Resonance Raman investigation and semi-empirical calculation of the natural carotenoid bixin

A detailed resonance Raman investigation of the natural carotenoid bixin (6, 6'-diapo-Psi-Psi'-carotenedioic acid monomethyl ester) was undertaken in chloroform solution. The excitation profiles of four fundamentals, one overtone and one combination band were obtained and calculated by the transform method within the standard assumptions. A simple model of displaced harmonic oscillators reproduced the profiles satisfactorily, in contrast to the more elaborate models previously used in the case of 1, 3, 5-hexatriene. In addition, the time-dependent formalism was used to reproduce the optical absorption spectrum of bixin, and together with the transform method, to calculate the displacement parameters. Use was made of semi-empirical calculations via MOPAC6 and ZINDO to gain further insight into the bond length variations in the excited electronic state. (C) 1997 Elsevier Science B.V.