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Combined theoretical and experimental research was conducted to study the effect of negatively charged phosphate groups on the reaction of water with oxidized DNA by their selective replacement with neutral methylphosphonate groups. It was found that the reaction had two critical enabling features, activation of a guanine radical cation by association with a neighboring [Na.sup.} counterion, and stabilization of products by interaction of a hydronium ion with a nearby phosphate group.

The measurement of temperature- and reaction-time-dependent ion intensities, obtained by ion trap mass spectrometry, has shown the promotion and activation of molecular oxygen by preadsorbed hydrogen. The results have provided insights into hydrogen and oxygen cooperative adsorption effects and consequent reaction mechanisms.

An investigation of the binding of Trioxatriangulenium ion (TOTA(super +)) to DNA oligomers in solution and the photophysical and photochemical reactions of bound TOTA(super +) is reported. Irradiation of bound TOTA(super +) results in damage at GG steps that is revealed as strand cleavage following treatment with piperidine.

The discovery that electron holes (radical cations) migrate long distances in DNA has fostered enormous interest in the mechanism of this processes, due in large part to its apparent similarity with oxidative damage in cells. Reaction of H subscript 2 O with a guanine radical cation in an AGGA sequence having two enabling features is showed but results reported revealed a third important feature - accessibility of reactants (H subscript O) to the reaction site, which is sequence dependent and governed by steric effects.

First-principles simulations are used to predict low-barrier CO oxidation reactions to occur through a Langmuir-Hinshelwood or an Eley-Rideal mechanism on two-dimensional (2D) gold nanocluster islands adsorbed on very thin MgO films that are supported on Mo(100). The excess electronic charge at the gold cluster/magnesia interface has resulted from penetration of metal states through the Mo thin film.

Research into the interactions of oxygen and carbon monoxide with the gold dimer anion is presented. It was established that the oxygen should bind molecularly to Au2-.

The article focuses on the joint experimental studies and theoretical investigations using first-principles simulations on size-selected small gold clusters, Gold (Au), adsorbed on a thin defect-free magnesium oxide film supported on a metal-oxide(100) surface under ultrahigh-vacuum conditions. In addition, the simulations were used to predict low-barrier carbon monoxide oxidation reactions to occur thru a Langmuir-Hinshelwood or an Eley-Rideal mechanism.

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au20 was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O2 molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O2 molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au20 nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects. [ABSTRACT FROM AUTHOR]

Temperature-dependent rf-ion trap mass spectrometry and first-principles simulations reveal the detailed reaction mechanism of the catalytic gas-phase oxidation of CO by free Au[sub 2, sup -] ions in the presence of O[sub 2]. A metastable intermediate with a mass of Au[sub 2]CO[sub 3, sup -] was observed at low temperatures. Two alternative structures corresponding to digold carbonate or peroxyformate are predicted for this intermediate. Both structures are characterized by low activation barriers for the formation of CO[sub 2]. These combined experimental and theoretical investigations provide a comprehensive understanding of the kinetics, energetics, and atomic arrangements along the reaction path, thus allowing a formulation of the catalytic cycle for the oxidation reaction. [ABSTRACT FROM AUTHOR]

Understanding the evolution of the structure and properties in metals from molecule-like to bulk-like has been a long sought fundamental question in science, since Faraday's 1857 work. We report the discovery of a Janus nanomolecule, Au 191 (SPh- t Bu) 66 having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au 191 has an anisotropic, singly twinned structure with an Au 155 core protected by a ligand shell made of 24 monomeric [-S-Au-S-] and 6 dimeric [-S-Au-S-Au-S-] staples. The Au 155 core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au 3 @Au 23 @Au 63 ]@Au 66 concentric shells of atoms. The inner core has a monotwinned/stacking-faulted face-centered-cubic (fcc) structure. Structural evolution in metal nanoparticles has been known to progress from multiply twinned, icosahedral, structures in smaller molecular sizes to untwinned bulk-like fcc monocrystalline nanostructures in larger nanoparticles. The monotwinned inner core structure of the ligand capped Au 191 nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multiple-twinning regime and the bulk-metal-like particles with untwinned fcc structure. The Janus nature of the nanoparticle is demonstrated by its optical and electronic properties, with metal-like electron-phonon relaxation and molecule-like long-lived excited states. First-principles theoretical explorations of the electronic structure uncovered electronic stabilization through the opening of a shell-closing gap at the top of the occupied manifold of the delocalized electronic superatom spectrum of the inner core. The electronic stabilization together with the inner core geometric stability and the optimally stapled ligand-capping anchor and secure the stability of the entire nanomolecule.

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.