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For the clusters H(H20) + , n = 1 to 5, geometry optimization and analytical second-derivative calculations have been carried out with two basis sets, 6-31 G* and 6-31 +G+ +, at both the Hartree-Fock and MP2 levels. Minimum energy structures, harmonic vibrational frequencies and stepwise hydration enthalpies have been obtained and compared with available experimental data. The computed minimum energy geometries of the protonated water clusters show a balance between covalent and ion-dipole hydrogen bonding, with dispersion effects being more important in the heavier clusters. Allowance for the effects of basis set superposition error (BSSE) in the calculations has been examined. The choice of methodology and basis sets in the calculations has been discussed and particular attention has been given to the incorporation of the gradient method in the counterpoise procedure for calculation of interaction energies.

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Ab initio molecular orbital calculations have been used to compute thermodynamic constants (ΔH°, ΔS°, ΔG°) for the stepwise hydration reactions of NO+(H2O) n and for the competing rearrangement reaction which produces HONO and H+(H2O) 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+(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.

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