A first-principles characterization of water adsorption on forsterite grains.

Numerical simulations examining chemical interactions of water molecules with forsterite grains have demonstrated the efficacy of nebular gas adsorption as a viable mechanism for water delivery to the terrestrial planets. Nevertheless, a comprehensive picture detailing the water-adsorption mechanisms on forsterite is not yet available. Towards this end, using accurate first-principles density functional theory, we examine the adsorption mechanisms of water on the (001), (100), (010) and (110) surfaces of forsterite. While dissociative adsorption is found to be the most energetically favourable process, two stable associative adsorption configurations are also identified. In dual-site adsorption, the water molecule interacts strongly with surface magnesium and oxygen atoms, whereas single-site adsorption occurs only through the interaction with a surface Mg atom. This results in dual-site adsorption being more stable than single-site adsorption.

Accretion disc origin of the Earth's water.

Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10⁻8 bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars.

Relationship between dye-iodine binding and cell voltage in dye-sensitized solar cells: a quantum-mechanical look.

It has been proposed that iodine binding to dyes may actually decrease the cell efficiency of a dye-sensitized solar cell. A previous experimental study showed that a two-atom change from oxygen to sulfur increased recombination of iodine with injected electrons by a factor of approximately 2. Here, it is shown that iodine binding is a plausible explanation for this effect. The steric and conjugation effects are quantified separately using a set of model compounds. Quantum-chemical calculations show that elongation of the hydrocarbon chain has only an insignificant effect on the iodine and bromine binding to the chalcogen atoms (O, S, Se). The conjugation, however, significantly disfavors the iodine and bromine interaction. Iodine and bromine binding to the dye and model compounds containing sulfur is significantly more favorable than to their oxygen containing counterparts. Bromine binding to dyes is shown to be stronger than that of iodine. Accordingly, bromine binding to dyes may contribute significantly to the observed lower efficiencies in cells using Br(3)(-)/Br(-) as the redox couple.

Adsorption of uranyl species onto the rutile (110) surface: a periodic DFT study.

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO(2) rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta- or hexa-coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H(2)O, OH(-), and CO(3)(2-) ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface-[UO(2)(H(2)O)(3)](2+) system were well reproduced by our calculations. With respect to adsorbates, [UO(2)(L1)(x)(L2)(y)(L3)(z)](n) (L1=H(2)O, L2=OH(-), L3=CO(3)(2-), x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H(2)O to OH(-) and CO(3)(2-) lengthens the U-O(surf) and U-Ti distances. As a result, the uranyl-surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO(2) decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U=O distances that are longer than those of aquouranyl species by 0.1-0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U=O bond lengths, shorter uranyl-surface separations in most cases, and stronger sorption interactions.

Interactions of the N3 dye with the iodide redox shuttle: quantum chemical mechanistic studies of the dye regeneration in the dye-sensitized solar cell.

The iodide/triiodide redox couple plays a unique role in the dye-sensitized solar cell (DSSC). It is a necessary and unique part of every highly efficient DSSC published to date; alternative redox couples do not perform nearly as well. Hence, a detailed molecular-level understanding of its function is desirable. A density-functional theory (DFT) study has been carried out on the kinetic and thermodynamic aspects of the dye regeneration mechanism involving the iodide/triiodide redox couple and the prototypical N3 dye in the DSSC. The intermediate complexes between the oxidized dye and iodide have been identified. These are outer-sphere complexes of the general formula [dye(+)···I(-)]. Solvent effects are seen to play a critical role in the thermodynamics, whereas relativistic spin-orbit effects are less important. Both the kinetic and thermodynamic data reveal that the formation of complexes between [dye(+)···I(-)] and I(-) is the rate limiting step for the overall dye regeneration process. The regeneration of the neutral dye proceeds with the liberation of I; processes involving atomic iodine or I(-) are inferior, both from thermodynamic and kinetic considerations. The overall dye regeneration reaction is an exothermic process.

Chalcogenophilicity of mercury.

Density-functional theory (DFT) calculations have been carried out to investigate the chalcogenophilicity of mercury (Hg) reported recently [J. Am. Chem. Soc. 2010, 132, 647-655]. Molecules of different sizes have been studied including ME, [M(EH)(4)](n), M(SH)(3)EH (M = Cd, Hg; E = S, Se, Te; n = 0, 2+) and [Tm(Y)]MEZ complexes (Tm = tris(2-mercapto-1-R-imidzolyl)hydroborato; Y = H, Me, Bu(t); M = Zn, Cd, Hg; E = S, Se, Te; Z = H, Ph). The bonding of Cd and Hg in their complexes depends on the oxidation state of the metal and nature of the ligands. More electronegative ligands form bonds of ionic type with Cd and Hg while less electronegative ligands form bonds that are more covalent. The Cd-ligand bond distances are shorter for the ionic type of bonding and longer for the covalent type of bonding than those of the corresponding Hg-ligand bonds. The variation of this Cd/Hg bonding is in accordance with the ionic and covalent radii of Cd and Hg. The experimentally observed (shorter) Hg-Se and Hg-Te bond distances in [Tm(Bu(t))]HgEPh (E = S, Se, Te) are due to the lower electronegativity of Se and Te, crystal packing, and the presence of a very bulky group. The bond dissociation energy (BDE) for Hg is the highest for Hg-S followed by Hg-Se and Hg-Te regardless of complex type.

