Ab initio study on quartet states of non-Kekulé-type molecules 1,3,5-trimethylenebenzene and 1,3,5-triaminobenzene trication

Abstract

The electronic structures of non-Kekulé-type isoelectronic molecules 1,3,5-trimethylenebenzene (TMB) and 1,3,5-triaminobenzene trication (TAB3+) having a threefold axis are discussed with the ab initio molecular orbital (MO) method. The quartet state with a D3h geometry is predicted to be the ground state of TMB and TAB3+. According to the Jahn–Teller theorem, the doublet 2E″ states of TMB and TAB3+ are subject to the first order distortions which remove the degeneracy. The quartet–doublet energy splittings are calculated for these non-Kekulé-type molecules at the second order Mo/ller–Plesset (MP2) perturbation level of theory. In TMB the 4A2″ state with a planar D3h geometry is predicted to lie well below the 2A2 state with a planar C2v geometry in which one of the methylene groups has a longer C–C bridge bond connecting with the benzene ring. Moreover, in TAB3+ the 4A2″ state with a planar D3h geometry also lies well below the 2A2 state. The 2A2–2B1 energy splittings on the pseudorotation potential surface overestimated at the unrestricted Hartree–Fock (UHF) level are corrected with the MP2 method.