The Csub5Hsub6NHsub2+ protonated shiff base: An ab initio minimal model for retinal photoisomerization

MC-SCF and multireference Moller-Plesset methods were used to compute the minimum energy path for photoisomerization of the minimal retinal protonated Shiff base model tZt-penta-3,5-dieniminium cation (cis-Csub4Hsub6NHsub2+). The molecule was shown to undergo a barrierless relaxation upon excitation to the spectroscopic state, where ground states are conically intersecting, providing an efficient nonadiabatic cis-trans isomerization. This indicates that cis-Csub5Hsub6NHsub2+ is a suitable ab initio model for the rationalization of the observed ultrafast isomerization of the retinal chromophore in rhodopsin.