Epr Publications

Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4′,4′′-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold ¹³C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research.

Radiation curing (photocuring) of thermosetting polymers, such as acrylate resins, is a common technology with diverse applications, such as in adhesives, coatings, advanced manufacturing, medical-related technologies, and more. Photocuring of acrylate thermosets is initiated via a radical-induced cleavage of a photocuring agent, like bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO), which activates the vinyl C═C double bond of the acrylate moiety. Here, we show that addition of semiconducting WS₂ nanoparticles (NPs) accelerates the photocuring process. Using electron paramagnetic resonance (EPR) of ethanol-water solutions, the mechanism of the radical reactions of BAPO and WS₂ NPs is investigated. It is found that the two photocuring agents operate according to entirely different mechanisms, which has been discussed in great detail. In contrast to BAPO, which is photocleaved during the curing process, the WS₂ NPs remain unchanged, leading to major mechanical reinforcements of the cured acrylate film.

We demonstrate that sequential disproportionation reactions can enable selective aggregation of two- or four electron-holes at a hypervalent iodine center. Disproportionation of an anodically generated iodanyl radical affords an iodosylbenzene derivative. Subsequent iodosylbenzene disproportionation can be triggered to provide access to an iodoxybenzene. These results demonstrate multielectron oxidation at the one-electron potential by selective and sequential disproportionation chemistry.

Photocatalytic processes are among the prime means for mitigating the pollution caused by toxic effluents. In this context, photocatalysis presents a promising path and is undergoing rapid evolution. Halide perovskite-nanocrystals (HP-NCs) are excellent candidates due to their negative conduction band minimum and low work function, essential for photocatalysis. Yet, HP-NCs face limitations within this domain because they are prone to chemical degradation when exposed to external factors like high temperature, polar solvents, oxygen, and light. A practical approach toward stabilizing HP-NCs involves hybridizing them with a chemically inert material that can provide steric stabilization and act as a cocatalyst. Transition-metal dichalcogenides emerge as outstanding candidates to sterically stabilize the HPs as they are stable, chemically inert, and can serve as co-catalysts, enabling suppressed charge recombination. Herein, the photocatalytic performance of Cs₄PbBr₆/WS₂-nanocomposites towards organic dye degradation in polar solvents under visible light illumination is investigated. We found that the presence of WS₂ nanostructures significantly stabilizes the HP-NCs and promotes dye degradation rate compared to pristine Cs₄PbBr₆-NCs. Using transient absorption measurements, we found that the WS₂-nanostructures act as an electron transport channel, effectively reducing charge recombination in the NCs. These findings pave the way for implementing Cs₄PbBr₆/WS₂-nanocomposites as stable and superior photocatalysts.

Copper(II) acetate reacted with 2,3-isatin bisthiosemicarbazone (2,3 H₂bitsc), 2,3-isatin bis-N¹-methyl thiosemicarbazone (2,3 H₂bitsc-N¹-Me) and 2,3-isatin N¹-phenyl thiosemicarbazone (2,3 H₂bitsc-N¹-Ph) in 1:1 (M: L) to form complexes of formula, [Cu(L)] (L = 2,3 bitsc 1, 2,3 bitsc-N¹-Me 2, 2,3 bitsc-N¹-Ph 3). The formation of bisthiosemicarbazone ligands and their complexes are supported by elemental analysis, FTIR, NMR (¹H and ¹³C), ESR, and Mass. For complexes 1–3, two g values obtained are: g_‖ = 2.24 (1), 2.17 (2), 2.26 (3) and g_⊥ = 2.09 (1), 2.11 (2), 2.08 (3) which confirm axial symmetry. The square planar geometry is confirmed from higher g_‖ values than g_⊥ and presence g value of free electron in d_x2-y2 ground term. Ligand to metal binding ratio of 1:1 is confirmed using UV–visible spectroscopy. The ligands (2,3 H₂bitsc, 2,3 H₂bitsc-N¹-Me and 2,3 H₂bitsc-N¹-Ph) and complexes (1–3) were assessed for anti-tubercular (M. tuberculosis H37RV strain ATCC27294) and antibacterial (against E. coli and S. aureus) activities. The anti- tuberculosis activity of ligands (50 µg/mL-2,3 H₂bitsc, 100 µg/mL-2,3 H₂bitsc-N¹-Me and H₂bitsc-N¹-Ph) gets enhanced on complexation with copper (25 µg /mL-1, 50 µg/mL 2 and 3). The minimal binding energies calculated through molecular modeling with mycobacterium tuberculosis enoyl reductase (PDB ID: 2H7M) is −7.6 (2,3 H₂bitsc), −7.3 (2,3 H₂bitsc-N¹-Me), −9.6 (2,3 H₂bitsc-N¹-Ph), −8.7 (1), −8.9 (2), and −10.7 (3) Kcal/mol. Higher negative binding energy indicates more stabilized structure in the docked state for 1–3 which supports the experimental data. The drug-protein binding study with human serum albumin (HSA) was used to explore effective transport of molecules to their target sites in 2,3 H₂bitsc and its complex 1. A high binding constant of complex 1 (6.09 × 10⁵ M⁻¹) with respect to the ligand 2,3 H₂bitsc (4.90 × 10⁵ M⁻¹) indicates its strong binding affinities with HSA.

