Together, Not Separately, OH and O3 Oxidize Hg(0) to Hg(II) in the Atmosphere.

Mercury, a highly toxic metal, is emitted to the atmosphere mostly as gaseous Hg(0). Atmospheric Hg(0) enters ecosystems largely through uptake by vegetation, while Hg(II) largely enters ecosystems in oceans and in rainfall. Consequently, the redox chemistry of atmospheric mercury strongly influences its fate and its global biogeochemical cycling. Here we report on the oxidation and reduction of Hg(I) (BrHg and HOHg radicals) in reactions with ozone and how the electronic structure of these Hg(I) species affects the kinetics of these reactions. The oxidation reactions lead to YHgO· + O2 (Y = Br and OH), while the reduction reactions produce Hg(0), OY, and O2. According to our calculations with CCSD(T), NEVPT2, and CAM-B3LYP-D3BJ, the kinetics of both oxidation reactions are very similar and much faster than their reduction counterparts. Past modeling of field data has supported the idea that OH and/or O3 (rather than Br) dominates Hg(II) production in the continental boundary layer. Almost all models invoking OH- and ozone-initiated oxidation of Hg(0) assume that these reactions produce Hg(II) in one step, despite the lack of plausible mechanisms. The two-step mechanism of formation of HOHg followed by its reaction with ozone helps reconcile modeling results with mechanistic insights.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury.

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.

Off-Center Gaussian Functions, an Alternative Atomic Orbital Basis Set for Accurate Noncovalent Interaction Calculations of Large Systems.

Proper description of noncovalent interactions requires, among other things, the use of diffuse atomic orbital (AO) basis sets. However, the presence of diffuse functions, especially in extended molecular systems, can lead to linear dependent AO basis sets. This in turn results, for example, in molecular orbital optimization problems or, when dependencies are removed in unpredictable and possibly geometry-dependent accuracy fluctuations. In this work, an alternative approach is proposed which suffers no linear dependence problems and delivers comparably accurate noncovalent interaction energies. An algorithm is proposed and implemented to construct a grid of off-center s-type Gaussian functions surrounding the molecule; substituting the presence of atom-centered diffuse basis functions. While the number of basis functions in the grid is comparable to the number of diffuse basis functions in aug-cc-pVXZ (for each cardinality number "X") basis sets for small molecular systems, the ratio becomes more favorable with increasing system size. The grid is constructed in a way that it is unique for a molecule (monomer) and, thus, independent of noncovalent complex/cluster geometry. The grid parameters, such as the density of grid points and s-function exponents, are obtained via optimization toward the S22 data set on the MP2 level. The quality, transferability, and versatility of the grid is tested on the S66 data set as well as on several cuts through the potential energy surface for noncovalent complexes, such as methyl-guanine···methyl-cytosine conversion from stacked to hydrogen-bonded structure.

Ab initio calculations of molecular properties of low-lying electronic states of 2-cyclopenten-1-one--link with biological activity.

Geometries, vibrational frequencies, vertical and adiabatic excitation energies, dipole moments and dipole polarizabilities of the ground and the three lowest electronic excited states, S1(n, π*), T1(n, π*), and T2(π, π*) of the 2-cyclopenten-1-one molecule (2CP) were calculated at the CCSD and CCSD(T) levels of approximation. Our results indicate that two triplets T1(n, π*) and T2(π, π*) are lying very close each to other, while the singlet S1(n, π*) is well above them. There are dramatic changes in dipole moments for (n, π*) excited states in respect to the ground state. On the other hand the T2(π, π*) state has a similar dipole moment as the ground state. These changes can be interpreted within the MO picture using electrostatic potential maps and changes in model IR spectra. Our CCSD(T) dipole moment data for the ground state and almost isoenergetic triplets T1(n, π*) and T2(π, π*) are 1.469 a.u., 0.551 a.u., and 1.124 a.u., respectively. Dipole polarizabilities of investigated excited states are much less affected by electron excitations than dipole moments. These are the first dipole moment and polarizability data of 2CP in the literature. The changes of molecular properties upon excitation to S1(n, π*) and T1(n, π*) correlate with the experimental data on the biological activity of 2CP related to the α, ß-unsaturated carbonyl group.

