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

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Abstract
Raman and infrared spectra in the region of 1800‐150 cm−1were recorded for a set ofcis‐transisomers of d̃‐carotene, i.e. the all‐trans, 7‐cis, 9‐cis, 13‐cisand 15‐cisisomers. Spectral comparison revealed Raman and infrared key bands which (1) distinguish stretched or terminal‐bent configurations (all‐trans, 7‐cisand 9‐cis) from central‐bent configurations (13‐cisand 15‐cis), and (2) distinguish unmethylated 7‐cisand 15‐cisconfiguratios. Keybands (1) include Raman bands at 1160 and 1140 cm−1and infrared bands at 825 and 775 cm−1(the intensity varies with the position of thecis‐bend) Key bands (2) include Raman bands at 1274 and 962 cm−1and an infrared band at 741 cm−1(characteristic of the 7‐cisconfiguration), and also a Raman band at 1247 cm−1and an infrared band at 775 cm−1(characteristic of the 15‐cisconfiguration). 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.