GEOMETRIES AND INTERCONVERSION PATHWAYS OF FREE AND PROTONATED BETA-IONONE SCHIFF-BASES - AN AB-INITIO STUDY OF PHOTORECEPTOR CHROMOPHORE MODEL COMPOUNDS

Ab initio calculations have been performed on the Schiff base of beta- ionone, both in its free and in its: N-protonated form. Geometries wer e fully optimized at the RHF/6-31G* level, with characterization of a ll stationary states as energy minima, transition states, or saddle po ints by calculation of second derivatives. Special attention has been given to the rotation about the C6-C7 single bond and the inversion of the cyclohexene ring. For the unprotonated species, two minima were e stablished, with C6-C7-twist angles of 60.7 degrees and 64.9 degrees, respectively, and the ring in the half-chair conformation. For the pro tonated species, an additional minimum was found, with the C6-C7 bond almost s-trans planar (twist angle 169.9 degrees). Calculated rotation al barriers are between 2 and 5 kcal mol(-1), the inversion barriers b etween 5 and 6 kcal mol(-1), making these systems conformationally hig hly labile at room temperature. Simoultaneous crossing of the rotation and inversion barrier involving a planar cyclohexene geometry is forb iddingly high in energy. (C) 1997 Elsevier Science B.V.