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

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 CH in‐plane bendings, coupled with the CC or CC stretching, or to the CH out‐of‐plane wagging vibrations, some of which coupled with the CC torsion.