CROATICA CHEMICA ACTA CCACAA, ISSN 0011-1643, e-ISSN 1334-417X Croat. Chem. Acta 86 (4) (2013) 453–462. http://dx.doi.org/10.5562/cca2314 Original Scientific Article Stability of Dimethylmercury and Related Mercury-containing Compounds † with Respect to Selected Chemical Species Found in Aqueous Environment Laimutis Bytautas Department of Chemistry, Rice University, Houston, Texas 77005, USA, Present address: Galveston College, Department of Chemistry, 4015 Avenue Q, Galveston, TX, 77550, USA. (E-mail: [email protected]) RECEIVED JUNE 23, 2013; REVISED OCTOBER 3, 2013; ACCEPTED OCTOBER 4, 2013 Abstract. Dimethylmercury (CH3−Hg−CH3) and other Hg-containing compounds can be found in atmos- pheric and aqueous environments. These substances are highly toxic and pose a serious environmental and health hazard. Therefore, the understanding of chemical processes that affect the stability of these sub- stances is of great interest. The mercury-containing compounds can be detected in atmosphere, as well as soil and aqueous environments where, in addition to water molecules, numerous ionic species are abun- dant. In this study we explore the stability of several small, Hg-containing compounds with respect to wa- + ter molecules, hydronium (H3O ) ions as well as other small molecules/ions using density functional theo- ry and wave function quantum chemistry methods. It is found that the stability of such molecules, most + notably of dimethylmercury, can be strongly affected by the presence of the hydronium H3O ions. Alt- hough the present theoretical study represents gas phase results, it implies that pH level of a solution should be a major factor in determining the degree of abundance for dimethylmercury in aqueous envi- + ronment. In particular, it is found that CH3−Hg−CH3 reacts readily with the H3O ion producing + CH3−Hg−OH2 and methane indicating that low-pH levels favor the decomposition of dimethylmercury. On the other hand, our study suggests that high-pH levels in aqueous environment would favor strongly- bound complexes of [CH3−Hg−CH3OH] species. Overall, the theoretical evidence presented in this study offers an explanation for the available experimental data concerning the stability of dimethylmercury and other mercury-containing compounds having the general structure X−Hg−Y (X,Y = + + CH3 and Cl) with respect to various ligands L (L = H2O, NH3, H3O , OH , Cl and NH4 ). (doi: 10.5562/cca2314) Keywords: dimethylmercury, ab initio quantum chemistry, potential energy surfaces, toxicology, mercu- ry-containing compounds INTRODUCTION population. A large body of experimental data document- ing effects of mercury to human health has accumulated Elemental mercury (Hg) is the only metal that is liquid over many years.8–12 In this context, theoretical studies at room temperature. This unique property evidently is are important in elucidating possible chemical mecha- related to relativistic effects.1,2 Over many years mercu- nisms involving Hg-containing species.13 It is well known ry has been classified as a transition element (see e.g. that mercury and its compounds, especially Ref. 3), however recently this classification has been methylmercury species, i.e. compounds containing 4 challenged. A recent paper by Jensen suggests that Hg CH3Hg-group, are extremely toxic to humans and other as well as Zn and Cd should not be classified as transi- living organisms.8–16 These compounds exist naturally in tion elements although some controversy still exists the environment, although human-related activities have regarding this issue.5,6 The ground state electronic con- contributed to the increased concentrations of mercury. It figuration of mercury atom is [Xe] 4f14 5d10 6s2 indicat- is understood that high affinity of the organomercurial ing that the Hg-dimer should be a weakly bound van der compounds for thiols and lipophilic characteristics are the Waals-type system in its ground state.7 critical factors in their toxicity and various living organ- Much attention has been dedicated to mercury- isms have developed unique neutralization mechanisms containing compounds containing halogens and methyl that counteract its effects. For example, bacteria devel- groups that are of practical significance to the human oped two resistance pathways which involve the cleavage † Dedicated to Professor Douglas Jay Klein on the occasion of his 70th birthday. 