Hazel Cox

Unexpected stability of [Cu⋅Ar]2+, [Ag⋅Ar]2+, [Au⋅Ar]2+, and their larger clusters

Experimental observations following the ionization of neutral group 11 metal/argon complexes have revealed the presence of doubly charged ions of the form [M . Ar-n](2+) for n in the range 1-6. Of particular interest are two features of the results. First, the unexpected stability of the dimer ions, [M . Ar](2+), since similar species involving a molecule rather than a rare gas atom are often unstable with respect to charge transfer. Ab initio calculations show the dimers owe their stability to a combination of a strong electrostatic interaction and the high ionization energy of argon. A second feature to the results is the high relative intensities of the [M . Ar-4](2+) and [M . Ar-6](2+) ions. Calculations show these complexes to consist of square-planar D-4h structures, with the additional two atoms in [M . Ar-6](2+) occupying axial sites, which are Jahn-Teller distorted. The calculated relative binding energies support the preferential stability of these two structures

Activation of Carbon Dioxide by Divalent Tin Alkoxides Complexes.

A series of terminal tin(II) alkoxides have been synthesized utilizing the bulky β-diketiminate ligand [{N(2,6-(i)Pr(2)C(6)H(3))-C(Me)}(2)CH] (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were observed. For instance, (BDI)SnOR only reacts with highly activated aliphatic electrophiles such as methyl triflate, but reacts reversibly with carbon dioxide. Both the rate of reaction and the degree of reversibility is dependent upon minor changes in the alkoxide ligand, with the bulkier tert-butoxide ligand displaying slower reactivity than the corresponding isopropyl ligand, although the latter system is a more exergonic reaction. Density Function Theory (DFT) calculations show that the differences in the reversibility of carbon dioxide insertion can be attributed to the ground-state energy differences of tin alkoxides while the rate of reaction is attributed to relative bond strengths of the Sn-O bonds. The mechanism of carbon dioxide insertion is discussed.

Time-dependent density functional study of the electronic potential energy curves and excitation spectrum of the oxygen molecule

Orbital energies, ionization potentials, molecular constants, potential energy curves, and the excitation spectrum of O(2) are calculated using time-dependent density functional theory (TDDFT) with Tamm-Dancoff approximation (TDA). The calculated negative highest occupied molecular orbital energy (-epsilon(HOMO)) is compared with the energy difference ionization potential for five exchange correlation functionals consisting of the local density approximation (LDAxc), gradient corrected Becke exchange plus Perdew correlation (B(88X)+P(86C)), gradient regulated asymptotic correction (GRAC), statistical average of orbital potentials (SAOP), and van Leeuwen and Baerends asymptotically correct potential (LB94). The potential energy curves calculated using TDDFT with the TDA at internuclear distances from 1.0 to 1.8 A are divided into three groups according to the electron configurations. The 1pi(u) (4)1pi(g) (2) electron configuration gives rise to the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) states; the 1pi(u) (3)1pi(g) (3) electron configuration gives rise to the c (1)Sigma(u) (-), C (3)Delta(u), and A (3)Sigma(u) (+) states; and the B (3)Sigma(u) (-), A (1)Delta(u), and f (1)Sigma(u) (+) states are determined by the mixing of two or more electron configurations. The excitation spectrum of the oxygen molecule, calculated with the aforementioned exchange correlation functionals, shows that the results are quite sensitive to the choice of functional. The LDAxc and the B(88X)+P(86C) functionals produce similar spectroscopic patterns with a single strongly absorbing band positioned at 19.82 and 19.72 eV, respectively, while the asymptotically corrected exchange correlation functionals of the SAOP and the LB94 varieties yield similar excitation spectra where the computed strongly absorbing band is located at 16.09 and 16.42 eV, respectively. However, all of the exchange correlation functionals yield only one strongly absorbing band (oscillator strength greater than 0.1) in the energy interval of 0-20 eV, which is assigned to a X (3)Sigma(g) (-) to (3)Sigma(u) (-) transition. Furthermore, the oxygen molecule has a rich spectrum in the energy range of 14-20 eV and no spin allowed absorption bands are predicted to be observed in the range of 0-6 eV.

Modelling of surface relaxation and melting of aluminium

An empirical potential energy function, comprising two-and three-body terms, has been derived for aluminium. This potential reproduces the experimental energies and relaxations of the (111), (110) and (100) surfaces of fcc Al to a high degree of accuracy. The melting of bulk fcc Al and its low index surfaces has been studied in detail by employing Monte Carlo simulation techniques. Melting has been defined in terms of a number of calculated quantities: order parameters, density profiles and radial distribution functions. The many-body potential overestimates the bulk melting temperature (Tm ? 1275±25 K) by approximately 37% but reproduces the sharp melting transition observed experimentally. To obtain a melting point in agreement with experiment we would need to lower the energy scaling parameter (determined from room temperature data) by 37%. The potential also indicates that the relatively open (110) surface melts about 200 K below the bulk, while the denser (100) surface does not melt until Tm. These findings are again in good agreement with experiment and with previous calculations.