Co²⁺-ZIF-67 metal-organic framework nanoparticles (NMOFs), act as nanozymes catalyzing diverse processes, including peroxidase, oxidase, and catalase activities. Peroxidase activities are reflected by the nanozyme-catalyzed H₂O₂ oxidation of dopamine to aminochrome, the H₂O₂ oxidation of NADH to NAD⁺, the H₂O₂-catalyzed generation of chemiluminescence through oxidation of luminol, and the H₂O₂-mediated oxidation of aniline to polyaniline. Oxidase activities of the nanozyme are demonstrated by the Co²⁺-ZIF-67 NMOFs catalyzed aerobic oxidation of dopamine to aminochrome and of NADH to NAD⁺, and catalase activities of the nanozyme are reflected by the catalytic decomposition of H₂O₂. Moreover, the Co²⁺-ZIF-67 catalyzed oxidation of aniline to polyaniline (PAN) by H₂O₂ is accompanied by the coating of NMOFs with PAN to yield a Co²⁺-ZIF-67/PAN hybrid material. Coating the particles in the presence of guest substrates leads to molecular imprinted PAN matrices revealing enhanced ZIF-67 catalyzed transformations as compared to non-imprinted PAN matrices. This is demonstrated by imprinting of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid), ABTS²⁻, in the PAN coating and the 3.3-fold enhanced catalyzed oxidation of ABTS²⁻ by H₂O₂, as compared to the non-imprinted PAN hybrid composite. Moreover, imprinting of L-/D-DOPA in the PAN coating of Co²⁺-ZIF-67 NMOFs leads to chiro-selective H₂O₂-guided oxidation of L-/D-DOPA to dopachrome by the NMOFs/PAN composite.

Reaction of copper (II) acetate with 2,5 thiophenedicarboxaldehyde bisthiosemicarbazone (2,5 H₂bttsc, ¹H₂L), 2,5 thiophenedicarboxaldehyde-N¹-methyl bisthiosemicarbazone (2,5 H₂bttsc-N¹-Me, ²H₂L) and 2,5 thiophenedicarboxaldehyde -N¹-phenyl bisthiosemicarbazone (2,5 H₂bttsc-N¹-Ph, ³H₂L) in 1:1 (M:L) molar ratio yielded complexes of stoichiometry, [Cu(L)] 1-3 (1L, 1; 2L 2; 3L 3). All the complexes are characterized using elemental analysis FTIR, Mass and ESR spectroscopy. The two distinct g values, g_‖ = 2.40 (1), 2.15 (2), 2.20 (3) and g_⊥ = 2.08 (1), 2.07 (2), 2.12 (3) indicate axial symmetry for the complexes. The greater g_‖ value than g_⊥, confirms the presence of free electrons in d_x2-y2 ground term in square planar geometry. The ligands (¹H₂L - ³H₂L) and their complexes (1–3) were tested for DPPH radical scavenging, ABTS radical scavenging, FRAP ferric reducing, and CUPRAC cupric reducing antioxidant capacity assays. In general, all the ligands and their metal complexes exhibited moderate antioxidant potential, however ³H₂L has shown best Cu²⁺ reducing activity (7.34 μg TE/ml) and ABTS radicals scavenging activity (IC₅₀, 16.95) amongst all. Ligands (¹H₂L - ³H₂L) and their complexes (1–3) were also evaluated for anti-tuberculosis activity against M. tuberculosis H37RV strain. It has been observed that with the increase of hydrophobicity of ligand due to substituent present at N¹ atom, anti-TB activity increased (H, MIC = 6.25 µg/ml < Me, MIC = 3.12 µg/ml < Ph, MIC = 1.6 µg/ml). Molecular modeling studies have shown considerable intermolecular interaction of these compounds with minimum binding energy −5.8 (¹H₂L), −5.8 (²H₂L), −9.6 (³H₂L), −6.9 (1), −6.6 (2) and −9.6 (3) Kcal/ mol, which also supports the experimental data. Pharmacokinetics (effective transport of molecule to their target sites) was studied using drug-protein binding study of molecules with human serum albumin (HSA) was performed using UV–vis and fluorescence spectroscopy. The ligand ²H₂L and complex 2 showed strong binding interactions with HSA having binding constant values of 4.24 × 10⁵ M⁻¹ and 4.92 × 10⁵ M⁻¹ respectively.

Microbial interactions govern marine biogeochemistry. These interactions are generally considered to rely on exchange of organic molecules. Here we report on a novel inorganic route of microbial communication, showing that algal-bacterial interactions between Phaeobacter inhibens bacteria and Gephyrocapsa huxleyi algae are mediated through inorganic nitrogen exchange. Under oxygen-rich conditions, aerobic bacteria reduce algal-secreted nitrite to nitric oxide (NO) through denitrification, a well-studied anaerobic respiratory mechanism. The bacterial NO is involved in triggering a cascade in algae akin to programmed cell death. During death, algae further generate NO, thereby propagating the signal in the algal population. Eventually, the algal population collapses, similar to the sudden demise of oceanic algal blooms. Our study suggests that the exchange of inorganic nitrogen species in oxygenated environments is a potentially significant route of microbial communication within and across kingdoms.

