Revisiting the Dependence of Cu K-Edge X-Ray Absorption Spectra on Oxidation State and Coordination Environment

Abstract

X-ray absorption spectroscopy (XAS) at the Cu K-edge is an important tool for probing the properties of copper centers in transition metal chemistry and catalysis. However, the interpretation of experimental XAS spectra requires a detailed understanding of the dependance of spectroscopic features on the local geometric and electronic structure, which can be established by theoretical X-ray spectroscopy. Here, we present a systematic computational study of the Cu K-edge XAS spectra of selected Cu complexes based on time-dependent density-functional theory in combination with a molecular orbital analysis of the relevant transitions. For a series of Cu ammine model complexes as well as a comprehensive test set of 12 Cu(I) and 5 Cu(II) complexes, we revisit the dependance of the pre-edge region in Cu K-edge XAS spectra on oxidation state and coordination geometry. While our calculations confirm earlier experimental assignments, we can also reveal additional signatures of the ligand orbitals and identify the underlying orbital interactions. The comprehensive picture revealed by this study will provide a reliable basis for the interpretation of <i>in situ</i> Cu K-edge XAS spectra of catalytic intermediates.

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

“…The number, amplitude, and separation of these peaks is keenly sensitive to the coordination geometry about the metal center and the degree of mixing between the Cu 3d/ 4p orbitals and unoccupied ligand orbitals. 27,28 The features in the region about the most intense peak in the spectrum (known as the white line) are typically attributed to multiple scattering events, 29,30 although there may also be some contribution from transitions to higher lying bound states. 31 An additional pre-edge band is present in d 9 Cu(II) species, which corresponds to nominally dipole-forbidden 1s → 3d transitions that gain intensity through 3d/4p mixing.…”

“…Perhaps the most important outstanding questions are those related to the metal−ligand bonding in the various intermediate species, and additional XTA experiments offer the possibility of addressing these directly. In particular, the loss of (3d → π*) back bonding that would accompany the change in hapticity on going to an alkyl copper species should be evident in the intensity of the pre-edge, 28,35 although many hours to even days of signal averaging would likely be required to resolve changes in such a weak feature with single X-ray bunch averaging (i.e., at early delay times). Extended X-ray absorption fine structure (EXAFS) analysis may also be possible with longer signal averaging, which could distinguish between linear and metallacyclic species and provide coordination numbers.…”

Resilience of Cuprous Oxide under Oxidizing Thermal Treatments via Magnesium Doping

“…36,37 The change in the oxidation state of the copper atom, from Cu + into Cu 2+ , leads to the observed 3.3 eV shift of the edge energy position, as confirmed by experimental 38 and theoretical studies. 39 The EXAFS spectra, extracted via the standard background removal procedure, are shown in Figure 5b. The XANES and EXAFS spectra versus the annealing temperature are identical for both Cu 5c, where the amplitude of the Fourier transform (FT) of the EXAFS data in the range 3.5−13 Å −1 is shown for the 450 °C annealing step, compared to that of the as-deposited and the 500 °C one.…”

Increasing Magnetic Hardness of Fe3Se4 via Cu Doping

“…Nevertheless, the Cu Kedge spectra of both A-samples and B-samples exhibit a weak pre-edge feature at 8979.5 eV (Figure 6), which can be assigned only to the dipole-forbidden 1s → 3d transition of the Cu 2+ ion with the single hole in the 3d orbitals. 44 This assignment is also supported by the fact that the XANES spectrum of Cu 2 Se does not show such a pre-edge feature, with the absorption edge appearing at a notably higher energy, 8982 eV. 45 In typical Cu K-edge spectra of Cu(I) compounds, the absorption edge characteristic of the Cu + ion appears in the 8983−8985 eV range.…”