Bond torsion affects the product distribution in the photoreaction of retinal model chromophores

Ab initio molecular dynamics (MD) calculations have been performed to study the photoisomerization of a 3-double-bond retinal model chromophore, the all-trans-4, 6-dimethylpenta-3, 5-dieniminium cation, and the possible influence of non-planar distortions on the product distribution. In total, 171 trajectories have been generated for four different conformations of the structure, a planar one and three in which the C4-C5 and the C5=C6 bonds were increasingly twisted out of plane. Starting geometries randomly distributed about the equilibrium geometry were generated by zero-point energy sampling; trajectories were calculated using CASSCF-BOMD methodology and were followed until the photoproduct and its configuration could be assigned. For the latter, two different approaches were applied, one involving the CASSCF configuration vectors, the other an analysis of the MD at the first possible hopping event. Isomerization was found to occur almost exclusively about the central C3=C4 double bond in the case of the planar model compound. Twisting the conjugated π-system shifts the isomerization site from the central double bond to the terminal C5=C6 double bond. With both the C4-C5 and the C5=C6 bonds twisted by 20°, about 35% of the trajectories lead to the configurationally inverted 5-cis product. The results are discussed with reference to the highly selective and efficient photo-induced isomerization of the retinal chromophore in rhodopsin.