Ab initio study of cyclobutadieno-p-benzoquinones: Kekule isomerization ornot?

The problem of the existence of two Kekule isomers 1a and 1b of cyclobutadi eno-p-benzoquinone is addressed by the CAS(10,10)/6-31G*//GVB(2)/6-31G* and CASPT2(10,10)/ANO(3s2p1d,2s1p)//GVB(2)/6-31G* theoretical models. It is sh own that the barrier separating these isomers on the Born-Oppenheimer surfa ce practically disappears if the zero-point vibrational energies are taken into account. The angular strain and antiaromaticity of the more stable iso mer 1a are estimated by employing the appropriate homodesmic reactions. It is concluded that 1a should be experimentally isolable, albeit in extreme c onditions.