Density functional theory study of the stability and vibrational spectra of the beta-carotene isomers.

A density functional theory analysis of the stability and vibrational spectra of the beta-carotene isomers is carried out. The study includes the 7-, 9-, 11-, 13-, and 15-monocis isomers and the 7,13'-, 9,13-, 9,13'-, 9,15-, 11,11'-, and 13,15-dicis isomers. The optimized geometries needed to study the stability of the isomers are calculated at the B3LYP/6-31G(d) level of theory, and their energies are further recalculated at the higher B3LYP/6-311+G(2d,2p) level. In addition, the Wiberg bond orders and the natural bond orbital charges of the isomers are computed study the effect of the torsion of the beta-ionone rings on the conjugation degree of the polyene chain. The infrared and Raman spectra of the beta-carotene isomers are then calculated at the B3LYP/6-31G(d) level, scaling the calculated frequencies with an overall factor to account for the anharmonicity effects. The calculated frequencies are shown to compare quite well with the experiment, and the normal modes of the key bands are theoretically interpreted.