Linear and nonlinear optical properties of a series of Ni-dithiolene derivatives.

Some linear and nonlinear optical (NLO) properties of Ni(SCH)(4) and several of its derivatives have been computed by employing a series of basis sets and a hierarchy of methods (e.g., HF, DFT, coupled cluster, and multiconfigurational techniques). The electronic structure of Ni(SCH)(4) has been also analyzed by using CASSCF/CASPT2, ab initio valence bond, and DFT methods. In particular we discuss how the diradicaloid character (DC) of Ni(SCH)(4) significantly affects its NLO properties. The quasidegeneracy of the two lowest-energy singlet states 1 (1)A(g) and 1 (1)B(1u), the clear DC nature of the former, and the very large number of low-lying states enhance the NLO properties values. These particular features are used to interpret the NLO properties of Ni(SCH)(4). The DC of the considered derivatives has been estimated and correlated with the NLO properties. CASVB computations have shown that the structures with Ni(II) are the dominant ones, while those with Ni(0) and Ni(IV) have negligible weight. The weights of the four diradical structures were discussed in connection with the weight of the structures, where all the electrons are paired. Comparative discussion of the properties of Ni(SCH)(4) with those of tetrathia fulvalene demonstrates the very large effect of Ni on the properties of the Ni-dithiolene derivatives. A similar remarkable effect on the NLO properties is produced by one or two methyl or C(3)S groups. The considered Ni-dithiolene derivatives have exceptionally large NLO properties. This feature in connection with their other physical properties makes them ideal candidates for photonic applications.

The quadrupole moment of the Sb nucleus from molecular microwave data and calculated relativistic electric-field gradients.

The recently determined accurate values of the nuclear quadrupole coupling constant of the Sb nucleus in SbN, SbP, SbF, and SbCl and the calculated electric-field gradients at Sb in these molecules are used to obtain the nuclear quadrupole moment of 121Sb and 123Sb. The calculation of the electric-field gradient has been carried out by using the infinite-order two-component relativistic method in the scalar approximation. The accompanying change of picture of the electric-field gradient operator has been accounted for by employing the shifted nucleus model of nuclear quadrupoles. The electron correlation effects are calculated at the level of the coupled cluster approximation. The present calculations give the "molecular" value of the nuclear quadrupole moment of 121Sb equal to -556+/-24 mb which is considerably different from the old "recommended" value of -360+/-40 mb and also differs from the recent "solid-state" result (-669+/-15 mb). The validation of the present data is comprehensively discussed.

The quadrupole moment of the 3/2+ nuclear ground state of 197Au from electric field gradient relativistic coupled cluster and density-functional theory of small molecules and the solid state.

An attempt is made to improve the currently accepted muonic value for the 197Au nuclear quadrupole moment [+0.547(16)x10(-28) m2] for the 3/2+ nuclear ground state obtained by Powers et al. [Nucl. Phys. A230, 413 (1974)]. From both measured Mossbauer electric quadrupole splittings and solid-state density-functional calculations for a large number of gold compounds a nuclear quadrupole moment of +0.60x10(-28) m2 is obtained. Recent Fourier transform microwave measurements for gas-phase AuF, AuCl, AuBr, and AuI give accurate bond distances and nuclear quadrupole coupling constants for the 197Au isotope. However, four-component relativistic density-functional calculations for these molecules yield unreliable results for the 197Au nuclear quadrupole moment. Relativistic singles-doubles coupled cluster calculations including perturbative triples [CCSD(T) level of theory] for these diatomic systems are also inaccurate because of large cancellation effects between different field gradient contributions subsequently leading to very small field gradients. Here one needs very large basis sets and has to go beyond the standard CCSD(T) procedure to obtain any reliable field gradients for gold. From recent microwave experiments by Gerry and co-workers [Inorg. Chem. 40, 6123 (2001)] a significantly enhanced (197)Au nuclear quadrupole coupling constant in (CO)AuF compared to free AuF is observed. Here, these cancellation effects are less important, and relativistic CCSD(T) calculations finally give a nuclear quadrupole moment of +0.64x10(-28) m2 for 197Au. It is argued that it is currently very difficult to improve on the already published muonic value for the 197Au nuclear quadrupole moment.