Quantum chemical analysis of heteroarylnitrenium ions and mechanisms for their self-destruction

Singlet-triplet state-energy splittings in arylnitrenium ions are substanti ally reduced when the aromatic system is heterocyclic with nitrogen atoms i ncorporated at ring positions that would otherwise better stabilize the for mally exocyclic positive charge through resonance. Ring contraction rearran gements of singlet arylnitrenium ions with a nitrogen atom at an ortho posi tion are predicted to proceed with barrier heights that become lower with i ncreasing substitution of nitrogen atoms at other positions. Predilection t o delocalize positive charge also controls the relative facility with which (i) N-acetylarylnitrenium ions undergo ring closure to form 2-oxazolium ca tions and (ii) arylnitrenium ions are subject to nucleophilic aromatic subs titution by water.