ERP studies of OH radical in ice and several crystal hydrates
Electron paramagnetic resonance studies of OH RADICAL in ice and several crystal hydrates after irradiation
FREE RADICAL↗
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Large Hyperfine Coupling Arising from Pseudo- 2 S Ground States in a Series of Lutetium(II) Metallocene Complexes
The synthesis of molecules with strong coupling between electronic and nuclear spins represents an important challenge in molecular quantum information science. Here, we report the synthesis and characterization of the divalent lutetium metallocene complexes Lu(Cp Me 5 )(Cp iPr 5 ) (Cp Me 5 = pentamethylcyclopentadienyl; Cp iPr 5 = pentaisopropylcyclopentadienyl), Lu(Cp iPr 4 Et ) 2 (Cp iPr 4 Et = ethyltetraisopropylcyclopentadienyl), and Lu(Cp iPr 4 ) 2 (Cp iPr 4 = tetraisopropylcyclopentadienyl). The molecular structures of these complexes, as determined through singlecrystal X-ray diffraction, feature a common bent sandwich geometry, with average Cp–Lu–Cp angles ranging from 159.9° to 152.6°. Analysis of continuous-wave electron paramagnetic resonance (EPR) spectra for the complexes reveals nearly isotropic g tensors with only a slight deviation from that of a free electron. Moreover, an extremely large splitting of the eight-line spectra indicates the presence of strong hyperfine coupling, and simulations provide isotropic hyperfine coupling constants of A iso = 4.38, 4.30, and 4.17 GHz across the series, where the value of A iso is found to decrease as the Cp–Lu–Cp angle becomes more acute. Notably, these values are the largest yet observed for any lanthanide complex. Moreover, EPR and computational analysis show that the large values of A iso stem from large s-orbital character up to 41.2% in the corresponding singly occupied molecular orbitals. To our knowledge, this degree of s-character in a molecular orbital is the largest yet reported for an open-shell isolable complex. These results outline a general strategy toward the isolation of paramagnetic molecules with strong hyperfine coupling and highly isotropic doublet electronic ground states.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Effect of ionizing radiation on ruby
Optical absorption and electron paramagnetic resonance techniques used to study ionizing radiation effect on ruby lasers
RADIATION EFFECT↗
Insights into Molecular Magnetism in Metal–Metal Bonded Systems as Revealed by a Spectroscopic and Computational Analysis of Diiron Complexes
A pair of bimetallic compounds featuring Fe–Fe bonds, [Fe( i PrNPPh 2 ) 3 FeR] (R = PMe 3 , ≡N t Bu), have been investigated using High-Frequency Electron Paramagnetic Resonance (HFEPR) as well as field- and temperature-dependent 57 Fe nuclear γ resonance (Mössbauer) spectroscopy. To gain insight into the local site electronic structure, we have concurrently studied a compound containing a single Fe(II) in a geometry analogous to that of one of the dimer sites. Our spectroscopic studies have allowed for the assessment of the electronic structure via the determination of the zero-field splitting and 57 Fe hyperfine parameters for the entire series. We also report on our efforts to correlate structure with physical properties in metal–metal bonded systems using ligand field theory guided by quantum chemical calculations. Through the insight gained in this study, we discuss strategies for the design of single-molecule magnets based on polymetallic compounds linked via direct metal–metal bonds.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Formation of CO2 Radical Ions When CO2 Is Adsorbed on Irradiated Magnesium Oxide
Electron paramagnetic resonance spectrum of carbon dioxide adsorbed on ultraviolet irradiated magnesium oxide and formation of negative carbon dioxide radical ions
MAGNESIUM OXIDE↗
Direct Identification of Mixed-Metal Centers in Metal–Organic Frameworks: Cu 3 (BTC) 2 Transmetalated with Rh 2+ Ions
Raman spectroscopy was used to establish direct evidence of heterometallic metal centers in a metal–organic framework (MOF). The Cu 3 (BTC) 2 MOF HKUST-1 (BTC 3– = benzenetricarboxylate) was transmetalated by heating it in a solution of RhCl 3 to substitute Rh 2+ ions for Cu 2+ ions in the dinuclear paddlewheel nodes of the framework. In addition to the Cu–Cu and Rh–Rh stretching modes, Raman spectra of (Cu x Rh 1– x ) 3 (BTC) 2 show the Cu–Rh stretching mode, indicating that mixed-metal Cu–Rh nodes are formed after transmetalation. Density functional theory studies confirmed the assignment of a Raman peak at 285 cm –1 to the Cu–Rh stretching vibration. Electron paramagnetic resonance spectroscopy experiments further supported the conclusion that Rh 2+ ions are substituted into the paddlewheel nodes of Cu 3 (BTC) 2 to form an isostructural heterometallic MOF, and electron microscopy studies showed that Rh and Cu are homogeneously distributed in (Cu x Rh 1– x ) 3 (BTC) 2 on the nanoscale.
