Stacey J. Borg

Maia X-ray fluorescence imaging: Capturing detail in complex natural samples

Motivated by the challenge of capturing complex hierarchical chemical detail in natural material from a wide range of applications, the Maia detector array and integrated realtime processor have been developed to acquire X-ray fluorescence images using X-ray Fluorescence Microscopy (XFM). Maia has been deployed initially at the XFM beamline at the Australian Synchrotron and more recently, demonstrating improvements in energy resolution, at the P06 beamline at Petra III in Germany. Maia captures fine detail in element images beyond 100 M pixels. It combines a large solid-angle annular energy-dispersive 384 detector array, stage encoder and flux counter inputs and dedicated FPGA-based real-time event processor with embedded spectral deconvolution. This enables high definition imaging and enhanced trace element sensitivity to capture complex trace element textures and place them in a detailed spatial context. Maia hardware and software methods provide per pixel correction for dwell, beam flux variation, dead-time and pileup, as well as off-line parallel processing for enhanced throughput. Methods have been developed for real-time display of deconvoluted SXRF element images, depth mapping of rare particles and the acquisition of 3D datasets for fluorescence tomography and XANES imaging using a spectral deconvolution method that tracks beam energy variation.

A di-iron dithiolate possessing structural elements of the carbonyl/cyanide sub-site of the H-centre of Fe-only hydrogenase

The synthesis and characterisation of the first {2Fe2S}-cluster bearing both CO and CN ligands is described; the iron atoms are linked by the bridging 1,3-propanedithiolate unit that has been identified in the crystallographic structure of the {2Fe2S} sub-unit of the H-centre of the all-iron hydrogenase from Desulfovibrio desulfuricans.

Xocolatlite, Ca2Mn24+Te2O12·H2O, a new tellurate related to kuranakhite: Description and measurement of Te oxidation state by XANES spectroscopy

Abstract Xocolatlite, Ca2Mn4+2 Te6+2O12·H2O, is a rare new mineral from the Moctezuma deposit in Sonora, Mexico. It occurs as chocolate-brown crystalline crusts on a quartz matrix. Xocolatlite has a copperbrown streak, vitreous luster, and is transparent. Individual crystals show a micaceous habit. Refractive indices were found to be higher than 2.0. Density calculated from the empirical formula is 4.97 g/cm3, and immersion in Clerici solution indicated a density higher than 4.1 g/cm3. The mineral is named after the word used by the Aztecs for chocolate, in reference to its brown color and provenance. The crystallographic characteristics of this monoclinic mineral are space group P2, P2/m, or Pm, with the following unit-cell parameters refined from synchrotron X-ray powder diffraction data: a = 10.757(3) Å, b = 4.928(3) Å, c = 8.942(2) Å, β = 102.39(3)°, V = 463.0(3) Å3, and Z = 2. The unavailability of a suitable crystal prevented single-crystal X-ray studies. The strongest 10 lines of the X-ray powder diffraction pattern are [d in Å (I) (hkl)]: 3.267(100)(012), 2.52(71)(303̄), 4.361(51) (002), 1.762(39)(323̄), 4.924 (34)(010), 2.244(32)(313̄), 1.455(24)(006), 1.996(21)(014), 1.565(20) (611), and 2.353(18)(411̄). XANES Te LIII-edge spectra of a selection of Te minerals (including xocolatlite) and inorganic compounds showed that the position of the absorption edge can be reliably related to the oxidation state of Te. XANES demonstrated that xocolatlite contains Te6+ as a tellurate group. Water has been tentatively included in the formula based on IR spectroscopy that indicated the presence of a small amount of water. Raman, IR, XANES, and X-ray diffraction data together with the chemical composition show a similarity of xocolatlite to kuranakhite. A possible series may exist between these two species, xocolatlite being the Ca-rich end-member and kuranakhite the Pb-rich one.

