Molecular Orbital Study on Quartet Molecules with Trigonal Axis of Symmetry

Abstract

Abstract The ESR spectrum of the randomly oriented cationic triradical of 1,3,5-tris(diphenylamino)benzene (TDAB) is shown to agree well with the theoretical prediction of a quartet (5 = 3/2) molecule. The electronic structures of non-Kekule-type isoelectronic molecules 1,3,5-trimethylenebenzene (TMB) and 1,3,5-triamino-benzene trication (TAB3+) are discussed by means of the ab initio molecular orbital (MO) method in the unrestricted Hartree-Fock scheme. In TMB the quartet state with planar D3h geometry is predicted to lie 26. 1 kcal/mol below the lowest doublet state with a planar C2V geometry in which one of the methylene groups has a longer C-Cbridge bond connecting the phenyl ring. Moreover, in TAB3+ the quartet state with planar D3h also lies 16. 9 kcal/mol below the lowest doublet state with an orthogonal geometry where one of the amino groups is twisted out of the molecular plane. These quartet ground states result from the nearly threefold-degenerate orbitals consisting of the nonbonding MOs. In a...