Phosphonitrenium, phosphonitrilium, and aminophosphenium cations. An ab initio study of the H3PN+ isomers and the decomposition of azidophosphonium salts

Abstract

Portions of the singlet and triplet [H₃PN⁺] energy surfaces are investigated by ab initio calculations. Local minima corresponding to structures H₃P–N⁺, H₂P–NH⁺, and HP–NH₂⁺, and transition structures connecting them via unimolecular 1,2-hydrogen shifts, were obtained at the HF/3-21G* level. Vibrational frequencies and thermochemical properties of the species considered are reported. The phosphonitrenium ion (H₃PN⁺) has a triplet ground state (³A₂) while its ¹A′ singlet state corresponds to a saddle point. The singlet ground state H₂PNH⁺ cation possesses a P–N triple-bond character and can thus be best regarded as a phosphonitrilium ion H₂P[graphic omitted]N⁺–H. the singlet aminophosphenium (HP–NH₂⁺) is the global minimum of the species considered. All the minima are separated from each other by large energy barriers. The intrinsic barriers (ΔG_o^‡, Marcus theory) are calculated to be 62 and 44 kcal mol^–1 for the singlet and triplet H₂PNH⁺→ HPNH₂⁺ rearrangements, respectively. The triplet–singlet energy separations are compared with those of neutral and parent species. Calculated results appear not to support the proposition that the photolysis of azidophosphonium salts (P–N₃⁺X^–) involve nitrene (P–N⁺) as intermediate. They suggest rather a concerted migration–nitrogen loss mechanism or a nucleophile substitution depending upon the nature of the counteranion (X^–).