A quantum mechanics, molecular dynamics and EXAFS investigation into the Hg2+ ion solvation properties in methanol solution

Abstract

The coordination properties of the Hg²⁺ ion in methanol solution have been investigated by combining extended X-ray absorption fine structure (EXAFS) spectroscopy and Quantum Mechanics/Molecular Dynamics (QM/MD) calculations. An effective Hg–methanol two-body potential has been generated from QM calculations in which the effect of bulk solvent is accounted for by the polarizable continuum model (PCM). This effective potential is then used in the MD simulation to obtain the structural and dynamic properties of the solution, and the reliability of the entire procedure is assessed by comparing the theoretical structural results with the EXAFS experimental data. The outstanding outcome of this work is that the Hg²⁺ ion forms a stable sevenfold complex in methanol solution, where the first shell solvent molecules are arranged in a distorted pentagonal bipyramid geometry. In this geometry five methanol molecules are not located on the equatorial plane, but are displaced above and below the plane, forming a “crown” around the ion. The Hg²⁺ first coordination shell has been found to be very flexible, and several transitions among coordination numbers of 7, 8 and 6 are observed during the simulation.