Density functional and vibrational spectroscopic analysis of β-carotene

We report a computational study on the structural, energetic and vibrational spectroscopic characteristics of A-carotene employing density functional theory (DFT). The optimized geometry and the complete vibrational spectrum calculated at the BPW91/6-31G* level, including infrared (IR) intensities and Raman activities, are presented. The centrosymmetric structure of beta-carotene is verified both theoretically and experimentally, by identifying a stable calculated structure with C-i symmetry and the mutually exclusive occurrence of bands in the experimental Fourier transform IR and Raman spectrum, respectively. The calculated vibrational spectra reflect the major characteristic features observed experimentally. Differences in the calculated IR intensities and Raman activities for a few dominant modes of two beta-carotene configuration isomers, the all-trans and the natural abundant (C-6-C-7) s-cis form, are explained qualitatively by the corresponding eigenvectors. At the level of theory employed, s-cis-beta-carotene was found to be 8.8 kJ mol(-1) more stable than the all-trans form. Calculations on beta-carotene model systems were performed to separate electronic from steric contributions. The higher stability of s-cis-beta-carotene is explained by an energetically favored beta-ionone ring conformation, compensating for its shorter conjugation length in comparison with the all-trans form. Copyright (C) 2003 John Wiley Sons, Ltd.