454 L. Bytautas, Stability of Dimethylmercury and Related Mercury-containing Compounds of Hg−C bond with a subsequent reduction of the mercu- and microbe-assisted transformations.44,45 The role ric residues to elemental mercury.15,16 The presence of played by humic acids that are abundant in aqueous methylmercurials in soil, aqueous environment and the environment has been recently discussed in Ref. 20. A atmosphere is well documented.17–27 Due to various study by Tossell44 examined the reaction energies for transport mechanisms such compounds accumulate in the formation of various CH3−Hg−L species resulting + living organisms, especially in aquatic environments. from CH3−Hg and ligands L. In particular, Tossell’s 8 44 + Methylmercury species are known to cross the blood- study indicates that (CH3−Hg−OH2 ) should be the brain barrier and are highly neurotoxic. One of the most most dominant species in natural water systems. toxic forms to humans among many mercury-containing Since methylmercury species frequently occur in chemical compounds is dimethylmercury (DMHg) with aquatic environments, one of important questions is its chemical formula CH3−Hg−CH3. DMHg is a non- investigating the stability of these compounds with polar compound and readily accumulates in fatty tis- respect to exposure of water molecules as well as hy- 8,14 + − sues. One of the important aspects in reducing the dronium (H3O ) and hydroxide (OH ) ions which, in long-term exposure of methylmecury compounds to turn are associated with pH levels of aqueous environ- human population relates to understanding of chemical ments. Of particular interest is the observed correlation and physical processes controlling its concentration. between low pH levels and instability of DMHg.32–34 Therefore, a significant volume of research has been Thus, the goal of the present theoretical study is to ex- dedicated to methylmercury, its demethylation as well as plore possible mechanisms that can contribute to the exploring other possible mechanisms involving mercu- chemical degradation of DMHg (CH3−Hg−CH3). 28–34 ry. Evidently, DMHg is the predominant methylated form of mercury in the deep ocean.23 It has been suggest- METHODOLOGY AND TECHNICAL DETAILS ed24 that the oceanic DMHg is the main source of monomethylmercury (MMHg) in the water column. The The geometry optimizations in this study were performed MMHg compound (i.e. methylmercury monocation for the following systems: H O, H O+, OH, Cl, NH +, [CH −Hg]+) plays a significant role in the aqueous envi- 2 3 4 3 CH −Hg−CH , CH −Hg−Cl, Cl−Hg−Cl, CH −Hg−OH +. ronment due to its ability to biomagnify in aquatic food 3 3 3 3 2 The ability of the Hg-containing compounds, i.e. chains leading to high toxicity levels in fish and other CH −Hg−CH , CH −Hg−Cl and Cl−Hg−Cl to form organisms. This seems to be the main pathway to the 3 3 3 complexes with the molecules of H O, H O+, OH, Cl, exposure of mercury to human population.8,12 Recent 2 3 NH + is explored by optimizing the geometries with the findings of Conaway et al.22 indicate unexpectedly high 4 inclusion of the counterpoise correction (CP) to correct concentrations of DMHg in costal surface waters. Some for the basis-set-superposition-error (BSSE).46 The basis studies35 suggest that one possible route for the degrada- set used in all the calculations is 6-31G(d,p) (spherical tion of DMHg into MMHg in aqueous environments is by harmonics) for all atoms except mercury. For the Hg evaporating of DMHg into the atmosphere and, after the atom the Stuttgart/Dresden pseudo-potential (effective- chemical transformation into MMHg, being brought back core-potential) MDF60 (see Refs. 47,48) was used to to the Ocean waters by aerosols or water vapor. An alter- treat core electrons while the MWB60 basis set was used native route for the transformation from DMHg to to treat valence electrons. The initial optimization step MMHg may involve mechanisms and chemical species uses PBE DFT-functional and the resulting optimized that can occur directly in aqueous environments. geometry is used as the starting point in the MP2 optimi- A number of theoretical studies investigating zation. After the geometry is optimized at the MP2 level chemistry of mercury-containing compounds have al- of theory, a single-point CCSD(T) calculation is per- ready been reported in the literature.36–44 For example, formed to obtain the total energy (electronic energy in- mechanisms associated with the Hg−C bond cleavage cluding nuclear repulsion). The vibrational analysis and by halogenic acids have been investigated by Barone the zero-point-energies correspond to the MP2 level of and co-workers.15 Ni et al.36 investigated possible theory. All calculations were performed using mechanisms for degrading chloromethylmercury GAUSSIAN package of ab initio programs.49 Only local (CH −Hg−Cl) and dimethylmercury (CH −Hg−CH ) 3 3 3 minima on the singlet ground-state potential energy sur- involving thiol and ammonium residues. Khalizov et face50–52 have been considered in this study. al.37 studied the reactions of Hg with halogens and dis- cussed their atmospheric implications. Shepler et al.38 have analyzed the effects of aqueous solvation on the RESULTS AND DISCUSSION thermochemistry of reactions between Hg and small halogen molecules. Furthermore, it has been proposed The strategy of this investigation is to explore possible that organic mercury-containing compounds can be complexes of the Hg-containing compounds, namely broken-down into less toxic species via photo-reduction CH3−Hg−CH3, CH3−Hg−Cl and Cl−Hg−Cl, with small Croat.