A theoretical study on the thermal ring opening rearrangement of 1H-bicyclo[3.1.0]hexa-3,5-dien-2-one: a case of two state reactivity†
Abstract
The molecular mechanism of the thermal rearrangement of singlet 1H-bicyclo[3.1.0]hexa-3,5-dien-2-one to give triplet 4-oxocyclohexa-2,5-dienylidene has been investigated using density functional theory (B3LYP and M05-2X functionals) as well as ab initioCASSCF and CASPT2 multiconfigurational methods. The reactant has a singlet ground state while the product can be found in three low lying electronic states P(3B1), P(1B1), and P(1A′). Therefore, the molecular mechanism of this ring opening rearrangement may involve up to three different potential energy surfaces of two spin multiplicities: two singlet (closed shell, CS, and open shell, OS) and one triplet. The stationary points on these surfaces have been characterized and two crossing regions have been found: one intersystem crossing region, ISC, connecting the CS singlet and triplet surfaces and a minimum energy conical intersection,