Raman and infrared spectra of the all‐trans, 7‐cis, 9‐cis, 13‐cis and 15‐cis isomers of β‐carotene: Key bands distinguishing stretched or terminal‐bent configurations form central‐bent configurations

Abstract Raman and infrared spectra in the region of 1800‐150 cm −1 were recorded for a set of cis‐trans isomers of d̃‐carotene, i.e. the all‐ trans , 7‐ cis , 9‐ cis , 13‐ cis and 15‐ cis isomers. Spectral comparison revealed Raman and infrared key bands which (1) distinguish stretched or terminal‐bent configurations (all‐ trans , 7‐ cis and 9‐ cis ) from central‐bent configurations (13‐ cis and 15‐ cis ), and (2) distinguish unmethylated 7‐ cis and 15‐ cis configuratios. Keybands (1) include Raman bands at 1160 and 1140 cm −1 and infrared bands at 825 and 775 cm −1 (the intensity varies with the position of the cis ‐bend) Key bands (2) include Raman bands at 1274 and 962 cm −1 and an infrared band at 741 cm −1 (characteristic of the 7‐ cis configuration), and also a Raman band at 1247 cm −1 and an infrared band at 775 cm −1 (characteristic of the 15‐ cis configuration). The normal modes for the key bands were determined by a set of normal coordinate calculations for the isomeric configurations of a simplified model of d̃‐carotene. The key bands were mainly related to the CH in‐plane bendings, coupled with the CC or CC stretching, or to the CH out‐of‐plane wagging vibrations, some of which coupled with the CC torsion.