Aqueous solvation of HgClOH. Stepwise DFT solvation and Born–Oppenheimer molecular dynamics studies of the HgClOH–(H2O)24 complex

Abstract

We address the aqueous solvation of HgClOH through a systematic study of stepwise hydration considering the HgClOH–(H₂O)_n structures with n = 1–24. After calibration of the DFT method, the electronic calculations have been carried out using the B3PW91 exchange–correlation functional. For n < 5 the main geometrical parameters and incremental binding energies are in agreement with counterpoise-corrected MP2/AVTZ static values and BO-MP2 dynamic averages. For n > 15 three direct water–Hg interactions appear during the hydration process and a pentacoordinated trigonal bipyramid apical pattern around Hg is found. 22 water molecules are needed to build the first solvation shell. Unlike microsolvated HgCl₂, no stable equatorial trigonal bipyramid was found. Optimizations with the Polarizable Continuum Model lead to structures with extremely large Hg–O(water) distances because of a dominant solvation effect on the explicit water molecules; however, this overestimation diminishes for large values of n. A DFT Born–Oppenheimer molecular dynamics simulation at T = 700 K revealed the stability of the HgClOH–(H₂O)₂₄ complex with an average trigonal bipyramid Hg-coordination pattern, in accordance with the static cluster description. After thermalization is achieved, the exchange rate of the Hg-coordinated water molecules is estimated to be ca. 10¹¹ s⁻¹.