We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII–organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds.

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The structures and chemical bonding of TaB(n)(-) (n = 3-8) clusters are investigated systematically to elucidate the formation of the planar metal-centred aromatic borometallic cluster, Ta©B10(-) (the © sign is used to designate the central position of the doped atom in monocyclic structures in M©B(n)-type planar clusters), which was found previously to have the highest coordination number for a metal atom in a planar geometry. Photoelectron spectroscopy is combined with ab initio calculations to determine the global minima of the TaB(n)(-) clusters. We find that from TaB3(-) to TaB5(-) the boron atoms nucleate around the central Ta atom to form fan-like structures. A structural transition occurs at TaB6(-), which is found to have a hexagonal structure, but with a boron atom in the centre and the Ta atom on the periphery. TaB7(-) is shown to have a three-dimensional boat-like structure, which can be viewed as a Ta atom coordinated to an elongated B7 cluster from above. The global minimum of the TaB8(-) cluster is found to be pyramidal with the Ta atom interacting with a B8 monocyclic ring. Starting from this structure, additional boron atoms simply enlarge the boron ring to form the slightly pyramidal TaB9(-) cluster and eventually the perfectly planar Ta-centred B10-ring aromatic cluster, Ta©B10(-). It is shown that boron atoms do not nucleate smoothly around a Ta atom on the way to the decacoordinated Ta©B10 (-) molecular wheel, but rather the competition between B-B interactions and Ta-B interactions determines the most stable structures of the smaller TaB(n)(-) (n = 3-8) clusters.

Six of the eleven possible cyclic isomers (CH) 2 B 2 N 2 are found, by geometric optimization at a correlated level of theory, to have planar closed-shell ground states. Current density maps calculated with distributed-origin coupled Hartree± Fock theory wavefunctions show that all six of these have delocalized p-electron systems supporting diamagnetic ring currents. In comparison with benzene at the same level of theory, the isomer with lowest energy, 1; 3; 2; 4 diazadiborinine, has a 5% greater centre of ring shielding, but 20± 25% smaller mean magnetizability, mean polarizability and polarizability anisotropy.

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