Avoided crossing of molecular excited states and photochemistry: Butadiene and unprotonated Schiff base

Consequences of the twisting motion around the CC bond of butadiene and around CC, CN, and CC bonds of the small unprotonated Schiff base (allylideneimine) on low-lying singlet and triplet states have been investigated employing large scale CI treatments. Characterization of the important features of the electronic wave functions in terms of VB-like ionic and covalent contributions in different twist intervals has been carried out. Importance of the zwitterionic singlet states with large charge separation in two different parts of molecule attached to the relaxed bond versus low-lying covalent excited state has been discussed. Photochemical implications of different minima on the energy hypersurfaces of the excited states with different features of electronic wave functions have been proposed.

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