XAFS, IR, and UV−vis (diffuse reflectance and photoluminescence) spectroscopies have been employed to investigate the local environment of CuI in CuI-ZSM-5 prepared by gas phase reaction of H-ZSM-5 (SiO2/Al2O3 = 90−50) with CuCl The measurements confirm that the absolutely predominant copper species formed with this procedure are CuI:Cu species in other oxidation states are less than 1% Direct proof of the absence of both unreacted CuCl microaggregates and of copper clusters in the zeolite channels is also given CuI-ZSM-5 prepared in this way represents a model solid for investigating the structure of isolated CuI sites in zeolites EXAFS analysis reveal that, in vacuum conditions, Cu+ cations are surrounded by 25 ± 03 oxygens at 200 ± 002 A The combined use of EXAFS, IR, and photoluminescence UV−vis spectroscopies and computer graphic simulations allows to infer that Cu+ ions in CuI-ZSM-5 are located in two families of sites, indicated as I and II, (site II having the highest coordinative unsatur

XAFS, IR, and UV−Vis Study of the CuI Environment in CuI-ZSM-5

A detailed account of the dependence of X-ray absorption spectra (XAS) on polarisation and light beam directions is given. Anisotropic XAS measured on single crystals, layered compounds, polymers and oriented powders are tabulated. The X-ray absorption cross section by non-magnetic samples is derived, including electric dipole and quadrupole contributions. Orders of magnitude are found for magnetic dipole and electric dipole-octupole terms. Using a spherical tensor expansion of the absorption cross section, simple analytical formulae are given for the angular dependence of the electric dipole and quadrupole contributions to XAS for all crystal point groups. These formulae are valid down to the edge and include many-body effects. The are applied to two experimental examples, the iron K-edge in haematite and the copper K-edge in CuCl42-. For the latter case, spherical tensor components are interpreted in terms of molecular orbitals. The influence of unpolarised and circularly polarised X-rays is discussed, as well as various experimental problems met in angle-resolved XAS. In an appendix, the quadrupole contribution to angular XAS is derived within the multiple-scattering formalism, along with closed expressions for the dipole and quadrupole spherical tensor components.

https://iopscience.iop.org/article/10.1088/0953-8984/2/3/018/pdf

Angular dependence of X-ray absorption spectra

The determination of the structure of transient molecules, such as photoexcited states, in disordered media (such as in solution) usually requires methods with high temporal resolution. The transient molecular structure of a reaction intermediate produced by photoexcitation of NiTPP-L2 (NiTPP, nickeltetraphenylporphyrin; L, piperidine) in solution was determined by x-ray absorption fine structure (XAFS) data obtained on a 14-nanosecond time scale from a third-generation synchrotron source. The XAFS measurements confirm that photoexcitation leads to the rapid removal of both axial ligands to produce a transient square-planar intermediate, NiTPP, with a lifetime of 28 nanoseconds. The transient structure of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same structure as the ground state in a noncoordinating solvent before it recombines with two ligands to form the more stable octahedrally coordinated NiTPP-L2.

https://science.sciencemag.org/content/sci/292/5515/262.full.pdf

Capturing a photoexcited molecular structure through time-domain x-ray absorption fine structure.