Ligand field spectroscopy of Cu(II) and Ag(II) complexes in the gas phase: theory and experiment

Ligand field spectra have been recorded in the gas phase for the two series of complexes containing either Cu(II) or Ag(II) in association with pyridine. Where comparisons are possible, the gas phase spectra match those recorded in the condensed phase; however, for Ag(II) systems the results differ in interpretation. The Ag(II) data are attributed to a ligand-to-metal charge transfer process, and the Cu(II) data (spectral region and extinction coefficient) match the characteristics of a d-d transition. A detailed theoretical analysis of two complexes. [Cu(py)4]2+ and [Ag(py)4]2+ provides evidence of a minimum energy, D4h structure and two less stable D2h and D2d structures within approximately 60 kJ mol(-1). From these structures it is possible to identify a range of optically and vibronically allowed transitions that could contribute to spectra observed in the gas phase. In the case of calculations on [Ag(py)4]2+ there is strong evidence of an electronic transition that would account for the observation of charge transfer in the experiments. Less detailed calculations on [Cu(py)6]2+ and [Ag(py)6]2+ show structural evidence of extensive Jahn Teller distortion. Taken together with incremental binding energies calculated for complexes containing between two and six pyridine molecules, these results show that the level of theory adopted is capable of providing a semi-quantitative understanding of the experimental data.

Potential-energy functions for platinum and palladium solids and their application to surfaces

An empirical potential energy function, comprising two- and three-body terms, has been derived for platinum and palladium, by fitting parameters to the phonon dispersion curves, elastic constants, lattice energy and lattice distance of the face-centred-cubic (fcc) solid, and the vacancy formation energy. These potentials reproduce the fcc strutural data and the experimetal energies and relaxation of the (111), (110) and (100) surfaces of fee Pt and Pd, to a high degree of accuracy, and correctly predicts the relaxation, pairing and buckling of the 1 × 2 reconstruction of the (110) Pt surface.

Recent advances in the visible and UV spectroscopy of metal dication complexes

Experimental techniques developed during the past 15 years have demonstrated that it is possible to prepare stable metal dication complexes, such as [Cu(NH3) N ]2+ and [Ni(H2O) N ]2+, in the gas phase. The significance of these complexes lies in the fact that they contain metal ions that are in their more common charge state, and therefore, a link between their properties and those found for the same ions in the condensed phase is readily accessible. In this review, we focus on one aspect of the study of these ion complexes, and that is their visible and ultraviolet (UV) spectroscopy. Current experimental techniques for recording spectra in the gas phase are discussed together with the theoretical methods required to interpret the spectra of metal dication complexes. An attempt is made to identify any barriers that might exist to measuring the optical spectra of metal dication complexes using current ion beam technology, where a typical experiment will involve measuring either photofragment yield or ion beam depletion as a function of laser wavelength. One very obvious area of spectroscopy to be explored, and one that is unique to transition metal complexes, is ligand field spectroscopy. Estimates of photofragment yields based on typical absorption cross sections and the kinetics of fragmentation highlight the difficulties involved in measuring such spectra. Of the theoretical techniques currently available for calculating spectral transitions, the method most commonly used for metal complexes is time-dependent density functional theory (TDDFT). Using selected examples, it is shown that although TDDFT is, for the most part satisfactory, extreme caution should be exercised when investigating the electronic states of open-shell complexes. An obvious conclusion to emerge is that a theoretical method that predicts the correct ground state geometry of an open-shell complex (and is free from spin contamination) does not necessarily yield the correct electronically excited states due to multi-electron character and/or spin contamination in the excited state manifold. It is anticipated that the development of experimental techniques that can record accurate electronic spectra will provide new and more demanding benchmarks for the refinement of theoretical methods.

Ligand field photofragmentation spectroscopy of [Ag(L)N]2+ complexes in the gas phase: Experiment and theory

Experiments have been undertaken to record photofragmentation spectra from a series of [Ag(L)N]2+ complexes in the gas phase. Spectra have been obtained for silver(II) complexed with the ligands (L): acetone, 2-pentanone, methyl-vinyl ketone, pyridine, and 4-methyl pyridine (4-picoline) with N in the range of 4-7. A second series of experiments using 1,1,1,3-fluoroacetone, acetonitrile, and CO2 as ligands failed to show any evidence of photofragmentation. Interpretation of the experimental data has come from time-dependent density functional theory (TDDFT), which very successfully accounts for trends in the spectra in terms of subtle differences in the properties of the ligands. Taking a sample of three ligands, acetone, pyridine, and acetonitrile, the calculations show all the spectral transitions to involve ligand-to-metal charge transfer, and that wavelength differences (or lack of spectra) arise from small changes in the energies of the molecular orbitals concerned. The calculations account for an absence in the spectra of any effects due to Jahn-Teller distortion, and they also reveal structural differences between complexes where the coordinating atom is either oxygen or nitrogen that have implications for the stability of silver(II) compounds. Where possible, comparisons have also been made with the physical properties of condensed phase silver(II) complexes.

MODELLING CU, AG AND AU SURFACES USING EMPIRICAL POTENTIALS

Empirical potential energy functions, comprising two- and three-body terms, have been derived for copper, silver and gold. These potentials reproduce the experimental energies and relaxations of the (111), (110) and (100) surfaces of the fcc solid, to a high degree of accuracy and correctly predict the 1 × 2 reconstruction of the (110) surface of gold.

A Gas-Phase Study of the Preferential Solvation of Mn2+ in Mixed Water/Methanol Clusters

The kinetic shift that exists between two competing unimolecular fragmentation processes has been used to establish whether or not gas-phase Mn2+ exhibits preferential solvation when forming mixed clusters with water and methanol. Supported by molecular orbital calculations, these first results for a metal dication demonstrate that Mn2+ prefers to be solvated by methanol in the primary solvation shell.