Ab initio molecular orbital calculations on NO+(H2O)n cluster ions.: 2.: Thermodynamic values for stepwise hydration and nitrous acid formation

Ab initio molecular orbital calculations have been used to compute thermodynamic constants (DeltaH degrees, DeltaS degrees, DeltaG degrees) for the stepwise hydration reactions of NO+(H2O)(n), and for the competing rearrangement reaction which produces HONO and H+(H2O)(n) (for n less than or equal to 4). Geometry optimizations and harmonic frequency calculations were performed at the MP2/6-311++G(2d,p) level, and relative energies were computed at the MP2/augcc-pVTZ level with MP2/6-311++G(2d,p) optimized geometries. The geometry changes and energetics of these competing solvation and rearrangement reactions have been studied, and reasons are proposed to explain why NO+(H2O)(n+1) formation is the dominant process for n = 1 and n = 2 but HONO + H+(H2O)(n) formation contributes for n = 3 and becomes more important for n = 4.