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.

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 initio CASSCF and CASPT2 multiconfigurational methods. The reactant has a singlet ground state while the product can be found in three low lying electronic states P((3)B(1)), P((1)B(1)), and P((1)A'). 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, CI, between both CS and OS singlet surfaces. The results point out that the reaction mechanism starts on the CS singlet surface and, after the transition structure is surmounted, the ISC takes place in the vicinity of the CS singlet product leading to the more energetically favourable P((3)B(1)) triplet product. A significant value (4.76 cm(-1)) of the spin orbit coupling term has been calculated at the point of the minimum energy path (MEP) where the ISC can take place. This behaviour indicates that the reaction can proceed through singlet-triplet coupling. The high energy value obtained for the CI allows discarding the participation of the OS singlet state in the thermal process. The reaction mechanism can be rationalized if the aromaticities of the final products are considered. Using NICS indexes it is shown that P((3)B(1)) and P((1)B(1)) are aromatic while the P((1)A(1)) presents a puckered conformation due to its antiaromatic character.