Browsing by Author "Szilagyi, Robert K."

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Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex
(Wiley, 2023-01) Stephens, David N.; Szilagyi, Robert K.; Roehling, Paige N.; Arulsamy, Navamoney; Mock, Michael T.
We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6-tri-tert-butylphenoxyl (tBu3ArO⋅) as a H atom acceptor to cleave the N−H bond of a coordinated NH3 ligand up to 56 equiv of N2 per Ni center can be generated. Employing the N-oxyl radical 2,2,6,6-(tetramethylpiperidin-1-yl)oxyl (TEMPO⋅) as the H-atom acceptor, up to 15 equiv of N2 per Ni center are formed. A bridging Ni-hydrazine product identified by isotopic nitrogen (15N) studies and supported by computational models indicates the N−N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]−NH2 fragments. Ni-mediated hydrazine disproportionation to N2 and NH3 completes the catalytic cycle.
Computational models for dual Cu sites in Pseudoazurin from Achromobacter cycloclastes (AcPAz) [dataset]
(Montana State University ScholarWorks, 2016-09) Szilagyi, Robert K.
Atomic positional coordinates (XYZ), computational log files (OUT), and binary (CHK) and formatted (FCHK) checkpoint files for electronic structure calculations are deposited. The levels of theory used in the simulations are denoted in the filenames. The full computational details can be obtained from the computational log files. The filenames are organised according to the peer reviewed publications. Fig.3 corresponds to the fully optimised inner sphere environment of the Type-1 Cu site i AcPAz for oxidised (panels A-C) and one-electron reduced (panels D-E) structures Fig.5 contains the refined structures for the oxidised axial and rhombic Type-1 Cu environment as quantum chemical refinement of the inner sphere environments in the presence of a single Cu site. Fig.6 contains the refined structures for the reduced axial and rhombic Type-1 Cu environment as quantum chemical refinement of the inner sphere environments in the presence of a single Cu site. This dataset was originally posted in September 2016; it was updated with additional files in January 2017. Additional electronic supporting information are also available upon request from the corresponding authors at szilagyi@montana.edu or takamitsu.kohzuma.qbs@vc.ibaraki.ac.jp
Methane Adsorption on Heteroatom-Modified Maquettes of Porous Carbon Surfaces
(American Chemical Society, 2021-07) Rowsey, Rylan; Taylor, Erin E.; Irle, Stephan; Stadie, Nicholas P.; Szilagyi, Robert K.
Experimental and theoretical studies disagree on the energetics of methane adsorption on carbon materials. However, this information is critical for the rational design and optimization of the structure and composition of adsorbents for natural gas storage. The delicate nature of dispersion interactions, polarization of both the adsorbent and the adsorbate, interplay between H-bonding and tetrel bonding, and induced dipole/Coulomb interactions inherent to methane physisorption require computational treatment at the highest possible level of theory. In this study, we employed the smallest reasonable computational model, a maquette of porous carbon surfaces with a central site for substitution and methane binding. The most accurate predictions of methane adsorption energetics were achieved by electron-correlated molecular orbital theory CCSD(T) and hybrid density functional theory MN15 calculations employing a saturated, all-electron basis set. The characteristic geometry of methane adsorption on a carbon surface (“lander approach”) arises due to bonding interactions of the adsorbent π-system with the proximal H–C bonds of methane, in addition to tetrel bonding between the antibonding orbital of the distal C–H bond and the central atom of the maquette (C, B, or N). The polarization of the electron density, structural deformations, and the comprehensive energetic analysis clearly indicate a ∼3 kJ mol–1 preference for methane binding on the N-substituted maquette. The B-substituted maquette showed a comparable or lower binding energy than the unsubstituted, pure C model, depending on the level of theory employed. The calculated thermodynamic results indicate a strategy for incorporating electron-enriched substitutions (e.g., N) into carbon materials as a way to increase methane storage capacity over electron-deficient (e.g., B) modifications. The thermochemical analysis was revised for establishing a conceptual agreement between the experimental isosteric heat of adsorption and the binding enthalpies from statistical thermodynamics principles.