3,3′,3′'-(Benzene-1,3,5-triyl)tris(1-phenyl-1H-benzo[e][1,2,4]triazin-4-yl) (1) is a C3-symmetrical triradical comprised of three Blatter radical units connected at the 1, 3, 5 positions of a central trimethylenebenzene core. This triradical has an excellent air, moisture, and thermal stability. Single-crystal XRD indicates that triradical 1 adopts a propeller-like geometry with the benzotriazinyl moieties twisted by 174.1(2)° and packs in 1D chains along the c axis to form an extensive network of weak intermolecular interactions. Frozen solution continuous wave (CW) EPR spectra and variable-temperature field-sweep echo-detected (FSED) spectra revealed an intramolecular ferromagnetic exchange within the spin system, supporting a quartet S = 3/2 ground state. DFT calculations further supported these experimental findings.

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li2OCaO·2SiO2 and Li2CaSiO4, respectively. Using Gd­(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both 6Li and 29Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

Reaction of copper(II) acetate with cyclohexanoneselenosemicarbazone (Hcysesc), 5-chloro isatinselenosemicarbazone (5-ClHIstsesc), 1-methyl isatinselenosemicarbazone (1-MeHIstsesc), 3-indole selenosemicarbazone (3-HInsesc), 3-acetylindole selenosemicarbazone (3-AcHInsesc and 2-naphthaldehyde selenosemicarbazone (2-Hnaphsesc) in 1:2 (M : L) molar ratio yielded complexes of stiochiometry, [Cu(L)2] 1-6 (L = cysesc 1; 5-ClIstsesc 2; 1-MeIstsesc 3; 3-Insesc 4; 3-AcInsesc 5; 2-naphsesc 6). All the complexes are characterized by IR, Mass and ESR spectroscopy. Two well defined g values (g‖ and g⊥) in ESR spectrum of complexes 1-4, 6 indicate axial symmetry, whereas symmetry around copper(II) metal in complex 5 is confirmed by three different g values (g1, 2.095; g2, 2.15; g3, 2.26), which supports rhombic distortion. The value of g‖ obtained is greater than g⊥, which is greater than ge, in 1-4, 6 is in well agreement in dx2-y2 ground term of square planar geometry. The value of empirical factor f for complexes 1-4, 6 lies in the range, 102-135 cm indicating a square planar geometry with small distortion, where as in complex 5 value (f = 146 cm) is in conformity of tetrahedral geometry with larger distortion. All the compounds (selenosemicabazones and their complexes) are tested for DPPH radical scavenging, ABTS radical scavenging, FRAP ferric reducing and CUPRAC cupric reducing antioxidant capacity assay and all the compounds have shown good antioxidant capacity in general. Best results are obtained for H1L with FRAP, 6.75±0.01 μg TE/ml and CUPRAC, 9.52±0.07μg TE/ml. All the compounds are also tested for their anti-tubercular activity against M. tuberculosis H37RV strain ATCC27294. Ligands, H1L, H3L, H6L and complex 6 have shown highest anti-TB activity (MIC= 1.6 µg/ml) which is similar to the standard drugs (Isoniazid, Ethambutol). Anti-TB activity of H2L and H5L (MIC = 12.5 µg/ml and 25 µg/ml) increased on complexation with copper(II) (MIC = 1.6 µg/ml, 2; 12.5µg/ml, 6). Experimental results are supported by molecular modeling studies, where a considerable intermolecular interaction of these compounds with the active site of amino acid of enoyl reductase from M. tuberculosis has been observed with minimum binding energy -6.2, -7.3, -7.7, -7.4, -7.2, -8.7 Kcal/mol for H1L, H2L, H3L, H4L, H5L, and H6L and -7.2, -9.4, -8.6, -9.3, -9.9, and -10.6 Kcal/mol for copper complexes 1, 2, 3, 4, 5 and 6 exhibited respectively.

