Characterization of a conical intersection between the ground and first excited state for a retinal analog

Ab initio complete active space SCF calculations have been carried out to i nvestigate the first excited electronic state of a retinal protonated Schif f base analog: all-trans-3,7-dimethylnona-2,1,6,8-tetraenmethylimminium cat ion. This model of the retinal chromophore in bacteriorhodopsin includes fi ve conjugated double bonds as well as both pertinent backbone methyl groups . The excited state minimum that is relevant for isomerization in bacterior hodopsin is investigated and is found to be in very close proximity to a Ja hn-Teller conical intersection. The two (global) coordinates that are most effective in promoting efficient internal conversion back to the ground ele ctronic state (by lifting the degeneracy between the ground and first excit ed state) are identified and discussed, and the distribution of the positiv e charge in the retinal analog as a function of these two coordinates is in vestigated. (C) 2000 Elsevier Science B.V. All rights reserved.