A QUANTUM-STATISTICAL MECHANICAL STUDY OF THE ENTHALPY OF FORMATION OF THE WATER DIMER

Monte Carlo simulations of quantum statistical mechanical properties u sing the Feynman path integral method were carried out over a temperat ure range from 50 to 400 K to study the energetics of the water dimer (H2O)(2). These results were then used to understand the relation betw een estimates of the enthalpy of formation obtained from recent ab ini tio electronic structure calculations and estimates of the enthalpy of formation deduced from experimental measurements of thermal conductiv ity, second virial coefficients and submillimeter spectroscopy. The fu ll quantum mechanical and anharmonic theoretical results were compared to results obtained from classical mechanical simulation and those ob tained from a quantum mechanical harmonic analysis. In performing the analysis for temperatures above 200 K, the definition of a water dimer becomes poorly defined as thermal activation leading to dissociation becomes more probable. The calculated enthalpy of the dimer is strongl y dependent on the manner in which trapped and independent monomer spe cies are defined. To address these issues we employ an energy threshol d as a dividing surface to separate trapped dimers from those that eve ntually dissociate on the time scale of an experiment. Approximate qua ntum mechanical expressions that are consistent with an energy definit ion of the water dimer were introduced and used in the simulation. It is found that experimental observations are consistent with theoretica l calculations once a characteristic time scale for the experimental t echnique is identified. (C) 1998 American Institute of Physics. [S0021 -9606(98)50515-6].