Transient FTIR spectroelectrochemical and stopped-flow detection of a mixed valence {Fe(I)–Fe(II)} bridging carbonyl intermediate with structural elements and spectroscopic characteristics of the di-iron sub-site of all-iron hydrogenase

Iron(I) in biology?: one-electron oxidation of an (Fe(I)-Fe(I)) carbonyl cyanide precursor bearing a proximal thioether group leads to an (Fe(I)-Fe(II)) bridging carbonyl transient with spectral features similar to the di-iron sub-site of the CO inhibited paramagnetic centre of all-iron hydrogenase.

Electrocatalytic proton reduction by dithiolate-bridged diiron carbonyl complexes: a connection to the H-cluster?

Spectroscopic and electrochemical investigation of electrocatalytic proton reduction by Fe(2)(mu-pdt)(CO)(6), 1, have been interpreted in terms of a reaction scheme involving sequential electron-proton reactions to give a two-electron, two-proton product that undergoes rate-limiting dihydrogen elimination. Further reduction, at slightly higher negative potentials, gives a more reactive product and this process dominates reactions conducted at higher acid concentrations. Inhibition of the electrocatalytic reaction by CO is due to the more efficient loss of catalyst and this is best modelled by a reaction that is second order in terms of 1(-). During electrocatalytic proton reduction a new species is observed, which features a bridging CO group and the wavenumbers of the nu(CO) modes of the terminally bound carbonyl groups are similar to those of the carbonyl groups bound to the oxidized form of the H-cluster.

Spectroelectrochemical cell for the study of interactions between redox-activated species and moderate pressures of gaseous substrates

The design and operation of a simple reflection/absorption spectroelectrochemical cell capable of operation at gas pressures up to 1 MPa is described. An important characteristic of the cell design is the small volume of solution (50 μl) required for the collection of spectroelectrochemical and electrochemical results. A multi-electrode assembly provides the compact electrode geometry required for operation with small solution volumes. These developments greatly facilitate the study of highly air-sensitive samples particularly in cases where sample quantity is a limiting factor, as is often the case for samples of biological origin. These techniques permit the elucidation of the dependence of the product distribution on the redox state and gas pressure. The application of this approach is demonstrated by the reduction of [Ni(mnt)2]2− (mnt=cis-1,2-dicyanoethylene-1,2-dithiolate) under varying pressures of CO, where the changes in electrochemistry and IR and UV–vis spectra are consistent with the formation of mono- and di-carbonyl adducts which may be reoxidised to recover the starting material. Changes to the equilibria of the metal-containing species can be effected by a change in the CO partial pressure and this provides important insights into the composition of the species formed in the course of the reaction.

Integration of EXAFS, Spectroscopic, and DFT Techniques for Elucidation of the Structure of Reactive Diiron Compounds

Strategies for modelling the EXAFS of a range of compounds with structural features common to the diiron subsite of the [FeFe] hydrogenase H-centre are compared, and this has allowed identification of highly constrained models that still permit expression of the main structural characteristics of the compounds. Despite giving self-consistent values of the iron–scatterer distances, the EXAFS analysis fails to give unambiguous identification of the stereochemistry and composition of the compounds, and this necessitates the integration of results obtained using other spectroscopic and computational approaches. The combination of infrared spectroscopy, EXAFS, and ab initio DFT calculations are shown to provide a particularly potent approach for the study of metal carbonyl compounds of this class. In this case the EXAFS-derived iron–scatterer distances provide the basis of the starting point for DFT geometry optimization calculations, and the final distances together with the calculated infrared spectrum provides a means of validating the computed geometry. The approach is applied both to compounds of known structure and to the examination of the unstable products of chemical or electrochemical reduction.

On the structure of a proposed mixed-valent analogue of the diiron subsite of [FeFe]-hydrogenase

We show that a dinuclear assembly apparently providing the first example of a synthetic molecule exhibiting key features of the diiron subsite of [FeFe] hydrogenase, viz. CO-bridging of a coordinatively unsaturated, dithiolate-bridged mixed-valence diiron centre, is in fact a diamagnetic tetranuclear complex.

Chapter 1:Infrared Spectroelectrochemistry

A set of electrochemical measurements may, with the aid of simulations, provide skeletal details of the redox-coupled reactions of a system, although the extent of the detail or uniqueness of the description depends on the complexity of the system and the relative rates of reaction. Since such an ap...