02 PETROLEUM↗
Direct Evidence of Visible Light-Induced Homolysis in Chlorobis(2,9-dimethyl-1,10-phenanthroline)copper(II)
Developments in the field of photoredox catalysis that leveraged the long-lived excited states of Ir(III) and Ru(II) photosensitizers to enable radical coupling processes paved the way for explorations of synthetic transformations that would otherwise remain unrealized. While first row transition metal photocatalysts have not been as extensively investigated, valuable synthetic transformations covering broad scopes of olefin functionalization have been recently reported featuring photoactivated chlorobis(phenanthroline) Cu(II) complexes. In this study, the photochemical processes underpinning the catalytic activity of [Cu(dmp) 2 Cl]Cl (dmp = 2,9-dimethyl-1,10-phenanthroline) were studied. The combined results from static spectroscopic measurements and conventional photochemistry, ultrafast transient absorption, and electron paramagnetic resonance spin trapping experiments strongly support blue light (λ ex = 427 or 470 nm)-induced Cu–Cl homolytic bond cleavage in [Cu(dmp) 2 Cl] + occurring in <100 fs. On the basis of electronic structure calculations, this bond-breaking photochemistry corresponds to the Cl → Cu(II) ligand-to-metal charge transfer transition, unmasking a Cu(I) species [Cu(dmp) 2 ] + and a Cl atom, thereby serving as a departure point for both Cu(I)- or Cu(II)-based photoredox transformations. No net photochemistry was observed through direct excitation of the ligand-field transitions in the red (λ ex = 785 or 800 nm), and all combined experiments indicated no evidence of Cu–Cl bond cleavage under these conditions. As a result, the underlying visible light-induced homolysis of a metal–ligand bond yielding a one-electron-reduced photosensitizer and a radical species may form the basis for novel transformations initiated by photoinduced homolysis featuring in situ-formed metal–substrate adducts utilizing first row transition metal complexes.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Cu(II) Stability and UV-Induced Electron Transfer in a Metal–Organic Hybrid: An EPR, DFT, and Crystallographic Characterization of Copper-Doped Zinc Creatininium Sulfate
Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopic experiments, complemented by quantum chemical DFT calculations, were carried out on the copper-doped metal–organic hybrid and Tutton salt analogue zinc creatininium sulfate to determine its crystal structure, to characterize the electronic structure of the doped Cu(II) binding site, and to propose a pathway for an excited-state, proton-coupled electron transfer (PCET) process in UV-exposed crystals. The crystal structure is isomorphous to that of cadmium creatininium sulfate, which has the transition ion, not in direct coordination with the creatinine, but forming a hexahydrate complex, which is bridged to a creatininium through an intervening sulfate ion. The EPR g (2.446, 2.112, 2.082) and copper hyperfine (A Cu : -327, -59.6, 10.8 MHz) tensor parameters are consistent with doped copper replacing host zinc in the metal–hexahydrate complex. These parameters are similar to those observed for copper hexahydrate in doped Tutton salt systems at low temperature, where the unpaired electron occupies mainly the copper 3d x 2 –y 2 orbital. At room temperature in the Tutton systems, vibration couplings stemming from a dynamic Jahn–Teller effect cause tensor averaging which results in a reduction in their maximum g-tensor and hyperfine tensor values. However, like for the doped isomorphous Cd creatinine crystal, the Cu(II) EPR exhibits little, or no room temperature averaging compared to its low temperature pattern. Samples exposed to 254 nm UV light generate a carbon-centered free radical species, characterized by an isotropic g-tensor (g = 2.0029) and an alpha-proton hyperfine coupling (-24 -14 +4 G). These parameters identify it as a creatinine radical cation formed by the oxidative release of one of its C2 methylene hydrogens. DFT calculations confirm the unpaired electronic structures of both the Cu(II) site and free radical. The growth in radical concentration with an increase in the UV exposure time coincides with a decrease in the copper EPR signal, indicating a coupled light-induced oxidation reduction process. A comparison of the crystal structure with the EPR parameters and DFT results provides evidence for a UV-induced PCET.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Spectroscopic Investigation of Shallow Hole Traps in Ga- and B-doped Czochralski Silicon: Insight into Light-Induced Degradation
