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

Ab Initio CASSCF results of 1,3,5-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 (ΔED–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.