Direct ab initio dynamics and MO studies on the formation reaction of ammonia dimer cations by NH_3~++ NH_3 reaction

摘要

The geometrical structures, the electronic states and hyperfine coupling constants (hfc) of the ammonia dimer cation [NH3–NH3]+, have been studied by means of ab initio MO method. The formation mechanism of the dimer cation was also studied by ab initio direct trajectory calculations. The dimer cation is known as an intermediate complex in the vibrational mode selected reaction, +NH3→+NH2. The direct trajectory calculation shows that the formation of two conformers of the dimer cation are possibly competitive. One is composed of a hydrogen bonded form (Cs structure, type I), the other is the complex having a N–N bond (C3v structure, type II). The hfcs are calculated for the two complexes at the MP4SDQ/6-311G(d,p) level of theory. The hfcs of type I are calculated to aH=?26.8 G for two protons, aH=?0.1 G for one proton and aH=0.3 G for three equivalent protons. The hfc of six equivalent protons was found to aH=?13.0 G for type II. The formation mechanism of the dimer cations is discussed on the basis of the theoretical results.