AB-INITIO STUDY ON QUARTET STATES OF NON-KEKULE-TYPE MOLECULES 1,3,5-TRIMETHYLENEBENZENE AND 1,3,5-TRIAMINOBENZENE TRICATION

The electronic structures of non-Kekule-type isoelectronic molecules 1 ,3,5-trimethylenebenzene (TMB) and 1,3,5-triaminobenzene trication (TA B(3+)) having a threefold axis are discussed with the ab initio molecu lar orbital (MO) method. The quartet state with a D-3h geometry is pre dicted to be the ground state of TMB and TAB(3+). According to the Jah n-Teller theorem, the doublet (2)E'' states of TMB and TAB(3+) are sub ject to the first order distortions which remove the degeneracy. The q uartet-doublet energy splittings are;calculated for these nonKekule-ty pe molecules at the second order Moller-Plesset (MP2) perturbation lev el of theory. In TMB the (4)A''(2) State with a planar D-3h geometry i s predicted to lie well below the (2)A(2) State with a planar C-2 upsi lon geometry in which One of the methylene groups has a longer C-C bri dge bond connecting with the benzene ring. Moreover, in TAB(3+) the (4 )A''(2) state with a planar D-3h geometry also lies well below the (2) A(2) state. The (2)A(2)-B-2(1) energy splittings on the pseudorotation potential surface overestimated at the unrestricted Hartree-Fock (UHF ) level are corrected with the MP2 method.