We report the minority carrier lifetime in B-doped Czochralski (Cz) Si declines upon carrier injection due to light-induced degradation (LID), which is attributed to the formation of a recombination-active boron-oxygen complex. Ga-doped Cz Si does not undergo LID. Previously, we showed that B-doped Cz Si undergoes a transition from paramagnetic to diamagnetic due to LID. Herein, we show that Ga-doped Cz Si remains paramagnetic upon carrier injection. This suggests that either the shallow hole traps that are formed in B-doped Cz Si are absent in Ga-doped Cz Si, or the negative-U centers in Ga-doped Cz Si do not transform into a recombination-active configuration, because the shallow acceptor trap state is shallower than the Ga acceptor level in the forbidden gap. In contrast to B-doped Cz Si, the defect signatures in Ga-doped Cz Si do not change upon light exposure, as detected by both electron paramagnetic resonance and deep-level transient spectroscopy.
14 SOLAR ENERGY↗
Room Temperature, Dip Coating Organic Passivation for c-Si Surfaces
Surface passivation of crystalline Si (c-Si) to reduce recombination rate is crucial to obtain high-efficiency c-Si solar cells. A fast passivation technique that can be performed at room temperature is needed when studying the degradation behaviors using various characterization methods to not destroy the degraded states. In this contribution, we explore Nafion via dip coating on n-type Cz wafers. We show that both the Nafion concentration and the conditions of surface and edge affect the passivation quality, and we achieve excellent passivation quality of iVoc of 716 mV, with a saturation current density (J0) of 7 fA/cm2. Additionally, electron paramagnetic resonance (EPR) shows Nafion can be used under cryogenic temperature and reveals the reduction of Si dangling bond after Nafion treatment.
dielectric solar cell↗
Resolving Chemical and Spatial Heterogeneities at Complex Electrochemical Interfaces in Li-Ion Batteries
The high specific capacities of Ni-rich transition metal oxides have garnered immense interest for improving the energy density of Li-ion batteries (LIBs). Despite the potential of these materials, Ni-rich cathodes suffer from interfacial instabilities that lead to crystallographic rearrangement of the active material surface as well as the formation of a cathode electrolyte interphase (CEI) layer on the composite during electrochemical cycling. While changes in crystallographic structure can be detected with diffraction-based methods, probing the chemistry of the disordered, heterogeneous CEI layer is challenging. In this work, we use a combination of ex situ solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide chemical and spatial information, on the nanometer length scale, on the CEI deposited on LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) composite cathode films. XPEEM elemental maps offer insight into the lateral arrangement of the electrolyte decomposition products that comprise the CEI and paramagnetic interactions (assessed with electron paramagnetic resonance (EPR) and relaxation measurements) in 13 C SSNMR provide information on the radial arrangement of the CEI from the NMC811 particles outward. Using this approach, here we find that LiF, Li 2 CO 3 , and carboxy-containing structures are directly appended to NMC811 active particles, whereas soluble species detected during in situ 1 H and 19 F solution NMR experiments (e.g., alkyl carbonates, HF, and vinyl compounds) are randomly deposited on the composite surface. We show that the combined approach of ex situ SSNMR and XPEEM, in conjunction with in situ solution NMR, allows spatially-resolved, molecular-level characterization of paramagnetic surfaces and new insights into electrolyte oxidation mechanisms in porous electrode films.
