Monte Carlo simulation of small hydrate clusters of NO−2

The ground-state Hartree–Fock (HF) potential for the NO−2:H2O dimer has been computed for 102 different intermolecular geometrical configurations and has been expressed in a computationally convenient analytical form. The main conclusion drawn from these calculations is that the ion–solvent attraction is mainly electrostatic for intermolecular distances between 6.0 and 7.0 bohr (N-to-O distance). Keeping the dipole vector of the H2O molecule oriented toward the NO−2 ion yields energetically favorable conformations. Rotations of the H2O molecule which do not change the dipole orientation of the H2O have been found to have small barriers (∼4 kcal/mole), whereas those that destroy proper dipole alignment encounter large (∼30 kcal/mole) barriers. The use of such ion–H2O intermolecular potentials together with the H2O:H2O pair potential of Clementi permits Monte Carlo techniques to be used to examine the nature of the inner hydration shells of NO−2. The results of Monte Carlo simulations of NO−2(H2O)n1⩽n⩽15 are discussed in some detail.