High-symmetry configurations Subject

Thus, with these proofs two important additions to the previous traditional understanding of the JTE emerged (1) Any polyatomic system may be subject to the JTE, and (2) if there are instabilities and distortions of high-symmetry configurations, they are due to, and only to the JTE. Together with the previously achieved understanding of the role of quadratic terms of the vibronic coupling, the extended formulation of the JTE that includes the latest achievements in this field is as follows [1,8] ... 

Hence the diradical states are exceptional in that the epikernel principle cannot be based on the usual first order JT section rule of Eq. (1) The reason is that the E state is essentially based on a half-filled shell configuration, with two electrons in a doubly degenerate e" orbital. As we have shown elsewhere such half-filled shell states are subject to a hole electron exchange symmetry, which counteracts the first order JT distorting forces [42], Moreover, in these highly reactive molecules the absence of a strong first order activity coincides with a pronounced second order effect, due to suitable low lying excited states. This limits the applicability of the epikernel principle in the case of diradicals. 

---