Effect of beta-ring rotation on the structures and vibrational spectra of beta-carotene: density functional theory analysis

The effect of beta-ring rotation on the structures and vibrational spectroscopic characteristics of beta-carotene, including infrared (IR) intensities and Raman activities, is analyzed using density functional theory. Two stable isomers having Ci symmetry are obtained. The reversion of bond lengths is ascribed to the hyperconjugation effect. The natural bond orbital (NBO) charge analysis suggests that the NBO charges of C5 can be used to estimate the degree of pi-electron delocalization. These structural variations are used to analyze and assign the vibrational spectra. It is concluded that (a) the similar rotational angle dependencies of nu1 and nu2 frequencies justify the contribution of C=C stretch vibrations to the nu2 mode and explain the same conjugation length dependencies of nu1 and nu2 frequencies in polyenes, (b) the nu1 mode can be assigned to the C=C stretching in the central part of polyene chain, whereas beta-rings play an important role in nu2 and IR1 bands, especially for the all-trans isomer, and (c) the transition dipole moment of the calculated IR1 absorption band is relevant to the conjugation degree and the crossing angle between the eigenvectors of the polyene chain and the C5=C6 stretching vibration. This theoretical analysis, together with our previous Raman spectral experiments, suggests that the C6-C7 bond is easier to be twisted than other parts of beta-carotene molecule and so provides an insight into the structures of carotenoids and the properties of binding sites in carotenoproteins.