Large-scale triage after major radiological events, such as nuclear reactor accidents, requires a method of ionizing radiation dose estimation called retrospective biodosimetry (RBD) to detect doses in the range of 0.5–8 Gy. A well-known technique for performing RBD is electron spin resonance (ESR), which can be used to measure radiation-induced paramagnetic defects in the enamel of the teeth. The concentration of these defects is linearly correlated with radiation doses in the applicable range. Despite its great potential and proven results when applied to extracted teeth, ESR still struggles to provide accurate in vivo readings. This is mainly because all available ESR-based RBD methods rely on quantitative signals for calculating the concentration of paramagnetic defects in tooth enamel to evaluate the dose. This requires an accurate knowledge of the volume of the measured enamel, which is very difficult to achieve in live subjects (since teeth also include dentin and possibly cavities). Here, we examine radiation-induced paramagnetic defects in the enamel layer of human teeth using advanced pulsed ESR methods, with the ultimate goal of supporting the development of an innovative practical RBD device for in vivo use. We employ a variety of pulsed ESR techniques, such as ESR measurements of spin–spin relaxation time (T₂), ESR monitoring of instantaneous diffusion decay time (T_ID), and dipolar ESR spectroscopy, to explore their possible use to quantify the irradiation dose. Moreover, we develop a special resonator for teeth measurements that make use of such pulse techniques to overcome the constrains of small signal magnitudes and short coherence times. Our results show a good correlation between measured values of T₂, T_ID, and the irradiated dose, but further work is required to improve the robustness, accuracy, and sensitivity of the methods presented before they could possibly be applied for in vivo measurements in typical doses of ~ 2–8 Gy. These findings and approaches may be used in the future for the development of a RBD device to evaluate ionizing radiation doses without prior knowledge of the measured enamel volume.

The oxoammonium cation (OAC) of 3-carboxy proxyl, a nitroxide radical (NitR), could be produced either by chemical or by electrochemical oxidation (0.8-1.0 V vs. NHE) of the radical. We have determined that in dilute aqueous basic solutions (pH >= 9), OAC is reduced quantitatively to the original radical with concomitant formation of molecular oxygen in a ratio ca. 4 : 1 (4 moles of OAC reduced per 1 mole of O-2), and the redox cycle can be repeated. The low electrolysis potential (0.8 V) contrasts with the high redox potential of the bare OH- anion (2-2.6 V vs. NHE for the first outer-sphere electron transfer). This apparent thermodynamic paradox was solved by a careful study of its possible mechanism. In our opinion, OAC/NitR's may represent a new class of redox mediators for a novel approach to water oxidation (and generation of molecular oxygen) at a low potential.

Aptamer-functionalized Ce4+-ion-modified C-dots act as catalytic hybrid systems, aptananozymes, catalyzing the H2O2 oxidation of dopamine. A series of aptananozymes function-alized with different configurations of the dopamine binding aptamer, DBA, are introduced. All aptananozymes reveal substantially enhanced catalytic activities as compared to the separated Ce4+-ion-modified C-dots and aptamer constituents, and structure-catalytic functions between the structure and binding modes of the aptamers linked to the C-dots are demonstrated. The enhanced catalytic functions of the aptananozymes are attributed to the aptamer-induced concentration of the reaction substrates in spatial proximity to the Ce4+-ion-modified C-dots catalytic sites. The oxidation processes driven by the Ce4+-ion-modified C-dots involve the formation of reactive oxygen species (center dot OH radicals). Accordingly, Ce4+-ion-modified C-dots with the AS1411 aptamer or MUC1 aptamer, recognizing specific biomarkers associated with cancer cells, are employed as targeted catalytic agents for chemodynamic treatment of cancer cells. Treatment of MDA-MB-231 breast cancer cells and MCF-10A epithelial breast cells, as control, with the AS1411 aptamer-or MUC1 aptamer-modified Ce4+-ion-modified C-dots reveals selective cytotoxicity toward the cancer cells. In vivo reveal that the inhibit MDA-MB-231 tumor

Polyadenine-stabilized Au nanoparticles (pA-AuNPs) reveal dual nanozyme catalytic activities toward the H2O2-mediated oxidation of dopamine to aminochrome and toward the aerobic oxidation of glucose to gluconic acid and H2O2. The conjugation of a dopamine-binding aptamer (DBA) to the pA-AuNPs yields aptananozyme structures catalyzing simultaneously the H2O2-mediated oxidation of dopamine to aminochrome through the aerobic oxidation of glucose. A set of aptananozymes consisting of DBA conjugated through the 5 '-or 3 '-end directly or spacer bridges to pA-AuNPs were synthesized. The set of aptananozymes revealed enhanced catalytic activities toward the H2O2-catalyzed oxidation of dopamine to dop-achrome, as compared to the separated pA-AuNPs and DBA constituents, and structure-function relationships within the series of aptananozymes were demonstrated. The enhanced catalytic function of the aptananozymes was attributed to the concentration of the dopamine at the catalytic interfaces by means of aptamer-dopamine complexes. The dual catalytic activities of aptananozymes were further applied to design bioreactors catalyzing the effective aerobic oxidation of dopamine in the presence of glucose. Mechanistic studies demonstrated that the aptananozymes generate reactive oxygen species. Accordingly, the AS1411 aptamer, recognizing the nucleolin receptor associated with cancer cells, was conjugated to the pA-AuNPs, yielding a nanozyme for the chemodynamic treatment of cancer cells. The AS1411 aptamer targets the aptananozyme to the cancer cells and facilitates the selective permeation of the nanozyme into the cells. Selective cytotoxicity toward MDA-MB-231 breast cancer cells (ca. 70% cell death) as compared to MCF-10A epithelial cells (ca. 2% cell death) is demonstrated.

Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of -0.23 vs. Fc/ Fc(+), and are partially oxidized in ambient conditions. Additional oxidation to the dication results in aromaticity switching to a global 30 pi e(-) aromatic state, as indicated by the formation of a strong diatropic current observed in the H-1 NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states.

Small molecule redox mediators convey interfacial electron transfer events into bulk solution and can enable diverse substrate activation mechanisms in synthetic electrocatalysis. Here, we report that 1,2-diiodo-4,5-dimethoxybenzene is an efficient electrocatalyst for C–H/E–H coupling that operates at as low as 0.5 mol % catalyst loading. Spectroscopic, crystallographic, and computational results indicate a critical role for a three-electron I–I bonding interaction in stabilizing an iodanyl radical intermediate (i.e., formally I­(II) species). As a result, the optimized catalyst operates at more than 100 mV lower potential than the related monoiodide catalyst 4-iodoanisole, which results in improved product yield, higher Faradaic efficiency, and expanded substrate scope. The isolated iodanyl radical is chemically competent in C–N bond formation. These results represent the first examples of substrate functionalization at a well-defined I­(II) derivative and bona fide iodanyl radical catalysis and demonstrate one-electron pathways as a mechanistic alternative to canonical two-electron hypervalent iodine mechanisms. The observation establishes I–I redox cooperation as a new design concept for the development of metal-free redox mediators.

The conversion of CO2 and epoxides to cyclic carbonates over a silica-supported di-iron­(III) complex having a reduced Robson macrocycle ligand system is shown to proceed at 1 atm and 80 °C, exclusively producing the cis-cyclohexene carbonate from cyclohexene oxide. We examine the effect of immobilization configuration to show that the complex grafted in a semirigid configuration catalytically outperforms the rigid, flexible configurations and even the homogeneous counterparts. Using the semirigid catalyst, we are able to obtain a TON of up to 800 and a TOF of up to 37 h–1 under 1 atm CO2. The catalyst is shown to be recyclable with only minor leaching and no change to product selectivity. We further examine a range of epoxides with varying electron-withdrawing/donating properties. This work highlights the benefit arising from the constraining effect of a solid surface, akin to the role of hydrogen bonds in enzyme catalysts, and the importance of correctly balancing it.

Carbonaceous materials are ubiquitous in energy storage and conversion systems due to their versatile chemical and physical properties. The surface chemistry of carbonaceous materials strongly affects their properties, and there is therefore great interest in determining the chemical composition of different carbon allotropes. Solid-state nuclear magnetic resonance spectroscopy is well suited for providing atomic-level structural information, especially when equipped with sensitivity from dynamic nuclear polarization (DNP). Exogenous DNP from nitroxide biradicals is the most efficient approach, typically providing 102–104 fold enhancement in surface sensitivity. Herein, we consider the application of DNP in the study of the surface chemistry of carbonaceous materials through natural abundance 13C detection. We found that with TEKPol biradicals, the polarization transfer via 1H-13C cross-polarization is limited to the solvent and does not propagate to the sample surface. We thus investigated in detail polarization transfer directly to 13C nuclei and found multiple interfering mechanisms when employing the exogenous DNP approach: (a) direct 13C polarization from the biradicals, (b) opposite enhancements due to heteronuclear cross-relaxation leading to the solvent Overhauser effect, and (c) solid effect from defects and delocalized electrons within the carbons. While the endogenous electron polarization interferes with the utilization of exogenous DNP, it provides significant surface sensitivity with signal enhancements of up to 50 and 20 for 13C and 1H, respectively. Moreover, we show that endogenous DNP can be used at a wide range of temperatures, providing close to a 10-fold increase in 13C and 1H signals at room temperature through differing DNP mechanisms. This approach opens the way for efficient detection of carbon surface chemistry under ambient conditions.

The olefin metathesis reaction is among the most widely applicable catalytic reactions for carbon–carbon double bond formation. Currently, Mo– and Ru–carbene catalysts are the most common choices for this reaction. It has been suggested that an iron-based catalyst would be a desirable economical and biocompatible substitute of the Ru catalysts; however, practical solutions in this regard are still lacking. Here, we report the discovery and mechanistic studies of three-coordinate iron(II) catalysts for ring-opening metathesis polymerization of olefins. Remarkably, their reactivity enabled the formation of polynorbornene with stereoregularity and high molecular weight (>107 g mol–1). The polymerization in the presence of styrene revealed cross metathesis reactivity with iron catalysts. Mechanistic studies suggest the possible role of metal–ligand cooperation in formation of the productive catalyst. This work opens the door to the development of iron complexes that can be economical and biocompatible catalysts for olefin metathesis reactions.