36 MATERIALS SCIENCE↗
Clock transitions guard against spin decoherence in singlet fission
Short coherence times present a primary obstacle in quantum computing and sensing applications. In atomic systems, clock transitions (CTs), formed from avoided crossings in an applied Zeeman field, can substantially increase coherence times. We show how CTs can dampen intrinsic and extrinsic sources of quantum noise in molecules. Conical intersections between two periodic potentials form CTs in electron paramagnetic resonance experiments of the spin-polarized singlet fission photoproduct. We report on a pair of CTs for a two-chromophore molecule in terms of the Zeeman field strength, molecular orientation relative to the field, and molecular geometry.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy
High-field magic angle spinning (MAS) dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (SSNMR) is becoming a common technique for improving the sensitivity in NMR by the hyperpolarization of nuclear spins. Recently, we have shown that gamma irradiation is capable of creating long-lived free radicals that are amenable to MAS DNP in quartz and a variety of organic solids. Here, we demonstrate that mechanochemical ball milling is able to generate millimolar concentrations of stable radical species in diverse materials, such as polystyrene, cellulose, borosilicate glass, and fused quartz. High-field electron paramagnetic resonance (EPR) was used to obtain further insight into the nature of the radicals formed in ball milled quartz and borosilicate glass. Finally, we further show that radicals generated in quartz by ball milling can be used for solid-effect DNP. We obtained 29 Si DNP enhancements of approximately 114 and 33 at 110 K and room temperature, respectively, from a sample of ball milled quartz.
36 MATERIALS SCIENCE↗
Zn acceptors in β-Ga 2 O 3 crystals
Electron paramagnetic resonance (EPR) is used to identify and characterize neutral zinc acceptors in Zn-doped β-Ga 2 O 3 crystals. Two EPR spectra are observed at low temperatures, one from Zn ions at tetrahedral Ga(1) sites (the $Zn$$^{0}_{Ga 1}$ acceptor) and one from Zn ions at octahedral Ga(2) sites (the $Zn$$^{0}_{Ga 2}$ acceptor). These Zn acceptors are small polarons, with the unpaired spin localized in each case on a threefold coordinated oxygen O(I) ion adjacent to the Zn ion. Resolved hyperfine interactions with neighboring 69 Ga and 71 Ga nuclei allow the EPR spectra from the two acceptors to be easily distinguished: $Zn$$^{0}_{Ga 1}$ acceptors interact equally with two Ga(2) ions and $Zn$$^{0}_{Ga 2}$ acceptors interact unequally with a Ga(1) ion and a Ga(2) ion. The as-grown crystals are compensated, with the Zn ions initially present as singly ionized acceptors ($Zn$$^{-}_{Ga 1}$ and $Zn$$^{-}_{Ga 2}$). Exposing a crystal to 325 nm laser light, while being held at 140 K, primarily produces neutral $Zn$$^{0}_{Ga 2}$ acceptors when photoinduced holes are trapped at $Zn$$^{-}_{Ga 2}$ acceptors. This suggests that there may be significantly more Zn ions at Ga(2) sites than at Ga(1) sites. Warming the crystal briefly to room temperature, after removing the light, destroys the EPR spectrum from the shallower $Zn$$^{0}_{Ga 2}$ acceptors and produces the EPR spectrum from the more stable $Zn$$^{0}_{Ga 1}$ acceptors. Furthermore, the $Zn$$^{0}_{Ga 2}$ acceptors decay in the 240–260 K region with a thermal activation energy near 0.65 eV, similar to $Mg$$^{0}_{Ga 2}$ acceptors, whereas the slightly deeper $Zn$$^{0}_{Ga 1}$ acceptors decay close to room temperature with an approximate thermal activation energy of 0.78 eV.
