Nonharmonic Vibrational Effects in HgClOH: An MP2 Born–Oppenheimer Molecular Dynamics Study Alejandro Ramı ´rez-Solı ´s* [a] and Laurent Maron [b] We report a Born–Oppenheimer molecular dynamics study of HgClOH to address the nonharmonic (NH) effects which are important in the development of classical interaction potentials. The electronic structure calculations were performed with the second order M€ oller–Plesset version of perturbation theory. For the Hg and Cl atoms, the Stuttgart– Dresden relativistic effective core potentials were used with extended-optimized valence basis, while for O and H the aug- cc-pVnZ (n ¼ D,T,Q) basis sets were used. Dynamic effects introduce non-negligible changes to the equilibrium HgAO distance and the ClAHgAO angle which shows rather large (but still harmonic) variations. The largest NH effects appear for the in-molecular-plane HAOAHg bending and the OAH stretching modes. The results for the isolated molecule are relevant to compare the geometrical parameters with the water-solvated species, both through cluster models or through condensed liquid phase studies. V C 2012 Wiley Periodicals, Inc. DOI: 10.1002/qua.24123 Introduction Mercury, one of the most toxic substances on Earth, can be found naturally in small amounts in the environment. However, human activities including fossil fuel combustions, metallurgy, and manufacturing industries have contributed to increase Hg levels in soil, sediments and aquatic ecosystems worldwide. [1] Even in remote and pristine areas high Hg levels have been detected in biota such as in fish, birds, mammals, and humans. [2,3] These elevated levels of Hg are driven, in large part, by the long range transport of mercury in the atmos- phere and its bioaccumulation and biomagnifications along the food chains. In spite that environmental concentrations are generally lower than levels encountered around contaminated sites, they may have significant adverse effects on humans and the ecosystems. Mercury reaches remote ecosystems mainly as elemental gaseous mercury, Hg(0). Once oxidized to more water soluble forms, Hg can be deposited after atmospheric scavenging by wet and dry deposition. To understand the mechanisms of food chain contaminations, the question of the chemical and biological fate of deposited Hg species is of utmost importance. As can be expected, the Hg 2þ ion is not inert once deposited. Hg 2þ complexes can be easily photode- gradated or reduced by compounds produced via photochem- ical reactions [4] and reemitted back to the atmosphere as Hg(0) from various environmental matrices including soil, [5] fresh water [6] sea water, [7] and snow. [8] The exact pathways are still unclear. The bioavailability of deposited Hg is followed by an important biochemical step in which Hg will cross a cell membrane by passive or active diffusion. The bioavailability is a critical issue in determining the Hg toxicity, the potential for Hg accumulation and for the production of organo-mercurial species, such as monomethylmercury. The bioavailability of Hg can be assessed in environmental samples by using biochemi- cal tools such as biosensors. [9–11] However, measurements of Hg bioavailability are to be carefully discussed due to the complex nature of environmental samples. Hg bioavailability will depend on pH variations that facilitate Hg uptake, changes in the proportion of ligands (more or less hydroxylated), and changes in the concentration of other metal ions that com- pete with Hg. [12] Among the many possibilities of Hg-complexes, the neutral closed-shell HgCl 2 , HgOHCl, and Hg(OH) 2 molecules appear to be the most abundant species in oxic environments if organic complexation is not considered. [13,14] Using laboratory-synthe- sized lipid bilayer membranes (lecithin–cholesterol–tetrade- cane), Gutknecht [15] studied the permeability of HgCl 2 , HgCl 3  , HgCl 4 2 , HgClOH, and Hg(OH) 2 , of which three are nonionic species and, therefore, more likely to pass across lipid bilayers and biological membranes. Based on the results of this study, lipid membranes are highly permeable to HgCl 2 with a perme- ability about 20 times higher than the permeability to water and more than a million times higher than the permeabilities to Na þ ,K þ , and Cl  . Barkay et al. [16] obtained similar results with uncharged HgCl 2 being more bioavailable than anionic forms of mercuric chloride. However, they also reported per- meability at significant rates for other nonionic forms of Hg, such as Hg(OH) 2 and HgClOH, in contrast with the findings of Gutknecht. [15] [a] A. Ramı´rez-Solı´s Departamento de Fı´sica Facultad de Ciencias, Universidad Aut  onoma del Estado de Morelos Cuernavaca, Morelos 62209, M exico E-mail: alex@uaem.mx [b] L. Maron Universit e de Toulouse, INSA, Laboratoire de Physicochimie de Nano-objets, 135 Avenue de Rangueil, F-31077 Toulouse, France Contract grant sponsor: ECOS-ANUIES/CONACYT Mexican-French cooperation program. Contract grant sponsor: CONACYT basic science; contract grant number: project No. 130931 (ARS). V C 2012 Wiley Periodicals, Inc. International Journal of Quantum Chemistry 2012, DOI: 10.1002/qua.24123 1 WWW.Q-CHEM.ORG FULL PAPER