THEORETICAL-STUDIES OF POSSIBLE PROCESSES FOR THE INTERSTELLAR PRODUCTION OF PHOSPHORUS-COMPOUNDS - REACTION OF P+ WITH AMMONIA

A theoretical study of the reaction of P+ with ammonia, as a possible source of interstellar phosphorus-nitrogen compounds, has been carried out. The geometries of the possible products of this reaction, (HNP)+ and (H2NP)+, as well as of the relevant species on the triplet (H3NP)+ potential energy surface have been optimized at the Hartree-Fock level. Relative energies were obtained through correlated calculations at the fourth-order Moller-Plesset theory. Calculations demonstrate that PNH+ is the lowest lying (HNP)+ triplet state, with HPN+ lying 50 kcal/mol higher at the correlated level. In the case of the (H2NP)+ system, PNH2+ is found to be the ground state with trans-HPNH+ and H2PN+ lying 44 and 93 kcal/mol, respectively, higher in energy at the correlated level. The PNH3+ ion-molecule complex is the lowest lying triplet (H3NP)+ structure by a relatively high value, i.e. 40 kcal/mol. The stability order of this system is PNH3+ > HPNH2+ < H2PNH+ > H3PN+. The reaction of P+ with ammonia has been found to be endothermic in all cases except for the production of PNH2+. In addition, two different mechanisms for producing PNH2+ that are barrier free have been found. Therefore, the reaction of P+ with NH3, leading to PNH2+, seems to be a plausible interstellar process, since it is exothermic and also proceeds with no activation barrier.