Theoretical studies of protonated and non-protonated Schiff bases of retinal: ground state structures and energies of the all-trans, 13-cis, 11-cis, 9-cis, 6,7-cis, and 6-s-cis isomers

The ground state structures of six isomers of the protonated and non-protonated Schiff bases of retinal were examined at the hybrid density functional B3LYP/6-31G(d) level. For the protonated Schiff bases, the all-trans isomer is the most stable, while for the non-protonated systems, the 6-s-cis isomer is lowest in energy. The protonated 11-cis isomer is 5.2 kcal/mol above the all-trans and this energy difference has been carefully analyzed. The geometry of the protonated species indicates a gradual increase in bond alternation beginning from the nitrogen end due to the stabilization of the positive charge in certain resonance structures.