) state is proposed. It is concluded that the method of analyzing the vibrational structure of the n-pi*-transition of high-resolution UV absorption spectra of vapors of the studied compounds is more reliable and accurate when studying the internal rotation in both electronic states.