Ab Initio CASSCF Study on Doublet and Quartet States of l,3,5-Tris(methylene)benzene and 1,3,5-Benzenetriamine Trication

Ab Initio CASSCF results of l,3,5-tris(methylene)benzene (TMB) and 1,3,5-benzenetriamine trication (BTA³⁺), which are model molecules of the actively researched triradicals with star-branched topology, are presented. This study included a geometry determination of the four low-lying states (⁴A₁″, ⁴A₂, ²A₂, ²B₁) of TMB and BTA³⁺. The doublet-quartet splitting energies (ΔE_D-Q) of TMB and BTA³⁺ were calculated to be 12.7 and 3.1 kcal mol^-1, respectively. Furthermore, the doublet-doublet splitting energies of TMB and BTA³⁺, which arise from a Jahn-Teller distortion of the doubly degenerate ²E" state, were suitably estimated to be 2.5 and 6.8 kcal mol^-1, respectively, due to obviation of the spin-contamination problem. As a result of configuration mixing, it was found that the doublet states for TMB and BTA³⁺ have to be represented at least by two configurations or more. In addition, the doublet states of TMB were regarded as being a doublet triradical, while those of BTA³⁺ were close to a doublet monoradical; associated with this result, the lowest doublet states of TMB and BTA³⁺ were found to be ²A₂ and ²B₁, respectively.