| Abstract
| - The minimum energy path for photoisomerization of theminimal retinal protonated Shiff base modeltZt-penta-3,5-dieniminium cation(cis-C5H6NH2+)is computed using MC−SCF and multireferenceMøller−Plessetmethods. The results show that, upon excitation to thespectroscopic state, this molecule undergoes abarrierlessrelaxation toward a configuration where the excited and ground statesare conically intersecting. Theintersectionpoint has a ∼80° twisted central double bond which provides a routefor fully efficient nonadiabatic cis →transisomerization. This mechanism suggests thatcis-C5H6NH2+provides a suitable “ab initio” model forrationalizingthe observed “ultrafast” (sub-picosecond) isomerization dynamics ofthe retinal chromophore in rhodopsin. Thedetailed analysis of the computed reaction coordinate providesinformation on the changes in molecular structureand charge distribution along the isomerization path. It is shownthat the initial excited state motion is dominatedby stretching modes which result in an elongation of the central doublebond of the molecule associated with thechange in bond order in the excited state. Thus, the actualcis → trans isomerization motion is inducedonly afterthe bond stretching has been completed. It is also demonstratedthat, along the excited state isomerization coordinate,the positive charge is progressively transferred from the-CHCHNH2 to the CH2CHCH- molecularfragment.Thus, at the intersection point, the charge is completelylocalized on the CH2CHCH- fragment. Thisresultsuggests that strategically placed counterions can greatly affect therate, specificity, and quantum yield of thephotoisomerization.
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