A conical intersection mechanism for the photochemistry of butadiene. A MC-SCF study

Abstract

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