Spectral and structural properties of carotenoids - DFT and thermochemical calculations

Abstract

The published data on Raman νC=C values of carotenoids and some polyenes with the number of conjugated C=C bonds (NC=C) up to 19 were tabulated. The experimental data were compared with the DFT calculated ones. It is shown that the dependences of νC=C on 1/NC=C for β-carotene homologues and linear carotenoids lie on two different straight lines, which tend to intersect with increasing NC=C. The DFT calculations with PBE, BPBE, BLYP and BPW91 functionals lead to the frequencies that are smaller than the experimental ones. However, with B3LYP one gets highly overestimated values. For PBE functional the difference between experimental and calculated values increases linearly with NC=C. One may believe that this is due to the increase of vibrations anharmonicity.The C=C bond strengths (DC=C) of 7 polyenes from butadiene to decapentaene were estimated and a linear dependence of DC=C on DFT calculated rC=C values was obtained. Using the dependence, the DC=C values for different С = С bonds of carotenoids were estimated and tabulated.The effective numbers of conjugated C=C bonds (NC=Ceff) of β-carotene homologues were estimated using the published spectral data by comparing the E0-0 values dependences on 1/NC=C for β-carotene homologues and linear carotenoids. It was shown that NC=Ceff increases linearly with NC=C, and for the longest β-carotene homologue – dodecapreno-β-carotene – NC=Ceff is nearly equal to NC=C. It follows from the result, that the longer is the conjugated chain of β-carotene homologue, the greater is the influence of its linear trans-part on the spectral properties of the compound.