Ab initio study of the ammonia ion–ammonia reaction paths

The three reactions NH+3+NH3→NH2+NH+4 (proton transfer), NH+3+NH3→NH+4+NH2 (atom transfer) and NH+3+NH3→NH3+NH+3 (charge transfer) are studied in an ab initio framework. All geometry optimizations are carried out at the MP2 level, and a SDCI(TQ) calculation is performed at the optimized geometry. For the charge transfer reaction, the energy is calculated as a function of the N–N internuclear distance. The intermediate complex is found to have D3d symmetry. The geometry of the NH3+NH+3 system is optimized for each value of the N–N distance. For the proton transfer reaction, the energy is calculated as a function of two variables which are the two N–H internuclear distances of the central part N–H–N of the complex. For each N–H–N configuration, other coordinates of the system are completely optimized. This approach shows that the atom transfer reaction can be interpreted as a charge transfer process followed by a proton transfer. The influence of the vibrational excitation of the NH+3 reagent on the reaction is discussed.