Teeth are usually targeted for dating archaeological sites because they are less prone to dissolution, in com-parison with bones. However, despite this apparent resistance, teeth do undergo diagenesis, which needs to beaccounted for in order to obtain accurate ages. In particular, the uptake of trace elements such as uranium indental tissues needs to be considered for dose rate determination when dated using electron spin resonance(ESR). Characterising the mineralogy and structural integrity of samples prior to dating may thus provideimportant information related to their state of preservation, especially in the case of teeth whose U content cansignificantly affect the dose rate.In this study, we dated five teeth of small-sized bovids using combined ESR/U-series dating. They werecollected at the Middle Stone Age site of Lovedale, located in the central interior of South Africa. Micromor-phology provided sedimentary context to the samples, which were recovered from a layer of gravel rich in faunalremains. Using cathodoluminescence, laser-induced fluorescence, Fourier transform infrared spectroscopy andRaman micro-spectroscopy we assessed the degree of preservation of the enamel. Results reveal that carbonatehydroxyapatite underwent post-depositional alteration, based on its molecular structure and elemental compo-sition. Although the teeth all originate from the same layer and were sampled in the same 1-m square and at asimilar elevation, U-content in the enamel differs highly from one tooth to the other, with values ranging from1.7 to 29.6 ppm. These values are correlated with equivalent doses (De) from 228 to 923 Gy and are consistentwith variations in crystallinity determined with vibrational spectroscopy. We also investigated the possiblesaturation of the ESR signal, by repeating measurements with microwave power values from 1 to 20 mW.Despite such diversity in U-content, the ages calculated assuming an early uptake of U all fall within the samerange, from 63 ± 8 ka to 68 ± 15 ka and may only represent a minimum estimate.

The creation of radicals via electrical discharge is a unique approach capable of facilitating dynamic nuclear polarization (DNP) in solids without the need to co-dissolve them with exogenous polarization agents. This method has previously been shown to generate high proton polarization in a variety of organic and inorganic solid compounds. In this work, we extend the scope of applicability for these so-called electrical discharge-induced radicals (EDIRs), by showing their use in DNP of 13C nuclei of glucose. We achieved significant NMR signal enhancement in low temperature solid-state DNP experiments. This enhancement was further improved by the use of a frequency modulated microwave pump instead of conventional DNP with single-frequency microwaves. We demonstrated successful dissolution DNP experiments with such hyperpolarized solid glucose, achieving 13C polarization levels of up to ∼0.6% in the resulting solution (∼0.95% at the exit of the polarizer). This is to be compared with the ∼6% polarization levels achieved with a trityl radical mixed with glucose in a frozen aqueous solution using the same DNP setup and experimental conditions. It is concluded that the use of EDIRs can serve as a suitable polarizing approach for metabolic MRI using glucose, avoiding the need to employ diluted solutions and eliminating the filtration stage of the polarizing agents.

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T1/T2, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMg1−xMnxPO4 and Fe(III) doped Li4Ti5O12, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300 K.

The covalent linkage of aptamer binding sites to nanoparticle nanozymes is introduced as a versatile method to improve the catalytic activity of nanozymes by concentrating the reaction substrates at the catalytic nanozyme core, thereby emulating the binding and catalytic active-site functions of native enzymes. The concept is exemplified with the synthesis of Cu2+ ion-functionalized carbon dots (C-dots), modified with the dopamine binding aptamer (DBA) or the tyrosinamide binding aptamer (TBA), for the catalyzed oxidation of dopamine to aminochrome by H2O2 or the oxygenation of l-tyrosinamide to the catechol product, which is subsequently oxidized to amidodopachrome, in the presence of H2O2/ascorbate mixture. Sets of structurally functionalized DBA-modified Cu2+ ion-functionalized C-dots or sets of structurally functionalized TBA-modified Cu2+ ion-functionalized C-dots are introduced as nanozymes of superior catalytic activities (aptananozymes) toward the oxidation of dopamine or the oxygenation of l-tyrosinamide, respectively. The aptananozymes reveal enhanced catalytic activities as compared to the separated catalyst and respective aptamer constituents. The catalytic functions of the aptananozymes are controlled by the structure of the aptamer units linked to the Cu2+ ion-functionalized C-dots. In addition, the aptananozyme shows chiroselective catalytic functions demonstrated by the chiroselective-catalyzed oxidation of l/d-DOPA to l/d-dopachrome. Binding studies of the substrates to the different aptananozymes and mechanistic studies associated with the catalytic transformations are discussed.

Metal–ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]−1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

Encapsulated atomic hydrogen in silsesquioxane cages is a promising candidate for applications in emerging technologies like spin-based quantum computing, magnetic field sensing, and atomic clock devices. Previous studies on different polyhedral octasilsesquioxanes (POSS) of the type Si8O12R8 have shown that key parameters for quantum computing like electron spin relaxation times T 1 and T M depend strongly on the type of peripheral organic substituents. Herein we examine for the first time the effect of deuterium isotopic substitution on the spin relaxation properties of H@h 72Q8M8, the derivative with R = OSi­(CH3)3, by applying pulsed electron paramagnetic resonance (EPR) methods on its deuterated analogues H@d 72Q8M8 and D@d 72Q8M8. For the latter species we measure a phase memory time of 60 μs at 190 K, the largest obtained so far for this family of molecular spins. We show that substitution of peripheral hydrogen atoms with deuterium reveals high-temperature relaxation mechanisms that were previously hidden by proton nuclear spin diffusion. Unusually short T M values observed for all deuterated species even at liquid helium temperatures are discussed in terms of tunneling reorientation of methyl groups.

