Unimolecular decay paths of electronically excited species. II. The C2H+4 ion

Abstract

The reaction paths of dissociation and the mechanisms of electronic relaxation of the ethylene cation have been calculated ab initio. Internal rotation is shown to bring about radiationless transition of states ? 2A and ? 2B2 to the ground state. Two competing channels are available for the first excited ? 2B3: it can either undergo internal conversion to the ground state ? 2B3 or lose a hydrogen atom by simple bond cleavage and give C2H+3+H fragments. The competition is governed by nonadiabatic interaction around a conical intersection. The coupling matrix element 〈?‖∂/∂q‖?〉 has been calculated and is shown to obey the linear model of nonadiabatic interaction quite well. The ground state ? 2B3 cannot eliminate directly a hydrogen molecule. It must first undergo rearrangement to a 2E state of a CH3CH+ structure via a bridged structure. Jahn–Teller interaction takes place at the doubly degenerate 2E state. Dissociation of CH3CH+ via 1,1-hydrogen elimination is then allowed and leads to C2H+2+H2. There is a small (possibly inexistent) energy barrier along this path. The complicated nature of this multistep reaction path brings about energy redistribution among many vibrational degrees of freedom and provides a plausible explanation for the success of statistical theories of unimolecular reactions (RRKM and QET).