Thermodynamic correlations and band gap calculations in metal oxides

Thermodynamic parameters and energy-band gap of a given metal oxide, are suitable measures of its stability. For this reason, direct correlations between energy-band gap or chemical hardness, and thermodynamic parameters (e.g. standard enthalpy of formation) have been constructed for different metal oxides. Furthermore, a simple relationship to calculate values of energy-band gaps, for metal oxides, from values of their enthalpies of formation, is presented here. When tested, an appreciable number of metal oxides from s- , p- and d-blocks well obeyed the relationship, while a number of metal oxides deviated. A qualitative theoretical account for such different behaviors is presented here.     

主族金属元素电子脱出功与拓扑指数~mP的相关性

利用Pauling电负性xpi、原子的价电子数mi和原子的价电子层数ni构建了点价iδ.由点价iδ构建了拓扑指数mP.利用其0阶指数0P与23种金属元素电子脱出功关联,拟合成3个线性回归方程.相关系数为0.9803,0.9870和0.9878,均优于文献值.预测取得了较好的结果.     

A general model of metal underpotential deposition in the presence of thiol-based additives based on an in situ STM study

Bis(3-sulfopropyl)disulfide (SPS) is a common additive in commercial copper electroplating baths. We have studied the influence of SPS on Cu underpotential deposition (UPD) on a Au(111) single crystal surface by means of cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). By combining our results with the results from the literature we propose a model that describes different stages of Cu UPD in the presence of SPS. Further analysis shows that our model is also applicable to a more general case of UPD of different metals, e.g. Cu and Ag, on a thiol-modified single-crystal surface, where the bond between the substrate and the thiol is adatom mediated. In addition, we have verified our model by in situ observation of the lifting of the Herringbone reconstruction on the Au(111) surface by Cu UPD.     

Scaling the Coulomb interaction in calculations of electron spectra of transition metal complexes

A simple technique of scaling two-electron integrals in ab initio calculations of the electronically excited states of transition metal complexes is proposed. This technique uses the fact that one-center two-electron integrals depend linearly on the scaling factor when Slater type functions are subjected to scaling transformation. This leads to a linear dependence of the d—d transition energy on the “scale” of Coulomb interaction, which allows one to affect the calculation result by varying the Slater exponential. To test the technique, ab initio configuration interaction and full active space calculations of the low excited states of the CrF ₆ ^3- , MnF ₆ ^2- , and VF ₆ ^3- complexes are performed. For transition elements, a basis of Slater type effective functions chosen from the optical spectra of the atoms and ions of transition elements is used. It is shown that in the STO-6G basis with effective exponentials, experimental transitions are reproduced with an accuracy of about 2000 cm^-1 even with the use of small active space determined by the orbitals localized on the central atom of the complex.     

Electromagnetic model and calculations of the surface-enhanced Raman-shifted emission from Langmuir-Blodgett films on metal nanostructures

We present a theoretical study of the electromagnetic contribution to surface-enhanced Raman scattering (SERS) from a Langmuir-Blodgett film close to a metal surface. This macroscopic dipolar model fully accounts for the Raman-shifted emission so that meaningful SERS (electromagnetic) enhancement factors that do not depend only on the local electromagnetic field enhancement at the pump frequency are defined. For a plane metal surface, analytical SERS enhancement factors that are consistent for all pump beam polarization and molecular orientation are obtained. In order to investigate SERS on complex nanostructured metal surfaces, we introduce this model into the formally exact, Green's theorem surface integral equation formulation of the scattered electromagnetic field. This formulation is thus employed to calculate numerically the near-field and far-field emissions at the Raman-shifted frequency for very rough, random nanostructured surfaces, with emphasis on the impact of collective processes for varying pump frequency and Raman shift. Our results reveal that the widely used |E|4 approximation tends to overestimate average SERS enhancement factors.     

Temperature dependence of the electronic factor in the nonadiabatic electron transfer at metal and semiconductor electrodes

The temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant (k_ET) at metal electrodes is discussed. It is found in these calculations that this contribution is proportional to the absolute temperature T. A simple interpretation is given. We also consider the nonadiabatic rate constant for electron transfer at a semiconductor electrode. Under conditions for the maximum rate constant, the electronic contribution is also estimated to be proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor versus Fermi–Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal).     

A mass-spectrometric determination of the work function of alkali metal halide crystals

Thermochemical cycles including experimentally measured enthalpies of sublimation of alkali metal halides in the form of M₂X⁺ and MX₂^∮ ionic clusters and MX molecules were used to calculate work functions for NaCl, NaBr, NaI, KCl, KBr, KI, RbCl, and RbI.     

The influence of metal adatoms deposited at underpotential on the kinetics of an outer-sphere redox reaction

PbO₂ anodes were used to oxidize cyclohexanone oxime to nitroso compounds in 1 M H₂SO₄ or 1 M HClO₄ in a two electron reaction:

Nu^? are nucleophiles like OH^?, NO^?₂ and Cl^?. The electrode material reacts chemically with the oxime and it is oxidized electrochemically at potentials very close to the Pb^{$\text{2+}\text{4+}$} redox potential. The chemical step has been studied separately by corrosion experiments in aqueous HClO₄. An upper limit of current density is observed, independent of convection, due to the formation of a film composed of Pb(II) products on the electrode. A reaction limitation was not attained under our conditions.Contrary to this, the PbO₂ electrode behaves inertly in the case of lower aliphatic alcohols (C₂C₄) in 1 M H₂SO₄. Potentials are about 0.5 V more positive. The inert anode generates the radical cation in the rate determining step:

Corrosion experiments disclose a rate, which is lower by four orders of magnitude (j_corr=2–3 μA cm^?2) with reference to the oxime experiment and only slightly above the corrosion rate in pure 1 M HClO₄.     

