Ab initio calculations of sulfur isotope fractionation factor for H2S in aqua–gas system

Abstract

High level ab initio calculations have been performed for hydrogen sulfide in aqua–gas system. Based on B3LYP density functional and MP2 methods with two basis sets, 6-31G(d) and 6-311++G(d,p), we have obtained the minimum energy structures of H2S molecule and H2S·nH2O hydrogen bonded molecular clusters, where n=1–5. For these structures the internal and intermolecular harmonic vibrational frequencies were calculated. The calibration of vibrational frequencies by appropriate scaling factor has been made.Using statistical mechanics approach, we have calculated the reduced partition functions ratios (beta-factors) for these clusters and respective sulfur equilibrium isotope fractionation factors as a function of temperature. The calculated beta-factors for H2Ssolution drop nearly linearly from about 1.013 to 1.008 in a temperature range of 0–100°C. The obtained magnitude of isotope fractionation between H2Ssolution and H2Sgas (about +0.8‰ at 25°C and +0.5 at 100°C) is consistent with existing experimental data. This is the second theoretical determination of sulfur isotope fractionation between gaseous and hydrated hydrogen sulfide (dissolved H2S). The previous theoretical calculations by Otake et al. (2008) yielded significantly lower values, ranging from −0.17 to +0.38‰ at 25°C, in dependence on the used model.