DIRECT AB-INITIO DYNAMICS AND MO STUDIES ON THE FORMATION REACTION OFAMMONIA DIMER CATIONS BY NH3++NH3 REACTION

The geometrical structures, the electronic states and hyperfine coupli ng constants (hfc) of the ammonia dimer cation [NH3-NH3](+), have been studied by means of ab initio MO method, The formation mechanism of t he dimer cation was also studied by ab initio direct trajectory calcul ations. The dimer cation is known as an intermediate complex in the vi brational mode selected reaction, NH3+ + NH3 --> NH4+ + NH2. The direc t trajectory calculation shows that the formation of two conformers of the dimer cation are possibly competitive. One is composed of a hydro gen bonded form (Cs structure, type I), the other is the complex havin g a N-N bond (C-3v structure, type II), The hfcs are calculated for th e two complexes at the MP4SDQ/6-311C(d,p) level of theory. The hfcs of type I are calculated to a(H)=-26.8 G for two protons, a(H)=-0.1 G fo r one proton and a(H)=0.3 G for three equivalent protons. The hfc of s ix equivalent protons was found to a(H)=-13.0 G for type II. The forma tion mechanism of the dimer cations is discussed on the basis of the t heoretical results. (C) 1997 Elsevier Science B.V.