Abstract
We propose a simple physical mechanism to explain the ultrafast first step of vision, a photoinduced cis to trans rotation of retinal. In the ground state, the torsional stability of π bonds is countered by Coulomb interactions acting between the π lobes; the torsional dependence for Coulomb interactions is absent in the often-used Ohno approximation, but restored with our formula. After photoexcitation, the bonding weakens causing the destabilizing terms to dominate. The twist in the ground state due to steric interactions surrounding the 11- cis bond increases the initial torque and thus the speed of the reaction.
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