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 (Δ H °, Δ S °, Δ G °) for the stepwise hydration reactions of NO + (H 2 O) n and for the competing rearrangement reaction which produces HONO and H + (H 2 O) n (for n ≤ 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/aug-cc-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 + (H 2 O) n +1 formation is the dominant process for n = 1 and n = 2 but HONO + H + (H 2 O) n formation contributes for n = 3 and becomes more important for n = 4.