A CONICAL INTERSECTION MECHANISM FOR THE PHOTOCHEMISTRY OF BUTADIENE - A MC-SCF STUDY

The excited state (2(1)Ag) reaction paths involved in the photochemica l transformations of butadiene have been studied via ab initio MC-SCF methods. It is demonstrated that the reaction funnel assumes the form of a conical intersection region where the ground (1(1)Ag) and first e xcited (2(1)Ag) potential energy surfaces are degenerate. This mechani sm is consistent with experimental results for the photochemical isome rization and is also consistent with the observed absence of fluoresce nce from the 2(1)Ag state. Thus the currently accepted mechanisms for butadiene photochemistry which involve radiationless decay at avoided crossing minima need to be replaced with a model that involves fully e fficient return to the ground state via a conical intersection. In add ition to the minima on the excited state surface, the lowest energy po ints on the conical intersection region have been fully optimized. The conical intersection points have been characterized by computing the gradient difference and non-adiabatic coupling vectors. Reaction paths from the excited state minima to these conical intersections have bee n computed. The lowest energy path from the s-trans minimum on the 2(1 )Ag potential energy surface involves the rotation of the central C-C bond coupled with asynchronous disrotatory motion of the terminal meth ylenes and leads to an s-transoid conical intersection region without passing over a barrier. The reaction path from the s-cis minimum leads to an s-cisoid conical intersection that lies some 4 kcal mol-1 above this minima. The nature of the possible reaction paths on the excited state is consistent with the fact that the major products of the phot ochemical reactions of butadiene are s-cis/s-trans isomerization and d ouble bond cis/trans isomerization. These findings are also consistent with the directions of the gradient difference and non-adiabatic coup ling vectors computed at a point where the system enters the conical i ntersection. In particular, the directions of these two vectors near t he s-cisoid conical intersection are consistent with the production of cyclobutane as minor product.