Vapor Pressure Isotope Effects of Water Studied by Molecular Orbital Calculations

Abstract

H/D and 16O/18O fractionation factors between liquid water and water vapor in the temperature range of 0 to 100°C were calculated using the reduced partition function ratios of small water clusters, (H2O) n with n=1 to 10, obtained at the HF/6-31G(d) and B3LYP/6-311G(d) levels of theory. The calculated fractionation factor values were heavily dependent on the choice of the MO theory and the basis set. Both the calculation levels showed that the formation of hydrogen bonds in liquid water caused the heavier isotopes of hydrogen and oxygen to be preferentially fractionated into the liquid water phase. While the HF/6-31G(d) level calculations yielded better results of the absolute values of the H/D and 16O/18O fractionation factors, the B3LYP/6-311G(d) level calculations reproduced the slopes of their temperature dependence better than the HF/6-31G(d) level calculations. As a whole, the quantitative agreements between the experiment and the present calculations were not satisfactory. It was indicated that a proper choice of MO theory, the basis set and the number of hydrogen bonds per molecule in liquid water may reproduce well the experimental vapor pressure isotope effect data of water.