Charge-transfer crystals exhibit unique electronic and magnetic properties with interesting applications. The charge-transfer single crystal formed by dibenzotetrathiafulvalene (DBTTF) together with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) presents a long-range ordered supramolecular structure of segregated stacks, with a unitary degree of charge transfer. Thus, the crystal structure is composed of dimerized radical molecules with unpaired electrons. The energy levels and the spin degrees of freedom of this material were investigated by solid-state electrochemistry and electron paramagnetic resonance (EPR) spectroscopy. The electrochemical data, supported by density functional theory calculations, show how this organic Mott insulator has an electronic gap in the range of hundreds of meV. EPR experiments show the presence of a ground-state S = 1 triplet spin state along with localized S = 1/2 spins. The calculations also predict a ground-state triplet configuration, with the singlet configuration at 170 meV higher energy. DBTTF/F4TCNQ seems to be a candidate material for organic electronic and spintronic applications.

The steady-state charge and spin transfer yields were measured for three different Ru-modified azurin derivatives in protein films on silver electrodes. While the charge-transfer yields exhibit weak temperature dependences, consistent with operation of a near activation-less mechanism, the spin selectivity of the electron transfer improves as temperature increases. This enhancement of spin selectivity with temperature is explained by a vibrationally induced spin exchange interaction between the Cu(II) and its chiral ligands. These results indicate that distinct mechanisms control charge and spin transfer within proteins. As with electron charge transfer, proteins deliver polarized electron spins with a yield that depends on the protein's structure. This finding suggests a new role for protein structure in biochemical redox processes.

Degradation processes at the cathode–electrolyte interface are a major limitation in the development of high-energy lithium-ion rechargeable batteries. Deposition of protective thin coating layers on the surface of high-energy cathodes is a promising approach to control interfacial reactions. However, rational design of effective protection layers is limited by the scarcity of analytical tools that can probe thin, disordered, and heterogeneous phases. Here we propose a new structural approach based on solid-state nuclear magnetic resonance spectroscopy coupled with dynamic nuclear polarization (DNP) for characterizing thin coating layers. We demonstrate the approach on an efficient alkylated LixSiyOz coating layer. By utilizing different sources for DNP, exogenous from nitroxide biradicals and endogenous from paramagnetic metal ion dopants, we reveal the outer and inner surface layers of the deposited artificial interphase and construct a structural model for the coating. In addition, lithium isotope exchange experiments provide direct evidence for the function of the surface layer, shedding light on its role in the enhanced rate performance of coated cathodes. The presented methodology and results advance us in identifying the key properties of effective coatings and may enable rational design of protective and ion-conducting surface layers.

Ecofriendly hydrothermal reduction of graphene oxide is widely used for producing hydrogels and aerogels, but it yields partly reduced graphene oxide (prGO) containing oxygen groups and some number of paramagnetic centers (PCs). In order to identify structural changes introduced by the reduction process, these PCs are studied by electron spin echo spectroscopy in the temperature range of 5-160 K. Two types of PCs with different spin-lattice (T1) and phase memory (Tm) relaxation times observed below 20 K result from a nonuniform distribution of magnetic defects. Above 20 K, only the PCs with the shorter T1 and Tm persist. Temperature dependences of T1 and the distribution of T1 for each type of the PCs reveal lattice distortions around the PCs and structural disorder in prGO. The unusually strong temperature dependence of the spin echo intensity is explained by the localization of conduction electrons. The localization is destroyed at high temperature, and exchange interactions decrease the number of the observed PCs. Every such PC is created by the sp3 defect induced by hydrogen covalently bonded to graphene. The obtained results indicate that the hydrothermal reduction is accompanied by partial hydrogenation of graphene. The presence of such hydrogen atoms is confirmed by infrared spectroscopy.

We present a thoroughgoing electron paramagnetic resonance investigation of polydopamine (PDA) radicals using multiple electron paramagnetic resonance techniques at the W-band (94 GHz), electron nuclear double resonance at the Q-band (34 GHz), spin relaxation, and continuous wave measurements at the X-band (9 GHz). The analysis proves the existence of two distinct paramagnetic species in the PDA structure. One of the two radical species is characterized by a long spin-lattice T1 relaxation time equal to 46.9 ms at 5 K and is assigned to the radical center on oxygen. The obtained data revealed that the paramagnetic species exhibit different electron spin relaxation behaviors due to different couplings to local phonons, which confirm spatial distancing between two radical types. Our results shed new light on the radical structure of PDA, which is of great importance in the application of PDA in materials science and biomedicine and allows us to better understand the properties of these materials and predict their future applications.

