Effect of protonation on the electronic structure of 1,3,5-trimethylenebenzene triradical

1,3,5-Trimethylenebenzene (TMB, C9H9) is a prototypical hydrocarbon triradical. CASPT2/ANO-L calculations confirm that TMB has a quartet ground state with a 2 B 1 – 4 A 1 ″ gap of 12.4 ± 2.0 kcal/mol; both 2B1 and 2A2 states are quasi-degenerate. C 9 H 9 - has nearly degenerate 3B2, 3A1 and 1A1 states and C 9 H 9 + nearly degenerate 3A1, 1A1 and 3B2 states. Exocyclic protonation is favoured over ring-protonation; both processes induce a low-spin doublet ground state. Ionization energy, electron affinity, proton affinity are: IEa(TMB) = 7.5 ± 0.2 eV, EA(TMB) = 22 ± 2 kcal/mol, PA(C9H9) = 235 ± 2 kcal/mol.