The 1,2,3-tridehydrobenzene triradical: 2B or not 2B? The answer is 2A!
The molecular and electronic structure of 1,2,3-tridehydrobenzene was investigated by a variety of computational methods. The two lowest electronic states of the triradical are the B2 and A1 doublet states characterized by different interactions of the unpaired electrons. Vertically, the two states are well separated in energy - by 4.9 and 1.4 eV, respectively. However, due to different bonding patterns, their equilibrium structures are very different and adiabatically the two states are nearly degenerate. The adiabatic energy gap between the B2 and A1 states is estimated to be 0.7-2.1 kcal/mol, in favor of the A1 state. Harmonic vibrational frequencies and anharmonic corrections were calculated for both states. Comparison with the three experimentally observed IR transitions supports the assignment of the A1 ground state for the triradical with a weakly bonding distance of 1.67-1.69 A between the meta radical centers.