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 β-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 β-carotene, but the v1 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 β-rings in β-carotene impedes the delocalization of π-electrons, shortens the effective conjugation length, and results in higher frequency and lower activity of the v1 mode in β-carotene than lycopene. The measured v1 bands of β-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. The v1 bands of β-carotene and lycopene shift respectively to higher and lower frequencies in solution compared with those in the solid state due to the release of the crystal packing forces. The B3LYP/6-31G(d) method can predict reasonable Raman spectrum of β-carotene, but the calculated v1 Raman activity of lycopene overflows. The solvent has slight (significant) effects on the Raman frequencies (intensities) of both carotenoids.