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

We report the result of comparison between two reaction coordinates [on the potential energy surface of the first excited state (S-1)] produced by CASSCF and these energies recalculated by MRMP2 in the Z to E photoisomerization of penta-2,4-dieniminium (PDI) as the minimal model of the retinal protonated Schiff base (RPSB). One coordinate is the S-1 state minimum-energy-path (MEP) in mass-weighted coordinates from the S-1 vertically excited point, where a strong hydrogen-out-of plane (HOOP) motion is not exhibited. The energy profile of the S-1 MEP at the MRMP2//CASSCF level shows a barrier for the rotation around the reactive C-C and hits the S-1/S-0 degeneracy space where the central C-C-C-C dihedral angle is distorted by 65 degrees. The other coordinate is an S-1 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 S-1 to S-0 transition may be possible without the large skeletal deformation by HOOP motion because the HOOP coordinate encounters the S-1/S-0 degeneracy space where the central C-C-C-C dihedral angle is distorted by only 40 degrees. Consequently, if PDI is a suitable model molecule for the RPSB as often assumed, the 11-cis to all-trans photoisomerization is predicted to be accelerated by the HOOP motion.