Degradation mechanism of methyl mercury selenoamino acid complexes: a computational study.

Density functional theory (DFT) calculations have been carried out on the possible degradation/demethylation mechanism of methyl mercury (CH(3)Hg(+)) complexes with free cysteine and seleonocysteine. The binding of CH(3)Hg(+) ions with one (seleno)amino acid is thermodynamically favorable. However, the binding with another acid molecule is a highly unfavorable process. The CH(3)Hg-(seleno)cysteinate then degrades to bis(methylmercuric)sulphide (selenide for the Se-containing complex) which in turn forms dimethyl mercury and HgS/HgSe, the latter being precipitated out as nanoparticles. The dimethyl mercury interacts with water molecules and regenerates the CH(3)HgOH precursor. The calculated free energies of formation confirm the thermodynamic feasibility of every intermediate step of the degradation cycle and fully support earlier experimental results. In completing the cycle, one unit of mercury precipitates out from two units of sources, and thereby Se antagonizes the Hg toxicity. The degradation of CH(3)Hg-L-cysteinate is thermodynamically more favorable than the formation of CH(3)Hg-L-cysteinate. The preferred degradation of the CH(3)Hg-L-cysteinate suggests that another mechanism for CH(3)Hg to cross the blood-brain barrier should exist.

Computational studies on the interactions among redox couples, additives and TiO2: implications for dye-sensitized solar cells.

One of the major and unique components of dye-sensitized solar cells (DSSC) is the iodide/triiodide redox couple. Periodic density-functional calculations have been carried out to study the interactions among three different components of the DSSC, i.e. the redox shuttle, the TiO(2) semiconductor surface, and nitrogen containing additives, with a focus on the implications for the performance of the DSSC. Iodide and bromide with alkali metal cations as counter ions are strongly adsorbed on the TiO(2) surface. Small additive molecules also strongly interact with TiO(2). Both interactions induce a negative shift of the Fermi energy of TiO(2). The negative shift of the Fermi energy is related to the performance of the cell by increasing the open voltage of the cell and retarding the injection dynamics (decreasing the short circuit current). Additive molecules, however, have relatively weaker interaction with iodide and triiodide.

Computational studies of structural, electronic, spectroscopic, and thermodynamic properties of methylmercury-amino acid complexes and their Se analogues.

Quantum chemical calculations have been carried out to study the structural, electronic, spectroscopic, and thermodynamic properties of five methylmercury-amino acid complexes and their selenium analogues. The structural properties of methylmercury-amino acids are very similar to their Se analogues except for those properties that are directly related to the Se atom which has a larger covalent radius. Characteristic stretching frequencies are observed for Hg-S/Se and Hg-C bonds. Electronic properties of both methylmercury-amino acids and their Se analogues are different from each other, with the S complexes showing stronger electrostatic attractions which leads to stronger bonds to mercury. The methylmercury complexes with selenoamino complexes, however, are thermodynamically more favorable (DeltaG of formation from suitable model reactants) than those of the corresponding amino acid complexes. This can be traced to the lower stability of the reactant selenoamino acids. Such different stability and favorability of formation might be responsible for the different physiological activity in biological systems such as the Hg-Se antagonism.

Synthesis, characterization and structures of methylmercury complexes with selenoamino acids.

Four new methylmercury-selenoamino acid complexes were synthesized, including methylmercury-L-selenoglutathionate, methylmercury-D,L-selenopenicillaminate, and two methylmercury-L-selenomethioninate complexes (one via a Hg-Se bonding and the other Hg-N bonding). All the complexes were characterized by NMR ((1)H, (13)C, (77)Se and (199)Hg), FT-IR and mass spectra. Their molecular structures were established by single crystal X-ray crystallography (for the Hg-N bonding methylmercury-L-selenomethioninate) and by quantum mechanical calculations using Gaussian-03 with the hybrid functional B3LYP/SDD. All four complexes were found to chemically and structurally resemble their sulfur analogues, with a slightly stronger binding affinity of Hg to Se than to S, suggesting chemical and structural mimicry might play a role in methylmercury-selenium antagonism in biological systems.

Properties of polythiophene and related conjugated polymers: a density-functional study.

Using a parameter-free, density-functional method that has been developed explicitly for the theoretical treatment of infinite, periodic, isolated, helical polymers we study various polymers related to polythiophene. In particular we discuss how the electronic properties of polythiophene are changed when replacing some of the H atoms by CH3 group, by incorporating vinylene bridges into the backbone, or when replacing some or all the CH units of the backbone by N atoms. We observe the weakest effects for the methyl-substitution and the strongest for the N-incorporation. The latter leads to an overall downward shift of all bands, but in contrast to the case for polyacetylene, the unrelaxed compound with N atoms does not have N lone-pair orbitals as the highest occupied ones. Instead these occur at somewhat deeper energies. When comparing the aromatic and quinoid forms we found for the pure compound as well as for the methyl-containing one that the gap closes when passing from the one to the other form which was not found for any of the other materials of the present study. Moreover, the energy of the HOMO was found to depend stronger on the bond-length alternation than the energy of the LUMO, ultimately giving that polarons will induce two asymmetrically placed gap states with the energetically lower one appearing deeper in the gap than the other one.