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.