An ab initio study of the photodissociation of the vinyl radical.

Photodissociation of the vinyl radical through pathways CH 2 CH → CH 2 C + H, CH 2 CH → CHCH + H, and CH 2 CH → CH 2 + CH is investigated by means of high-level ab initio calculations. Potential-energy curves (PECs) along the corresponding dissociating bond distance associated with the ground and several excited electronic states involved in the above fragmentation pathways, as well as the nonadiabatic couplings connecting the different states, are obtained. The findings of several experiments on vinyl photodissociation performed at different excitation wavelengths are analyzed and explained qualitatively in the light of the present PECs. A two-dimensional representation (consisting of radial and angular coordinates to represent one of the H atoms of the CH 2 group) is also used to calculate the electronic states. The surfaces obtained reflect a rich variety of conical intersections, exit barriers, and nonadiabatic couplings leading to predissociation in different regions of energy and of the two coordinates, suggesting a complex photodissociation dynamics of the CH 2 CH → CHCH + H pathway, with rather different fragmentation mechanisms involved. The two-dimensional results also provide interesting information on the mechanism of in-plane hydrogen migration from the CH 2 group to the CH one through a high-lying transition state.