Reaction mechanism of platinum dimer cation with ammonia based on the relativistic density functional study

The gas-phase reactions between Pt-2(+) and NH3 have been investigated using the relativistic density functional approach (ZORA-PW91/TZ2P). The quartet and doublet potential energy surfaces of Pt-2(+) + NH3 have been explored. The minimum energy reaction path proceeds through the following steps: Pt-2(+) ((4)Sigma(u)) + NH3 -> q-1 -> d2 -> d-3 -> d-4 -> d-Pt2NH+ + H-2. In the whole reaction pathway, the step of d-2 -> d-3 is the rate-determining step with a energy barrier of 36.1 kcal/mol, and exoergicity of the whole reaction is 12.0 kcal/mol. When Pt2NH+ reacts with NH3 again, there are two rival reaction paths in the doublet state. One is degradation of NH4+ and another is loss of H-2. In the case of degradation of NH', the activation energy is only 3.4 kcal/ mol, and the overall reaction is exothermic by 8.9 kcal/mol. Thus, this reaction is favored both thermodynamically and kinetically. However, in the case of loss of H21 the rate-determining step's energy barrier is 64.3 kcal/mol and the overall reaction is endothermic by 8.5 kcal/mol, so it is difficult to take place. Predicted relative energies and barriers along the suggested reaction paths are in reasonable agreement with experimental observations. (c) 2007 Wiley Periodicals, Inc.