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 (S₀) and excited (S₁) electronic states in the following series of α,β-unsaturated carbonyl compounds: R₄R₃C = CR₂–COR₁, where R₁ = H, R₁ = F, R₁ = C_I; R₂ = H, R₂ = CH₃, R₂ = F; and R₃ = R₄ = 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-π*-transition of high-resolution UV absorption spectra and Fourier-transform IR spectra used in the study of the internal rotation in the (S₀) electronic state. The reasons for such differences are established. It is shown that in the (S₀) 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-π*-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 (S₁) state is proposed. It is concluded that the method of analyzing the vibrational structure of the n-π*-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.