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 d-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 d-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.

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