Quantum chemical study of the electronic structure of the 1-methylene-3,5-didehydrobenzene triradical (C7H5)

We investigated the electronic structure of 1-methylene-3,5-didehydrobenzene triradical (MDB, C7H5) containing a σ1σ1 biradical benzene ring coupled with an exocyclic π1 CH2. MS-CASPT2(9,9)/ANO-L calculations reveal that MDB exhibits a low-spin doublet 2B1 (2Π) ground state, followed by an open-shell doublet 2A2 state with a 2A2-2B1 gap of 17 ±  3 kcal mol−1. The energy ordering of MDB is (kcal mol−1): X2B1 (0) < A2A2 (17) < a4A2 (20) < B2B2 (39) < b4A1 (59) < C2A1 (61) < c4B1 (75) < d4B2 (79). The doublet states are lower in energy than the quartets, violating Hund's rule, due to a coupling of the two unpaired σ-electrons transforming the triradical into a radical.