The C₅H₆NH₂⁺ Protonated Shiff Base:  An <i>ab Initio</i> Minimal Model for Retinal Photoisomerization

The minimum energy path for photoisomerization of the minimal retinal protonated Shiff base model tZt-penta-3,5-dieniminium cation (cis-C5H6NH2+) is computed using MC−SCF and multireference Møller−Plesset methods. The results show that, upon excitation to the spectroscopic state, this molecule undergoes a barrierless relaxation toward a configuration where the excited and ground states are conically intersecting. The intersection point has a ∼80° twisted central double bond which provides a route for fully efficient nonadiabatic cis → trans isomerization. This mechanism suggests that cis-C5H6NH2+ provides a suitable “ab initio” model for rationalizing the observed “ultrafast” (sub-picosecond) isomerization dynamics of the retinal chromophore in rhodopsin. The detailed analysis of the computed reaction coordinate provides information on the changes in molecular structure and charge distribution along the isomerization path. It is shown that the initial excited state motion is dominated by stretching modes which result in an elongation of the central double bond of the molecule associated with the change in bond order in the excited state. Thus, the actual cis → trans isomerization motion is induced only after the bond stretching has been completed. It is also demonstrated that, along the excited state isomerization coordinate, the positive charge is progressively transferred from the -CHCHNH2 to the CH2CHCH- molecular fragment. Thus, at the intersection point, the charge is completely localized on the CH2CHCH- fragment. This result suggests that strategically placed counterions can greatly affect the rate, specificity, and quantum yield of the photoisomerization.