Triplet state e.s.r. studies of aryl cations. Part 2. Substituent factors influencing net stabilisation of the triplet level

Abstract

E.s.r. study of triplet state aryl cations generated photochemically (hν at 77 K) by the process ArN₂+BF₄^–→ Ar⁺+ N₂+ BF₄ indicates that the lowering of the triplet state energy level of Ar⁺(relative to the almost isoenergetic singlet level) to become the ground state, is promoted by the presence of electron-releasing groups, such as NR₂, OR, SR, and SAr; the stabilising effect of a given substituent is positionally dependent, viz. 4 > 2 3 substitution, whilst for a given site of substitution the stabilisation effect is NR₂ > OR ∼ SR ∼ SAr. The magnitude of the D parameter reflects both this sequence and the presence of other electron-withdrawing substituents. It is concluded that ab initio calculations cannot deal adequately with the subtlety of the effects promoting stabilisation of triplet Ar⁺.