Acceleration of the Z to Ephotoisomerization of penta-2,4-dieniminium by hydrogen out-of-plane motion: theoretical study on a model system of retinal protonated Schiff base

Abstract

We report the result of comparison between two reaction coordinates [on the potential energy surface of the first excited state (S₁)] produced by CASSCF and these energies recalculated by MRMP2 in the Z to Ephotoisomerization of penta-2,4-dieniminium (PDI) as the minimal model of the retinal protonated Schiff base (RPSB). One coordinate is the S₁ state minimum-energy-path (MEP) in mass-weighted coordinates from the S₁ vertically excited point, where a strong hydrogen-out-of plane (HOOP) motion is not exhibited. The energy profile of the S₁MEP at the MRMP2//CASSCF level shows a barrier for the rotation around the reactive C–C and hits the S₁/S₀ degeneracy space where the central C–C–C–C dihedral angle is distorted by 65°. The other coordinate is an S₁ coordinate obtained by the relaxed scan strategy. The relaxed coordinate along the central C–C–C–C dihedral angle, which we call the HOOP coordinate, shows strong HOOP motion. According to the MRMP2//CASSCF calculation, there is no barrier on the HOOP coordinate. Furthermore, the S₁ to S₀ transition may be possible without the large skeletal deformation by HOOP motion because the HOOP coordinate encounters the S₁/S₀ degeneracy space where the central C–C–C–C dihedral angle is distorted by only 40°. Consequently, if PDI is a suitable model molecule for the RPSB as often assumed, the 11-cis to all-transphotoisomerization is predicted to be accelerated by the HOOP motion.