Strategies for O2 activation by copper enzymes were recently expanded to include mononuclear Cu sites, with the discovery of the copper-dependent polysaccharide monooxygenases, also classified as auxiliary-activity enzymes 9–11 (AA9-11). These enzymes are finding considerable use in industrial biofuel production. Crystal structures of polysaccharide monooxygenases have emerged, but experimental studies are yet to determine the solution structure of the Cu site and how this relates to reactivity. From X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies, we observed a change from four-coordinate Cu(II) to three-coordinate Cu(I) of the active site in solution, where three protein-derived nitrogen ligands coordinate the Cu in both redox states, and a labile hydroxide ligand is lost upon reduction. The spectroscopic data allowed for density functional theory calculations of an enzyme active site model, where the optimized Cu(I) and (II) structures were consistent with the experimental data. The O2 reactivity of the Cu(I) site was probed by EPR and stopped-flow absorption spectroscopies, and a rapid one-electron reduction of O2 and regeneration of the resting Cu(II) enzyme were observed. This reactivity was evaluated computationally, and by calibration to Cu-superoxide model complexes, formation of an end-on Cu-AA9-superoxide species was found to be thermodynamically favored. We discuss how this thermodynamically difficult one-electron reduction of O2 is enabled by the unique protein structure where two nitrogen ligands from His1 dictate formation of a T-shaped Cu(I) site, which provides an open coordination position for strong O2 binding with very little reorganization energy.

https://www.pnas.org/content/pnas/111/24/8797.full.pdf

Spectroscopic and computational insight into the activation of O2 by the mononuclear Cu center in polysaccharide monooxygenases

Molecular scale characteristics of Cu(II) bonding in goethite–humate complexes

Polarized x-ray absorption edge spectroscopy of single-crystal copper(II) complexes

The sulfur and chlorine centers in sensitizing dyes and surface-modifying agents of importance to the photographic system were characterized by S and Cl K-edge X-ray absorption spectroscopy (XAS). Dramatic changes in the sulfur-ligand spectral features are produced from complexation with Ag and Au atoms, which permits the distinction of covalent vs ionic interactions with the metals encountered in photographic systems. Covalent metal interaction with an exocyclic S-containing ligand resulted in the disappearance of the low-energy [pi][sup *] pre-edge feature, while the intensity of the predominant [sigma][sup *] feature in all compounds investigated was greatly enhanced upon complexation with Au and Ag metals. In contrast to the uncomplexed ligands, this predominant feature was found to be most intense for polarizations along the S-metal direction and perpendicular to the C-S bond, indicating a direct bonding interaction with the Au metal, as seen, for example, in a bis(ethylenethiourea) Au(I) complex. The results of these studies form the basis for the interpretation of polarized surface measurements at a glancing angle configuration of a dyed AgBr sheet crystal, which suggest that there is no covalent interaction between the terminal sulfur centers in the merocyanine dye molecules and the silver ions on the AgBr substrate surface.more » 26 refs., 11 figs., 4 tabs.« less

Sulfur and Chlorine K-Edge X-ray Absorption Spectroscopic Studies of Photographic Materials

The X-ray edge and near-edge region, typically 0–50 eV above the threshold energy, is often rich in structure which reflects the local environment of the absorbing atom. Edge and near-edge features are poorly resolved in the K-edge spectra of most amorphous samples. Polarized studies result in the enhancement of the edge and near-edge feature intensities, and the symmetry properties of the final states responsible for these features may be determined from their angular properties. We have utilized the high flux and plane-polarized nature of synchrotron radiation to study the polarized K-edge spectra of oriented single crystals of a number of square-planar or flattened tetrahedral (Td) Cu(II) and linear Cu(I) model compounds. The intensity, shape and location of observed polarized edge features are examined as probes of the electronic and geometric environment of the metal center.

Polarized Cu K-Edge Studies

X-ray absorption edge spectra constitute a very tantalizing set of data since they are usually the easiest part of the x-ray absorption spectrum to measure in a complex chemical system. In this talk we focus on the measurement of absorption spectra for heavy atoms (principally copper) in low-symmetry environments. Our motivation is to understand how metallo-proteins “do their thing.” Before getting to specific model systems, we discuss some of the theoretical problems in general.

Teresa A. Smith

Papers

Polarized x-ray absorption edge spectroscopy of single-crystal copper(II) complexes

Sulfur and Chlorine K-Edge X-ray Absorption Spectroscopic Studies of Photographic Materials

Polarized Cu K-Edge Studies

Theory of X-Ray Absorption Edge Spectra