Zachary Mathe

Bio: Zachary Mathe is an academic researcher from Max Planck Society. The author has contributed to research in topics: Non-covalent interactions & Molecular orbital. The author has an hindex of 2, co-authored 2 publications receiving 15 citations.

Calcium Valence-to-Core X-ray Emission Spectroscopy: A Sensitive Probe of Oxo Protonation in Structural Models of the Oxygen-Evolving Complex.

Abstract: Calcium is an abundant, nontoxic metal that finds many roles in synthetic and biological systems including the oxygen-evolving complex (OEC) of photosystem II. Characterization methods for calcium centers, however, are underdeveloped compared to those available for transition metals. Valence-to-core X-ray emission spectroscopy (VtC XES) selectively probes the electronic structure of an element's chemical environment, providing insight that complements the geometric information available from other techniques. Here, the utility of calcium VtC XES is established using an in-house dispersive spectrometer in combination with density functional theory. Spectral trends are rationalized within a molecular orbital framework, and Kβ2,5 transitions, derived from molecular orbitals with primarily ligand p character, are found to be a promising probe of the calcium coordination environment. In particular, it is shown that calcium VtC XES is sensitive to the electronic structure changes that accompany oxo protonation in Mn3CaO4-based molecular mimics of the OEC. Through correlation to calculations, the potential of calcium VtC XES to address unresolved questions regarding the mechanism of biological water oxidation is highlighted.

Phosphorus Kβ X-ray emission spectroscopy detects non-covalent interactions of phosphate biomolecules in situ

Abstract: Phosphorus is ubiquitous in biochemistry, being found in the phosphate groups of nucleic acids and the energy-transferring system of adenine nucleotides (e.g. ATP). Kβ X-ray emission spectroscopy (XES) of phosphorus has been largely unexplored, with no previous applications to biomolecules. Here, the potential of P Kβ XES to study phosphate-containing biomolecules, including ATP and NADPH, is evaluated, as is the application of the technique to aqueous solution samples. P Kβ spectra offer a detailed picture of phosphate valence electronic structure, reporting on subtle non-covalent effects, such as hydrogen bonding and ionic interactions, that are key to enzymatic catalysis. Spectral features are interpreted using density functional theory (DFT) calculations, and potential applications to the study of biological energy conversion are highlighted.

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

Abstract: A series of organometallic copper complexes in formal oxidation states ranging from +1 to +3 have been characterized by a combination of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core X-ray emission spectroscopies (VtC XES). Each formal oxidation state exhibits distinctly different XAS and VtC XES transition energies due to the differences in the Cu Zeff, concomitant with changes in physical oxidation state from +1 to +2 to +3. Herein, we demonstrate the sensitivity of XAS and VtC XES to the physical oxidation states of a series of N-heterocyclic carbene (NHC) ligated organocopper complexes. We then extend these methods to the study of the [Cu(CF3)4]- ion. Complemented by computational methods, the observed spectral transitions are correlated with the electronic structure of the complexes and the Cu Zeff. These calculations demonstrate that a contraction of the Cu 1s orbitals to deeper binding energy upon oxidation of the Cu center manifests spectroscopically as a stepped increase in the energy of both XAS and Kβ2,5 emission features with increasing formal oxidation state within the [Cun+(NHC2)]n+ series. The newly synthesized Cu(III) cation [CuIII(NHC4)]3+ exhibits spectroscopic features and an electronic structure remarkably similar to [Cu(CF3)4]-, supporting a physical oxidation state assignment of low-spin d8 Cu(III) for [Cu(CF3)4]-. Combining XAS and VtC XES further demonstrates the necessity of combining multiple spectroscopies when investigating the electronic structures of highly covalent copper complexes, providing a template for future investigations into both synthetic and biological metal centers.

Probing a Silent Metal: A Combined X-ray Absorption and Emission Spectroscopic Study of Biologically Relevant Zinc Complexes.

Abstract: As the second most common transition metal in the human body, zinc is of great interest to research but has few viable routes for its direct structural study in biological systems. Herein, Zn valen...

Challenges and Opportunities for Applications of Advanced X-ray Spectroscopy in Catalysis Research

Abstract: X-ray spectroscopy has had a significant and continually growing impact on catalysis research for nearly 50 years. In particular, the ability to obtain element selective electronic and geometric structural information via the X-ray absorption (XAS) edge and extended X-ray absorption fine structure regions, respectively, has been a major asset for catalysis research. In the last two decades, the development of dedicated synchrotron-based X-ray emission spectrometers has greatly expanded the range of possible experiments, enabling both nonresonant and resonant X-ray emission spectroscopy experiments that can provide greater selectivity and more detailed electronic and structural information. Herein, we briefly review the range of hard X-ray photon-in, photon-out experiments that are presently possible, and highlight their recent applications in catalysis research. We also discuss the ongoing need for conventional XAS applications, either in standalone applications or in combination with more advanced approaches. The open opportunities and ongoing challenges for applying these methods, and ultimately for analyzing and interpreting the data, are also discussed.

Ruthenium 4d-to-2p X-ray Emission Spectroscopy: A Simultaneous Probe of the Metal and the Bound Ligands.

Abstract: Ruthenium 4d-to-2p X-ray emission spectroscopy (XES) was systematically explored for a series of Ru2+ and Ru3+ species. Complementary density functional theory calculations were utilized to allow f...

CaMn3IVO4 Cubane Models of the Oxygen Evolving Complex: Spin Ground States S < 9/2 and the Effect of Oxo Protonation

Abstract: We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn3 IV O4 and YMn3 IV O4 complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC) The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated In contrast to previous computational models, we show that the spin ground state of CaMn3 IV O4 complexes and variants with protonated oxo moieties need not be S=9/2 Desymmetrization of the pseudo-C3 -symmetric Ca(Y)Mn3 IV O4 core leads to a lower S=5/2 spin ground state The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn3 IV O3 (OH) Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S2 state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes