RESONANCE RAMAN INVESTIGATION AND SEMIEMPIRICAL CALCULATION OF THE NATURAL CAROTENOID BIXIN

A detailed resonance Raman investigation of the natural carotenoid bix in (6, 6'-diapo-Psi-Psi'-carotenedioic acid monomethyl ester) was unde rtaken in chloroform solution. The excitation profiles of four fundame ntals, one overtone and one combination band were obtained and calcula ted by the transform method within the standard assumptions. A simple model of displaced harmonic oscillators reproduced the profiles satisf actorily, in contrast to the more elaborate models previously used in the case of 1, 3, 5-hexatriene. In addition, the time-dependent formal ism was used to reproduce the optical absorption spectrum of bixin, an d together with the transform method, to calculate the displacement pa rameters. Use was made of semi-empirical calculations via MOPAC6 and Z INDO to gain further insight into the bond length variations in the ex cited electronic state. (C) 1997 Elsevier Science B.V.