Low-energy electron capture by 6-Aza-2-thiothymine: investigations by electron attachment and electron transmission spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.     

Mechanistic study of partial oxidation of methane to syngas using in situ time-resolved FTIR and microprobe Raman spectroscopies

In situ time-resolved Fourier transform infrared (FTIR) and microprobe Raman spectroscopies were used to characterize the reaction mechanisms of the partial oxidation of methane to syngas over SiO(2)- and gamma-Al(2)O(3)-supported rhodium and ruthenium catalysts. The interaction of both pure methane and a methane/oxygen mixture at a stoichiometric feed ratio with an oxygen-rich catalyst surface led to the formation of CO2 and H(2)O as the primary products. For the H(2)-pretreated samples, the reaction mechanisms with the catalysts differ. Only Rh/SiO(2) is capable of catalyzing the direct oxidation of methane to syngas, while syngas formation over Rh/gamma-Al(2)O(3), Ru/SiO(2), and Ru/gamma-Al(2)O(3) can be achieved mainly via a combustion-reforming scheme. The significant difference in the mechanisms for partial oxidation of methane to syngas over the catalysts can be correlated to the differences in the concentration of oxygen species (O(2-)) on the catalyst surface during the reaction, mainly due to the difference in the nature of the metals and supports.     

Electron-acceptor properties of phospha-alkynes as probed by electron transmission and dissociative electron attachment spectroscopies

Electron transmission spectroscopy is applied for the first time to a phospha-alkyne derivative, (CH₃)₃CCP. Electron attachment to the π^* LUMO in the gas-phase occurs at 0.63 eV. Formation of the temporary π^* anion state is also observed in measurements of the total anion current, as a function of the incident electron energy. Comparison with the ethyne analogue shows that replacement of a CH group with a P atom causes a very strong increase (2.3 eV) of the vertical electron affinity. Simple Koopmans’ theorem calculations reproduce the experimental data.     

A study of Roman glass by reflectance and electron paramagnetic resonance spectroscopies

Reflectance and electron paramagnetic resonance (EPR) spectroscopies were used to study 25 fragments of Roman glass. Colour coordinates were used for an unbiased classification of the glasses in colour groups, which accounted for the presence of blue, blue-green, green, yellow-green, yellow and purple samples. Reflectance spectra were recorded in the 250–2500 nm wavelength range and showed absorption bands characteristic of Fe^II, Fe^III and Mn^III ions; furthermore, Co^II and Cu^II bands were observed in the spectra of the blue glasses. A decrease of the absorbance ratio of Fe^II to Fe^III ions was observed moving from blue-green to green and yellow-green glasses; however, yellow fragments still proved to be reduced glasses. EPR spectra displayed the characteristic patterns of Fe^III and Mn^II ions, with g-values in the 2–5 interval and spectral features depending on the relative content of the two elements. The characteristic pattern of the V^IV ion (g ≈ 2) and signals due to the formation of iron-sulphur complexes (g ≈ 6) appeared in the spectrum of a dark yellow glass, recorded at 77 K.     

An investigation of the chromium oxidation state of a monoanionic chromium tris(catecholate) complex by X-ray absorption and EPR spectroscopies