Thermodynamic implication of the dependence of mesomorphic transition entropy on chain-length

Meaning of the alkyl chain-length dependence of transition entropy in liquid crystal is discussed and clarified in a long-chain region. A method to deduce contributions of the alkyl chain and the molecular core is proposed. Application of the method suggests that, in most smectic A (SmA) - nematic (N) phase transitions, not only a molecular core (modeled by a hard-rod) but also a (terminal) alkyl chain is disordered upon the SmA→N transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

The additivity of polarization function and electron correlation effects in ab initio molecular-orbital calculations

Several examples are preseted to show that estimated third-order Møller-Plesset (MP3) relative energies obtained from schemes which assume additivity of correlation and polarization function effects are likely to provide the most reliable energy comparisons in cases where full MP3 calculations with polarization basis sets are not feasible.     

Thermodynamic calculations of the interaction of scandium halides with aluminum

The fundamental characteristics of the reduction of scandium trifluoride and trichloride with aluminum were studied by thermodynamic modeling over wide temperature and pressure ranges (1100–1400 K and 1–10⁶ Pa for ScF₃ and 800–1200 K and 1–10⁵ Pa for ScCl₃). Calculations of the equilibrium compositions of the systems were performed to draw conclusions about the contents of condensed and gaseous components and determine the temperatures of the complete reduction of trihalides with the formation of a two-phase alloy containing a saturated solution of scandium in aluminum and the Al₃Sc intermetallic compound. The results of modeling were in agreement with the available experimental data.     

An empirical potential function for metal centers: Application to molecular mechanics calculations on metalloproteins

At present, most molecular mechanics programs that model metalloproteins do not allow for specific geometric requirements at metal centers. An analysis of small-molecule crystal structures containing four-, five-, and six-coordinated zinc, retrieved from the Cambridge Structural Database (CSD), leads to a new metal-center potential function for use in molecular mechanisc programs. This potential function includes as variables the metal-ligand separations and the angles subtended at the metal and allows specifically for distortions from frequently occurring types of coordination geometries (e.g. tetrahedron square pyramid, trigonal bipyramid, and octahedron). The combination of such a metal-center potential function and one for hydrogen bonds allowing for Ione-pair directionality makes monopole electrostatic contributions to the force-field energy superfluous, thus circumventing many problems associated with the assignment to the force-field energy superfluous, thus circumventing many problems associated with the assignment of atomic apartial charges and a dielectric constant. The molecular mechanics program ‘YETI’, containing both types of potential functions, has been used to refine details of substrate binding of 16 complexes of human carbonic anhydrase II with small molecules. The stereochemistry of the refined complexes is in good agreement with data retrived from the CSD and hence allows realistic structure activity relationships.     

Correlation between electron localization and metal ion mutagenicity in DNA synthesis from QM/MM calculations

DNA polymerases require two divalent metal ions in the active site for catalysis. Mg(2+) has been confirmed to be the most probable cation utilized by most polymerases in vivo. Other metal ions are either potent mutagens or inhibitors. We used structural and topological analyses based on ab initio QM/MM calculations to study human DNA polymerase λ (Polλ) with different metals in the active site. Our results indicate a slightly longer O3'-Pα distance (～3.6 ?) for most inhibitor cations compared to the natural and mutagenic metals (～3.3-3.4 ?). Optimization with a larger basis set for the previously reported transition state (TS) structures (Cisneros et al., DNA Repair, 2008, 7, 1824.) gives barriers of 17.4 kcal mol(-1) and 15.1 kcal mol(-1) for the Mg(2+) and Mn(2+) catalyzed reactions respectively. Relying on the key relation between the topological signature of a metal cation and its selectivity within biological systems (de Courcy et al., J. Chem. Theor. Comput., 2010, 6, 1048.) we have performed electron localization function (ELF) topological analyses. These analyses show that all inhibitor and mutagenic metals considered, except Na(+), present a "split" of the outer-shell density of the metal. This "splitting" is not observed for the non-mutagenic Mg(2+) metal. Population and multipole analyses on the ELF basins reveal that the electronic dipolar and quadrupolar polarization is significantly different with Mg(2+) compared to all other cations. Our results shed light at the atomic level on the subtle differences between Mg(2+), mutagenic, and inhibitor metals in DNA polymerases. These results provide a correlation between the electronic distribution of the cations in the active site and the possible consequences on DNA synthesis.     

Adsorption of carbon monoxide and oxygen on transition metal surfaces: A comparative study of the results from electron spectroscopy and theoretical calculations

Results of investigations on the adsorption of CO andO ₂ on transition metal surfaces by employinguv and x-ray photoelectron spectroscopy and electron energy loss spectroscopy (eels) are presented. Results of molecular orbital calculations on adsorbed CO and O₂ are also discussed. Some of the interesting aspects discussed are, satellites in the O(ls) region due to adsorbed CO, vibrationaleels of adsorbed O₂ and dissociation energy profiles of adsorbed O₂ on clean surfaces as well as surfaces covered with potassium or presorbed atomic oxygen. Contribution No 245 from the Solid State and Structural Chemistry Unit.     

Characterization of tungsten surfaces by simultaneous work function and secondary electron emission measurements.

The changes of the work function (Phi) and the secondary electron emission (SEE) of oxygen covered polycrystalline tungsten occurring after ion sputtering and heat treatments have been investigated. The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS), and the electron emission properties by work function spectroscopy (WFS). We observed in what manner the chemical changes of the surface are reflected in the work function and SEE. The simultaneous change of Phi and SEE in the case of oxygen covered tungsten have been pointed out and a direct relationship between them can be supposed.