Computer simulation of molecular complexes H3O+(H2O)n under conditions of thermal fluctuations: I. Interactions in the system

Abstract

The simplest pair model of intermolecular interactions fails to reproduce known experimental free energy and entropy of hydration of H 3 O + ions in water vapor. A fit to experiment is attained only when covalent bonds and nonpair interactions, which are of particular importance at contact distances from the ion, are taken into account. An interaction model was constructed, which allows the experimental free energies of cluster formation to be reproduced to fractions of k B T by the Monte3Carlo method. Numerical values of interaction parameters were obtained by fitting simulated results to refined experimental data. Problem of description of interactions in ion3water clusters. Though the boundary between clusters and molecules is rather conventional, it is a common practice to associate formation of molecules with formation of chemical bonds. A distinctive feature of such interactions is their nonpair nature whose extreme manifestation is the effect of saturation of chemical bonds according to atomic valence. The nature of interactions (pair or nonpair) might serve as a criterion in attributing a certain molecular complex to clusters or molecules. However, this scheme, too, has its disadvantages. Nonpair interactions also can be of essential importance in typical clusters, especially in clusters formed in a strong electric field of an ion. Quantum-chemical calculations of the energies of various molecular configurations were fulfilled by Kistenmacher et al. [1] for clusters with 2310 water molecules on the Li + , Na + , K + , F 3 , and Cl 3 ions. The numerical results were approximated by analytic functions that were then used in statistical calculations at finite temperature. The nonpair interaction is most contributed by water3ion3water terms. Water3water3 water interactions are negligibly small. Three-body interactions make up~10% of the energy of a system (tens k B T, where k B is Boltzmann constant and T is absolute temperature), and four-body interactions make up 132%. The contributions from many-body interactions are approximately proportional to the amount of water3ion3water triplets in the first coordination shell. Interactions with the ion contribute most to cluster energy.There are two basic techniques for constructing model intermolecular potentials: quantum-chemical calculations by the Hartree3Fock self-consistent field (SCF) method with subsequent corrections for electron correlations and a reconstruction of the potential from measurable thermal and structural properties of real substances. Inclusion of nonpair interactions involves multiple solution of the Schroedinger equation for a great number of molecular configurations followed by approximation of the numerical results by multidimensional functional relationships. Sequential fulfillment of this program requires huge computing expenditures. Systematic investigations aimed at the reconstruction of intermolecular potentials from thermal and structural properties of water had been initiated in early 1970s [2310]. A series of models of intermolecul...