An ab initio potential-energy function for NH +4· H2O and its use in the study of ionic coordination in solution

Abstract

Ab initio quantum-mechanical calculations with a ‘double zeta plus polarization’ basis set have been carried out for the complex NH⁺₄· H₂O. The calculated dimer energies have been fitted to an analytical form based on atom–atom potentials and a point-charge representation of the electrostatic interactions. The fitted potential has been used in a molecular-dynamics simulation of [NH⁺₄]_aq with results for the structure that are in fair agreement with those obtained experimentally by neutron-diffraction methods. Examination of the atom—atom distribution functions shows that the first coordination shell of the ion contains ca. 13 water molecules; four of these are tetrahedrally coordinated to the ion in a manner which resembles the packing of water molecules in the pure liquid.