Theoretical studies of hydride transfer reactivity: transition structures and the origin of chemical reaction barriers

The second-order Jahn-Teller effect provides a viewpoint on the origin of a chemical reaction barrier which is entirely complementary to the view that the barrier arises from an avoided crossing between reactant-like and product-like wavefunctions. The link between these alternative pictures is the existence of a low lying excited state of a transition structure on the ground state potential energy surface: this excited state species is at a minimum of energy with respect to displacement along the reaction coordinate. These ideas are illustrated by ab initio restricted open-shell Hartree-Fock (ROHF) and complete active space self-consistent field (CASSCF) calculated results for transition structures of the symmetrical hydride transfer reaction between methanimine and formonitrilium cation.