Spectral and structural properties of carotenoids - DFT and thermochemical calculations

The published data on Raman nu(C=C) values of carotenoids and some polyenes with the number of conjugated C=C bonds (N-C=C) up to 19 were tabulated. The experimental data were compared with the DFT calculated ones. It is shown that the dependences of nu(C=C) on 1/NC=C for beta-carotene homologues and linear carotenoids lie on two different straight lines, which tend to intersect with increasing N-C=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 N-C=C. One may believe that this is due to the increase of vibrations anharmonicity. The C=C bond strengths (D-C=C) of 7 polyenes from butadiene to decapentaene were estimated and a linear dependence of D-C=C on DFT calculated r(C=C) values was obtained. Using the dependence, the D-C=C values for different C = C bonds of carotenoids were estimated and tabulated. The effective numbers of conjugated C=C bonds (N-C=C(eff)) of beta-carotene homologues were estimated using the published spectral data by comparing the E0-0 values dependences on 1/N-C=C for beta-carotene homologues and linear carotenoids. It was shown that N(C=C)(eff )increases linearly with NC=C, and for the longest beta-carotene homologue - dodecapreno-beta-carotene - N-C=C(eff) is nearly equal to N-C=C. It follows from the result, that the longer is the conjugated chain of beta-carotene homologue, the greater is the influence of its linear trans-part on the spectral properties of the compound. (C) 2019 Elsevier B.V. All rights reserved.