Optimized molecular structures for nanokaolinite dehydration and dehydroxylation toward the formation of nanometakaolinite [dataset]
(MSU ScholarWorks, 2017-03) Szilagyi, Robert K.; Taborosi, Attila
The data depository contains XYZ atomic positional coordinates for all relevant structures to describe the dry-grinding, mechanochemical activation of nanokaolinite. During this process the kaolinite undergo dehydration, followed by edge dehydroxylation, dissociation of the surface trapped water, and finally dehydroxylation of the surface hydroxides. The overall five step procedure results in amorphous metakaolinite particle with highly activated surface sites. The theoretical results are in close correlation with SEM/TEM, FTIR, and TG/DTG experimental measurements.
Surface Characterization of Mechanochemically Modified Exfoliated Halloysite Nanoscrolls
(2017-03) Zsirka, Balazs; Tabarosi, Attila; Szabo, Peter; Szilagyi, Robert K.; Horvath, Erzsebet; Juzsakova, Tatjana; Fertig, David; Kristof, Janos
The surface modifications fundamentally influence the morphology of kaolinite nanostructures as a function of crystallinity and presence of contaminants. Beside morphology, the catalytic properties of 1:1-type exfoliated aluminosilicates are also influenced by the presence of defect sites that can be generated in a controlled manner by mechanochemical activation. In this work, we investigated exfoliated halloysite nanoparticles with quasi-homogeneous, scroll-type secondary structure toward developing structure/function relationships for composition, atomic structure, and morphology. The surface properties of thin-walled nanoscrolls were studied as a function of mechanochemical activation expressed by the duration of dry-grinding. The surface characterizations were carried out by N2, NH3, and CO2 adsorption measurements. The effects of grinding on the nanohalloysite structure were followed by thermoanalytical (TG/DTG) and infrared spectroscopic (FTIR/ATR) techniques. Grinding results in partial dehydroxylation with similar changes as observed for heat treatment above 300ºC. The employed mechanochemical activation shows decrease in dehydroxylation mass loss and the DTG peak temperature, decrease in specific surface area and number of mesopores, increase of surface acidity, blue-shift of surface hydroxide bands, and decrease in intensity in FTIR/ATR bands as a function of grinding time. The experimental observations were used to guide atomic-scale structural and energetic simulations using realistic molecular cluster models for a nanohalloysite particle. A full potential energy surface description was developed for mechanochemical activation and/or heating toward nanometahalloysite formation that aid the interpretation of experimental results. The calculated differences upon dehydroxylation show remarkable agreement with the mass-loss values from DTG measurements.
Uranium exerts acute toxicity by binding to pyrroloquinoline quinone cofactor
(2010-12) VanEngelen, Michael R.; Szilagyi, Robert K.; Gerlach, Robin; Lee, Brady D.; Apel, William A.; Peyton, Brent M.
Uranium as an environmental contaminant has been shown to be toxic to eukaryotes and prokaryotes; however, no specific mechanisms of uranium toxicity have been proposed so far. Here a combination of in vivo, in vitro, and in silico studies are presented describing direct inhibition of pyrroloquinoline quinone (PQQ)-dependent growth and metabolism by uranyl cations. Electrospray-ionization mass spectroscopy, UV-vis optical spectroscopy, competitive Ca2+/uranyl binding studies, relevant crystal structures, and molecular modeling unequivocally indicate the preferred binding of uranyl simultaneously to the carboxyl oxygen, pyridine nitrogen, and quinone oxygen of the PQQmolecule. The observed toxicity patterns are consistent with the biotic ligand model of acute metal toxicity. In addition to the environmental implications, this work represents the first proposed molecular mechanism of uranium toxicity in bacteria, and has relevance for uranium toxicity in many living systems.
X-ray crystallographic evidence for the simultaneous presence of axial and rhombic sites in cupredoxins: atomic resolution X-ray crystal structure analysis of pseudoazurin and DFT modelling
(2016-09) Yamaguchi, T.; Akao, K.; Takashina, A.; Asamura, S.; Unno, M.; Szilagyi, Robert K.; Kohzuma, T.
Crystal structure refinement of pseudoazurin from Achromobacter cycloclastes (AcPAz) was carried out at atomic (1.10 angstrom) resolution. The copper ion was localized in two positions at the metal binding site of AcPAz. The occupancies of the copper sites are consistent with the ratio of axial and rhombic signals from EPR spectra and the intensity ratios for blue (axial) and green (rhombic) copper sites from UV-VIS spectroscopy. Computational modelling using an approximately 6 angstrom protein environment around the Cu site for both the oxidized and reduced forms showed that a small scale inner sphere rearrangement can account for the co-existence of two different redox active sites for a mononuclear cupredoxin independently from the employed density functionals.