Exploring the potential energy surfaces of the reactions of O +( 4S) and O +( 2D) with ammonia

Abstract

High-level ab initio calculations in the framework of the G2 and CBS-Q theories have been performed for the [H 3,N,O] + doublet and quartet state cations. The geometries of the different stationary points of both potential energy surfaces (PESs) were optimized at the QCISD/6-31+G(df,p) level. The bonding characteristics of doublet and quartet state cations are rather different. The latter are weakly bound species involving one-electron linkages, while the former present normal covalent bonds. The global minimum of the doublet PES is the HO-NH 2 + 2D species, which has a N–O linkage with a partial double-bond character. This means that the ionization of hydroxylamine implies an important enhancement of the stability of the N–O bond, which upon ionization becomes much shorter, although its stretching frequency is significantly blue-shifted. For the quartets the global minimum can be viewed roughly as the interaction between O in its 3P ground state and NH 3 +. From the characteristics of both [H 3,N,O] + doublet and quartet state potential energy surfaces we conclude that the products of O +( rS) and O +( 2D) reactions with ammonia are significantly different. For reactions involving O + in its 4S ground state, only the charge transfer process should be observed, in agreement with the experimental evidence. In reactions involving O + in its 2D first excited state, the charge transfer process is energetically disfavored with respect to H and H 2 loss. Hence, the major product ions should be H 2NO +, HNO +, NOH +, HNOH + and in a much smaller proportion NH 3 +, NH 2 +( 1A 1), and NH +( 2Π). The exothermicity of H 3 + reactions with NO to yield NOH +, HNO +, and H 2NO + (which may be important depletion mechanisms for NO in the interstellar clouds) is also discussed.