The well-known monoanionic Cr tris(3,5-di-tert-butylcatecholato) complex, [Cr(DTBC)3]-, has been studied by X-ray absorption spectroscopy. The multiple-scattering fit to the XAFS gave good correlation (R = 19.8%) and good values for all of the bond lengths, angles, and Debye-Waller factors. The principal bond lengths and angles around the metal center (Cr-O, 1.96 A; O-C, 1.28 A; O-Cr-O, 81.8 degrees; Cr-O-C, 113.3 degrees) were most consistent with the XRD structure for [Cr(X4C6O2)3]- (X = Cl, Br), compared to those in other oxidation states, [Cr(DTBC)3], [Cr(Cl4C6O2)3], and [Cr(O2C6H4)3]3-. The XANES spectrum shows the main K edge at 6003.3 eV and a preedge peak at 5992.9 eV, which is approximately 8% of the intensity of the main K edge. The XANES data were compared to those for Cr-ehba complexes (ehbaH2 = 2-ethyl-2-hydroxybutanoic acid) of known oxidation states (III, IV, and V) and show, in conjunction with EPR spectroscopy and a reevaluation of XRD structures and theoretical calulations, that the complex is best described as a Cr(V) center with delocalization from the catechol ligands. The [Cr(catecholato)3]n+ (n = 1, 0) complexes have similar EPR spectroscopic and structural properties, respectively, to the 1- complex and are also best described as Cr(V) complexes. Such intermediates are important in the redox reactions of catechol(amine)s, and oxidized amino acids (e.g., DOPA), with carcinogenic Cr(VI) and may have relevance in Cr-induced cancers.     

The adsorption and faradaic processes of formylferrocene thiosemicarbazone monitored by in situ SERS and UV-VIS spectroscopies

The adsorption and faradaic processes of formylferrocene thiosemicarbazone (TFF) on gold electrode in 0.1 mol L⁻¹ NaClO₄ acetonitrile solution were monitored by surface-enhanced Raman scattering (SERS) and ultraviolet-visible (UV-VIS) techniques. SERS data indicate that TFF adsorbs through the iminic N(2′) and S atoms on the gold electrode. The reduction product formed on the gold surface was aminomethylferrocene, whose experimental spectrum was supported by density functional theory calculations. In solution, thiourea was detected by the UV-VIS technique. Although there was an oxidation wave in the TFF cyclic voltammogram, no spectral changes were observed after the oxidation process. This work is dedicated to the memory of Prof. Francisco C. Nart.     

Surface-amplified ligand disorder in CdSe quantum dots determined by electron and coherent vibrational spectroscopies

This Article reports measurements of the intra- and intermolecular ordering of tight-binding octylphosphonate ligands on the surface of colloidal CdSe quantum dots (QDs) within solid state films, and the dependence of this order on the size of the QDs. The order of the organic ligands, as probed by vibrational sum frequency generation (SFG) spectroscopy, decreases as the radius of the QDs decreases; this decrease is correlated with a decrease in the order of underlying Cd(2+), as detected by X-ray photoelectron spectroscopy (XPS) line width measurements, for radii of the QDs, R > 2.4 nm, and is independent of the disorder of the Cd(2+) for R < 2.4 nm. We believe that, for R < 2.4, the decreasing order of the ligands with decreasing size is due to an increase in the curvature of the QD surfaces. Disorder in the Cd(2+) results from the presence of a shell of Cd(2+)-surfactant complexes that form during synthesis, so this work demonstrates the possibility for chemical control over molecular order within films of colloidal QDs by changing the surfactant mixture.     

In situ examination of uranium contaminated soil particles by micro-X-ray absorption and micro-fluorescence spectroscopies

Two complimentary spectroscopic techniques, X-ray absorption and fluorescence spectroscopy have been conducted at spatial scales of 1 to 25 μm on uranium contaminated soil sediments collected from two former nuclear materials processing facilities of the DOE: Fernald, OH and Savannah River Site, SC. A method of imbedding particles in a non-reactive Si polymer was developed such that individual particles could be examined before and after extraction with a wide range of chemicals typically used in sequential extraction techniques and others proposed forex situ chemical intervention technologies. Using both the micro-X-ray fluorescence (XRF) and micro-X-ray Absorption Near Edge Structure (XANES) techniques, both elemental and oxidation state distribution maps were generated on individual particles before and following chemical extraction. XANES can determine the relative proportion of U(VI) and U(IV) in phases comprising individual particles before and after extraction and showed that greater than 85% of the uranium existed as hexavalent U(VI). Fluorescence spectra of contaminated particles containing mainly U(VI) revealed populations of uranyl hydroxide phases and demonstrated the relative efficacy and specificity of each extraction method. Correlation of XAS and fluorescence data at micron scales provides information of U oxidation state as well as chemical form in heterogeneous samples.     

