| Abstract
| - CASPT2//CASSCF photoisomerization path computations have been used to unveil the effectsof an acetate counterion on the photochemistry of two retinal protonated Schiff base (PSB) models: the2-cis-penta-2,4-dieniminium and the all-trans-epta-2,4,6-trieniminium cations. Different positions/orientationsof the counterion have been investigated and related to (i) the spectral tuning and relative stability of theS0, S1, and S2 singlet states; (ii) the selection of the photochemically relevant excited state; (iii) the controlof the radiationless decay and photoisomerization rates; and, finally, (iv) the control of the photoisomerizationstereospecificity. A rationale for the results is given on the basis of a simple (electrostatic) qualitative model.We show that the model readily explains the computational results providing a qualitative explanation fordifferent aspects of the experimentally observed “environment” dependent PSB photochemistry. Electrostaticeffects likely involved in controlling retinal photoisomerization stereoselectivity in the protein are alsodiscussed under the light of these results, and clues for a stereocontrolled electrostatically drivenphotochemical process are presented. These computations provide a rational basis for the formulation ofa mechanistic model for photoisomerization electrostatic catalysis.
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