Geometries and interconversion pathways of free and protonated β-ionone Schiff bases. An ab initio study of photoreceptor chromophore model compounds Original Research Article

Ab initio calculations have been performed on the Schiff base of β-ionone, both in its free and in its N-protonated form. Geometries were fully optimized at the RHF/6-31G** level, with characterization of all stationary states as energy minima, transition states, or saddle points 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 established, with C6–C7-twist angles of 60.7° and 64.9°, respectively, and the ring in the half-chair conformation. For the protonated species, an additional minimum was found, with the C6–C7 bond almost s-trans planar (twist angle 169.9°). Calculated rotational barriers are between 2 and 5 kcal mol−1, the inversion barriers between 5 and 6 kcal mol−1, making these systems conformationally highly labile at room temperature. Simoultaneous crossing of the rotation and inversion barrier involving a planar cyclohexene geometry is forbiddingly high in energy.