Phase transition induced changes in β-ring rotation and methyl group asymmetric deformation of all-trans-β-carotene

The Raman spectra of all-trans-β-carotene (β-Car) liquefied in dimethyl sulfoxide (DMSO) with the decrease of temperature from 333 K to 263 K were recorded with the aim to reveal the molecular structure properties during phase transition. The CC stretching mode of β-Car shows red shift during liquid and solid phase, while blue shift during the liquid-solid phase transition. The 1425 cm−1 Raman band of DMSO shows the inverse characters for β-Car. Raman shift and intensity in coalescence caused by phase transition are studied by means of density functional theory and the molecular vibration mechanism is found out. The calculated Raman spectra of β-Car and DMSO accurately assign the vibrational modes. The potential energy was calculated to investigate the effect of β-ring rotation on Raman shift during phase transition. The latent 1452 cm−1 Raman band of β-Car crops up when temperature drops to the transformation point of 288 K and the 1425 cm−1 Raman band of DMSO split into two peaks at 1417 cm−1 and 1427 cm−1 after phase transition. Phase transition inducing individual methyl groups vibrations helps to interpret the dwindling intensity of 1425 cm−1 Raman peak of DMSO and the increase of 1452 cm−1 Raman peak of β-Car. A visualisation of nuclear motions is illustrated for better understanding the characteristics of vibronic variations of β-Car molecule in critical phenomena. The implication of these analyses is expected to be helpful for the understanding of the effect of phase transition on biomolecules in solution.