Stability of thin film glasses of toluene and ethylbenzene formed by vapor deposition: an in situ nanocalorimetric study

Vapor deposited thin films (~100 nm thickness) of toluene and ethylbenzene grown by physical vapor deposition show enhanced stability with respect to samples slowly cooled from the liquid at a rate of 5 K min(-1). The heat capacity is measured in situ immediately after growth from the vapor or after re-freezing from the supercooled liquid at various heating rates using quasi-adiabatic nanocalorimetry. Glasses obtained from the vapor have low enthalpies and large heat capacity overshoots that are shifted to high temperatures. The stability is maximized at growth temperatures in the vicinity of 0.8 T(g) for both molecules, although glasses of ethylbenzene show superior stabilization. Our data is consistent with previous results of larger organic molecules suggesting a generalized behavior on the stability of organic glasses grown from the vapor. In addition, we find that for the small molecules analyzed here, slowing the growth rate below 0.1 nm s(-1) does not result in increased thermodynamic stability.     

Uranyl coordination environment in hydrophobic ionic liquids: an in situ investigation

Different inner-sphere coordination environments are observed for the uranyl nitrate complexes formed with octyl-phenyl-N,N-diisobutylcarbamoylmethylphosphine oxide and tributyl phosphate in dodecane and in the hydrophobic ionic liquids (ILs) [C(4)mim][PF(6)] and [C(8)mim][N(SO(2)CF(3))(2)]. Qualitative differences in the coordination environment of the extracted uranyl species are implied by changes in peak intensity patterns and locations for uranyl UV-visible spectral bands when the solvent is changed. EXAFS data for uranyl complexes in dodecane solutions is consistent with hexagonal bipyramidal coordination and the existence of UO(2)(NO(3))(2)(CMPO)(2). In contrast, the complexes formed when uranyl is transferred from aqueous nitric acid solutions into the ILs exhibit an average equatorial coordination number of approximately 4.5. Liquid/liquid extraction results for uranyl in both ILs indicate a net stoichiometry of UO(2)(NO(3))(CMPO)(+). The concentration of the IL cation in the aqueous phase increases in proportion to the amount of UO(2)(NO(3))(CMPO)(+) in the IL phase, supporting a predominantly cation exchange mechanism for partitioning in the IL systems.     

Stability of an ultrathin plasma polymerized film in aqueous solution: in situ detection by surface plasmon resonance

Recently, surface plasmon resonance (SPR) has been successfully implemented to characterize the film stability in real time: a radio frequency (RF) plasma polymerized film was monitored by swelling or extracting behaviors in aqueous solution. The high-resolution SPR revealed that film stability strongly depended on the monomers, plasma mode, and substrate locations as well as plasma polymerization conditions: incident power and working pressure. By Fourier transform infrared (FTIR), the possible reasons of film stabilization affected by plasma conditions and sample locations are explained. It is recommended that as an adhesive layer for future applications the plasma polymerized polymer shall be prepared in low working pressure pulsed plasma.     

Characterization of chromodulin by X-ray absorption and electron paramagnetic resonance spectroscopies and magnetic susceptibility measurements

The biologically active form of the essential trace element chromium is believed to be the oligopeptide chromodulin. Chromodulin binds four chromic ions before binding at or near the active site of activating insulin receptor and subsequently potentiating the tyrosine kinase activity of the receptor. Charge balance arguments and preliminary spectroscopic studies suggested that the chromic centers might be part of a multinuclear assembly. Using a combination of X-ray absorption and electron paramagnetic resonance spectroscopies and variable-temperature magnetic susceptibility measurements, we found that holochromodulin is shown to possess an antiferromagnetically coupled trinuclear assembly which probably weakly interacts with a fourth chromium center. The chromium centers possess octahedron coordination comprised of oxygen-based ligation, presumably derived primarily from oligopeptide-supplied carboxylate groups. X-ray absorption data cannot be reproduced with the presence of sulfur atom(s), indicating that the cysteine thiolate group does not coordinate to the chromium centers. Thus, chromodulin possesses a unique type of multinuclear assembly, distinct from those known in other bioinorganic systems.     

