Thermodynamics of OHgX, XHgOH, XHgOCl, XHgOBr, and HOHgY Gaseous Oxidized Mercury Molecules from Isodesmic, Isogyric, and Atomization Work Reactions (X = Halogen, Y = OH, OCl, OBr).

Isogyric, isodesmic, and atomization reactions were used to determine the standard enthalpies of formation (Δ H°f) for a series of OHgX, XHgOH, XHgOCl, XHgOBr, and HOHgY molecules (X = halogen, Y = OH, OCl, OBr) from single-point CCSD(T)/AV nZ//M06-2X/AVTZ ( n = D, T, Q) energies extrapolated to the complete basis set limit using a three-point mixed Gaussian exponential scheme. Standard enthalpies of formation from isogyric and isodesmic reactions are in good agreement with experimental data and values from higher level calculations but at significantly reduced computation time. The established template work reaction scheme and methodology can potentially be extended for Hg species outside the scope of this work. The standard error for the Δ H°f for the studied molecules averaged over several isodesmic/isogyric reactions is 1.67 kcal/mol or less. The X-Hg-Y bond dissociation energies (BDEs) were determined from the Δ H°f and used as a metric of the molecular stability. The BDE decreases from F to I for the X-HgOH and OHg-X series. The trend is reversed for X-HgOCl and X-HgOBr where the BDE increases from F to Br but decreases with I. All studied molecules have significant BDEs in the range of 50-90 kcal/mol. Assuming that the concentrations of halogens OH, OCl, or OBr are reasonably high near the vicinity of a mercury emission source, the studied molecules can potentially exist long enough to affect the mercury oxidation-reduction rate and impact the global mercury budget.