Calculation of the energetics for the oligomerization of gas phase HgO and HgS and for the solvolysis of crystalline HgO and HgS.
Recent experimental studies indicate that gaseous elemental Hg (GEM) is rapidly oxidized to Hg(II) compounds, known collectively as reactive gaseous Hg (RGM), in Arctic and Antarctic regions after polar sunrise. The reduction in GEM is correlated with a reduction in surface O(3) concentration, which is thought to be caused by photochemically initiated catalytic reactions involving halogen species, particularly Br and BrO. Initially, the reaction of Hg(0) and BrO to produce HgO and Br was thought to be the dominant reaction, but recent theoretical studies have decisively shown that this reaction is highly endoergic due to the low stability of monomeric gas-phase HgO. This result is in conflict with experimental data on the energetics of the species existing in the vapor over heated HgO (s). One possible explanation for this discrepancy is the existence of highly stable oligomers formed from HgO. Recent high-level quantum calculations on the dimers of HgO and HgS support this concept. In the present work, we systematically examine the structures, stabilities, and other properties of closed (HgX)(n)() ring-type oligomers, n = 2, 3, 4, and 6, X = O, S, as well as infinite one-dimensional (1D) polymers of HgX (studied by using the periodic boundary condition DFT implementation in GAUSSIAN03). We find that the HgX ring oligomers become systematically more stable (per HgX unit) as n increases but that this stability levels off around n = 4-6. We also find that the 1D chain polymers are only marginally more stable than the n = 6 oligomers. To estimate the energies of interaction between the chains in the 3-dimensional (3D) crystal structures of HgX (s), we adopt a cluster model and use the MP2 method to describe the interchain dispersion interactions. We have also obtained optimized geometries for open chain triplets for the dimers, finding them to be substantially more stable than the closed ringlike dimeric species previously described. Trends in relative energies and structures indicate that the higher n oligomers are fairly normal Hg(II) compounds that can be accurately described at low computational levels, as opposed to the monomer and dimer, which possess highly unusual bonding properties and require high-level methods for their description. Nonetheless, even the high n ring, oligomers show close approach of Hg atoms, consistent with a metallophilic-type stabilization. Calculated free energies for the interaction of HgO with H(2)O and with simple models for silicate surfaces are highly favorable, indicating that hydration and surface effects will greatly promote the formation of such species. Molecular cluster models of the HgX surface such as Hg(2)X(XH)(2) are used to calculate the energetics for solvolysis reactions with H(2)O or H(2)S, obtaining good agreement with experiment for the energetics of the dissolution reaction of HgS (s, cinnabar) with H(2)S.
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA. tossell@chem.umd.edu
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D000465:Algorithms; D002384:Catalysis; D005740:Gases; D006219:Halogens / Q000737:chemistry; D017669:Mercury Compounds / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D010087:Oxides / Q000737:chemistry; D010126:Ozone / Q000737:chemistry; D010777:Photochemistry; D011108:Polymers / Q000737:chemistry*; D017640:Silicates / Q000737:chemistry; D012997:Solvents / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
D005740:Gases; D006219:Halogens; D017669:Mercury Compounds; D010087:Oxides; D011108:Polymers; D017640:Silicates; D012997:Solvents; D013440:Sulfides; D014867:Water; D010126:Ozone