G2 AB-INITIO CALCULATIONS ON THE THERMOCHEMISTRY OF [P,N,HN] (N = 0-2) AND [P,N,HN](+)(N = 0-3) SPECIES AND ON THE POTENTIAL-ENERGY SURFACES OF [P,N,H3](+) SINGLET-STATE AND TRIPLET-STATE CATIONS

Ab initio molecular orbital calculations at G1 and G2 levels of theory have been used to study the structures, relative stabilities, ionization potentials, heats of formation and proton affinities of neutral and cationic [P,N,H(n)] (n = 0-2) systems, for which there is a complete lack of experimental information. Some of the compounds investigated have been detected as products of the P+ + NH3 reaction in the gas phase. Hence, the singlet and triplet potential energy surfaces for [P,N,H-3]+ have been also investigated. The global minimum corresponds to the HPNH2+(1A1) species, being the lowest triplet (PNH3+(3A1)) 18.5 kcal/mol above it. Since the calculated triplet-singlet energy gap for P+ is ca. 25.0 kcal/mol, the reactive channels on the singlet potential energy surface should be taken into account, at least for high-temperature gas-phase reactions. In this respect some possible mechanisms related to the formation of PN are discussed. A topological analysis of the electronic charge densities permits a systematization of the characteristics of the PN bonding along the series, as well as a quantification of bond activation effects upon ionization and protonation.