Density Functional Theoretical Analysis of the Molecular Structural Effects on Raman Spectra of β-Carotene and Lycopene

The molecular structural and Raman spectroscopic characteristics of beta-carotene and lycopene are investigated by density functional calculations. The effects of molecular structure and solvent environment on the Raman spectra are analyzed by comparing the calculated and measured results. It is found that the B3LYP/6-31G(d) method can predict the reasonable result for beta-carotene, but the nu 1 Raman activities of lycopene overflow at all the used theoretical methods because of the longer conjugation length. The calculated results indicate that the rotation of beta-rings in beta-carotene impedes the delocalization of pi-electrons, shortens the effective conjugation length, and results in higher frequency and lower activity of the nu 1 mode in beta-carotene than lycopene. The measured nu 1 bands of beta-carotene and lycopene shift respectively to higher and lower frequencies in solution compared with that in crystals since the crystal packing forces can lead to different conformational variations in the carotenoids molecules. The polarized continuum model theoretical analysis suggests that solvent has slight (significant) effects on the Raman frequencies (intensities) of both carotenoids.