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

The excited state (2(1)Ag) reaction paths involved in the photochemical 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 excited (2(1)Ag) potential energy surfaces are degenerate. This mechanism is consistent with experimental results for the photochemical isomerization and is also consistent with the observed absence of fluorescence 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 efficient return to the ground state via a conical intersection. In addition to the minima on the excited state surface, the lowest energy points 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 been 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 methylenes 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 photochemical reactions of butadiene are s-cis/s-trans isomerization and double bond cis/trans isomerization. These findings are also consistent with the directions of the gradient difference and non-adiabatic coupling vectors computed at a point where the system enters the conical intersection. In particular, the directions of these two vectors near the s-cisoid conical intersection are consistent with the production of cyclobutane as minor product.