The photodissociation mechanisms of acrylonitrile: Ab initio calculations on reaction channels and surface intersections.

The dissociations of CH(2)CHCN into CH(2)CH+CN and CH(2)C+HCN in the S(0), T(1), and (1)pi(2)pi(C[triple bond]N) ( *) (definitions of pi orbitals can refer to computational details) states, have been explored at the complete active space self-consistent field level of theory employing the Dunning correlation consistent triple-zeta basis set. The lowest energy points of the surface crossing seams have been searched. Two conical intersections, from (1)pi(C[triple bond]N)pi(1) ( *) to (1)pi(2)pi(1) ( *) (CI(1)) and from (1)pi(2)pi(1) ( *) to S(0) (CI(2)), and one intersystem crossing point (T(1)/S(0)) have been located. The energies of all critical points have been recomputed with the multiconfigurational second-order perturbation method. At each conical intersection, derivative coupling and unscaled gradient difference vectors have been analyzed to determine the relaxation channels that the molecule may evolve in after nonradiative decay. Once the molecule is photoexcited to the (1)pi(2)pi(1) ( *) or (1)pi(C[triple bond]N)pi(1) ( *) state, it would relax along the similar pathway: funneling through CI(1) and then CI(2), and finally populate the ground state. Our results show that upon 193 nm photoexcitation, the most probable reaction channel is the ground-state HCN elimination following radiationless decays from excited states through surface crossings, which consists with experimental results J. Chem. Phys. 108, 5784 (1998). The investigated dissociation channels on the (1)pi(2)pi(C[triple bond]N) ( *) surface, which are inaccessible upon 193 nm photoexcitation, may provide information for reactions induced by higher energy excitations.