In situ transmission electron microscopy observation of silver oxidation in ionized/atomic gas

The interaction between silver and ionized and atomic gas was observed directly by in situ transmission electron microscopy with an environmental cell for the first time. The electron beam provides dual functions as the source of both gas ionization and imaging. The concentration of ionized gas was tuned via adjusting the current density of the electron beam. Oxidation of the silver is observed in situ, indicating the presence of ionized and/or atomic oxygen. The evolution of microstructure and phase constituents was characterized. Then the oxidation rate was measured, and the relationships among grain size, mass transport rate, and electron flux were characterized. The role of the electron beam is discussed, and the results are rationalized with respect to ex situ results from the literature.     

Mechanism of heterogeneous oxidation of carbonyl sulfide on Al2O3: an in situ diffuse reflectance infrared fourier transform spectroscopy investigation

Heterogeneous reaction of carbonyl sulfide (OCS) on the surface of different types of alumina (Al(2)O(3)) at 298 K was investigated in a closed system and a flowed system using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The effects of calcination temperature of the Al(2)O(3) on its catalyzed reactivity were studied. The crystal structure and surface area of the Al(2)O(3) were characterized using X-ray diffraction (XRD) and the Brunauer-Emmett-Teller (BET) method. This paper revealed that adsorbed OCS could be catalytically oxidized on the surface of Al(2)O(3) to form gas-phase CO(2) and surface hydrogen carbonate (HCO(3)(-)) and sulfate (SO(4)(2-)) species at 298 K. The surface hydroxyl (OH) species on the Al(2)O(3) had been found to be the key reactant for the heterogeneous oxidation of OCS. Furthermore, the surface hydrogen thiocarbonate (HSCO(2)(-)) species, an intermediate formed in the reaction of OCS with OH, can be observed on the thermal-treated Al(2)O(3). On the basis of these results, the reaction mechanism of heterogeneous oxidation of OCS on Al(2)O(3) is discussed.     

Spectroelectrochemical investigation of direct electron transfer between resting horseradish peroxidase and its oxidation states promoted by DNA

Direct electron transfer between resting horseradish peroxidase (HRP) and its oxidation states was observed at a gold mesh electrode in a spectroelectrochemical cell in the presence of DNA. The conversion between HRP and the oxidized species induced electrochemically was found to be reversible and parallel to that initiated chemically. DNA played an important role as electron carrier and promoted the electron transfer between HRP and the electrodes. Voltammetric results and CD spectra indicated an interaction between HRP and DNA. Moreover, the secondary structure of HRP was slightly disturbed upon mixing with DNA. The direct spectroelectrochemistry of HRP at a gold mesh electrode presented new information on its bioelectrochemical characteristics.     

Spectroelectrochemical investigation of direct electron transfer between resting horseradish peroxidase and its oxidation states promoted by DNA

Direct electron transfer between resting horseradish peroxidase (HRP) and its oxidation states was observed at a gold mesh electrode in a spectroelectrochemical cell in the presence of DNA. The conversion between HRP and the oxidized species induced electrochemically was found to be reversible and parallel to that initiated chemically. DNA played an important role as electron carrier and promoted the electron transfer between HRP and the electrodes. Voltammetric results and CD spectra indicated an interaction between HRP and DNA. Moreover, the secondary structure of HRP was slightly disturbed upon mixing with DNA. The direct spectroelectrochemistry of HRP at a gold mesh electrode presented new information on its bioelectrochemical characteristics. Received: 19 October 1999 / Revised: 7 December 1999 / Accepted: 10 December 1999