36 MATERIALS SCIENCE↗
Spectroscopic investigation of historical uranium glasses
Here, we present spectroscopic investigations of uranium in two historical glasses. A 1920's-1930's Art Deco (Bagley Carnival) green soda-lime-silica uranium glass bowl and a 1930's-1940's (Thomas Webb) yellow full lead crystal uranium glass vase were studied using optical absorption spectroscopy, XRF and SEM-EDX, X-band EPR and High- Energy Resolution Fluorescence-Detected (HERFD)-XANES. Uranium speciation may be different in the two glasses. Uranium occurs as uranyl groups (UO 2 2+ ) with minority reduced uranium species, mostly as U(V), more important in the Webb glass than in the Bagley glass. The differences between the two glasses arise from the fining procedure and result from the presence of other multivalent elements and differences in base glass composition. Electron Paramagnetic Resonance (EPR) spectra show the presence of an incipient signal close to g~2, partially annealed by heating to 550 °C, which we assign to a small concentration of radiation-induced defects caused by uranium decay. The presence of multivalent glass components in addition to U (As + Cu + Fe in the Bagley glass and Sb + Fe in the Webb glass) can trap the electron-hole pairs generated by the presence of uranium. This may explain the weakness of this signal in these glasses that anyway received a limited radiation dose since their fabrication. The low activity of these glasses, close to the background radiation, confirms that there is no danger to exhibiting them in museums.
36 MATERIALS SCIENCE↗
Biophysical Characterization of Iron-Sulfur Proteins
Iron-sulfur proteins are primordial catalysts and biological electron carriers that today drive major metabolic pathways across all forms of life. They can access a diversity of oxidation states and can mediate electron transfer over an extended range of reduction potentials spanning more than 1 V. Depending on the protein micro-environment and geometry of ligand, co-ordination the iron-sulfur clusters can occur in different forms [2Fe-2S], [3Fe-4S], HiPIP [4Fe-4S], and [4Fe-4S]. There are several spectroscopic methods available to characterize the composition and electronic configuration of the iron-sulfur clusters, such as optical methods and electron paramagnetic resonance. This paper presents the protocols used to characterize the metal center of Coiled-Coil Iron-Sulfur (CCIS), an artificial metalloprotein containing one [4Fe-4S] cluster. It is expected that these protocols will be of general utility for other iron-sulfur proteins.
Iron-sulfur proteins↗
Chloride facilitates Mn(III) formation during photoassembly of the Photosystem II oxygen-evolving complex
The Mn 4 Ca oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ from free Mn 2+ , Ca 2+ , and water. In an early light-driven step, Mn 2+ in a protein high-affinity site is oxidized to Mn 3+ . Using dual-mode electron paramagnetic resonance spectroscopy, we observed that Mn 3+ accumulation increases as chloride concentration increases in spinach PSII membranes depleted of all extrinsic subunits. At physiologically relevant pH values, this effect requires the presence of calcium. Here, when combined with pH studies, we conclude that the first Mn 2+ oxidation event in OEC assembly requires a deprotonation that is facilitated by chloride.
59 BASIC BIOLOGICAL SCIENCES↗
Electronic Spin Relaxation and Clustering in High-Pressure High-Temperature Synthesized Microcrystalline Diamond Particles with Reduced Nitrogen Content
The negatively charged nitrogen-vacancy (NV – ) color center in diamonds is widely studied because of numerous applications of this unique quantum system in sensing and quantum information sciences. While substitutional nitrogen is required to form the NV – centers in diamond, it also yields other paramagnetic defects─primarily the neutrally charged substitutional nitrogen centers (P1)─that decrease NV – spin coherence, which in turn degrades performance in applications. Herein, we investigate high-pressure high-temperature synthesized diamond microparticles (ca. 140–185 μm) having lower─ranging from 3 to 38 ppm─than the typical nitrogen content of type 1b diamond ( ca. 100 ppm and higher) typically used for the production of fluorescent diamond particles with NV – centers. A suite of electron paramagnetic resonance, optically detected magnetic resonance, and nuclear magnetic resonance methods are used to characterize spin properties of P1 and NV – centers in the particles. Upon decreasing the nitrogen content from 29 to 3 ppm, the ensemble NV – T 2 relaxation time increased by about 3-fold as measured directly in the Hahn Echo experiment at magnetic field of 1.2 T. Analysis of electronic relaxation of P1 centers revealed the existence of at least two distinct populations of P1 centers, consisting of fast and slower relaxing spins and allowed for an estimation of local concentrations. Even with <10 ppm nitrogen contents, the analysis indicated a highly heterogeneous distribution of P1 centers, suggesting the possibility of P1 spin clustering even at low nitrogen concentrations. The combined data demonstrate that the particles prepared from HPHT diamond with a low nitrogen content offer improved spin properties that are beneficial for NV – sensing applications.
Carbon↗