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal–organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above ∼50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

The arrangement of two-dimensional graphene oxide sheets has been shown to influence physico-chemical properties of the final bulk structures. In particular, various graphene oxide microfibers remain of high interest in electronic applications due to their wire-like thin shapes and the ease of hydrothermal fabrication. In this research, we induced the internal ordering of graphene oxide flakes during typical hydrothermal fabrication via doping with Calcium ions (similar to 6 wt.%) from the capillaries. The Ca2+ ions allowed for better graphene oxide flake connections formation during the hydrogelation and further modified the magnetic and electric properties of structures compared to previously studied aerogels. Moreover, we observed the unique pseudo-porous fiber structure and flakes connections perpendicular to the long fiber axis. Pulsed electron paramagnetic resonance (EPR) and conductivity measurements confirmed the denser flake ordering compared to previously studied aerogels. These studies ultimately suggest that doping graphene oxide with Ca2+ (or other) ions during hydrothermal methods could be used to better control the internal architecture and thus tune the properties of the formed structures.

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li 4Ti 5O 12. In addition for being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.

Acridine-based PNP pincer complexes have been previously utilized for several environmentally benign catalytic processes. In light of the recent growth in interest in base-metal catalysis, we report here the synthesis of acridine-PNP pincer complexes of Ni, Co, Fe, and Mn. We also report here the noninnocent redox nature of these complexes that results in the dimerization of pincer complexes by forming a C-C bond at the C9 position of the acridine ring.

It has been hypothesized that the cytotoxicity of secondary organic aerosols (SOA) is mediated through the formation of reactive oxygen species (ROS) in the exposed cells. Here, lung epithelial cells (A549) residing at the air-liquid interface were exposed to proxies of anthropogenic and biogenic SOA that were photochemically aged under varying nitrogen oxide (NOx) concentrations in an oxidation flow reactor. The total organic peroxides and ROS radical content in the SOA were quantified by the iodometric spectrophotometric method and by continuous-wave electron paramagnetic resonance. The effect of the exposure was evaluated by measuring cell viability and cellular ROS production following the exposure. The results demonstrate that SOA that aged in the absence of NOx contained more ROS than fresh SOA and were more toxic toward the cells, while varying NOx conditions had no significant influence on levels of the ROS content in fresh SOA and their toxicity. Analysis of ROS in the exposed cells using flow cytometry showed a similar trend with the total ROS content in the SOA. This study provides a first and direct observation of such association.

Paramagnetic centers in graphene oxide (GO) were studied by continuous wave and pulsed electron paramagnetic resonance (EPR) in the temperature range 4.2-300 K. Saturation of the EPR signal indicates long spin relaxation times of the paramagnetic centers and show that the relaxation times increase with lowering temperature as well as with decreasing the number of water molecules by drying. Long spin-lattice and phase memory relaxation times in deeply dried GO are quantitatively studied by the pulsed EPR techniques. The spin-lattice relaxation time is found to decrease from 52 ms at 5 K to 0.153 ms at 240 K and is dominated by the direct process below 100 K. The T-5 dependence observed at higher temperatures can result from Raman processes. The phase memory time is about 1 mu s at 240 K, changes non-monotonically with lowering temperature, and reaches its maximum of 2.2 mu s at 5 K. Molecular motions of the functional groups and the adsorbed water molecules are employed to explain this dependence. The paramagnetic centers are attributed to the unfunctionalized carbons in the highly functionalized regions of GO. Their interactions with protons are confirmed by ESEEM. Slow relaxation extends possible applications of GO for quantum computing and spintronics. (C) 2019 Elsevier Ltd. All rights reserved.

A new and simple design of electrochemical setup for in-situ generation of free radicals to be measured using X-band (9.5 GHz) electron paramagnetic resonance (EPR) spectrometer is described. The cell parts are all from commercially available components and requires no specially made glassware. The EPR performance of the setup is demonstrated by in-situ electrochemical generation of organic radical cations and radical anions such as quinones, perylene-diimide, pyrene, flavin, tryptophan and tyrosine through oxidation or reduction in solution. The well-resolved EPR spectra of the radical products were simulated and analyzed and the hyperfine coupling constants have been assigned for the interactions of the unpaired electron with the various ring protons and the nitrogen nuclei.

Triarylmethyl (TAM or trityl) radicals are becoming important for measuring distances in proteins and nucleic acids. Here, we report on a new trityl spin label CT02MA, which conjugates to a protein via a redox stable thioether bond. The performance of the new spin label was demonstrated in W-band double electron-electron resonance (DEER) distance measurements on doubly trityl-labelled mutants of immunoglobulin G-binding protein 1 (GB1) and ubiquitin. For both doubly CT02MA-labelled proteins we measured, by applying chirped pump pulse(s), relatively narrow distance distributions, comparable to those obtained with the same protein mutants doubly labelled with BrPy-DO3MA-Gd(III). We noticed, however, that the sample contained some free CT02MA that was difficult to remove at the purification step. Dual labelling of ubiquitin with one CT02MA tag and one BrPy-DO3MA-Gd(III) tag was achieved as well and the trityl-Gd(III) distance distribution was measured, facilitated by the use of a dual mode cavity in combination with a chirped pump pulse. We also measured the Gd(III)-Gd(III) distance distribution in this sample, showing that the labelling procedure was not fully selective. Nevertheless, these measurements demonstrate the potential of the high sensitivity Gd(III)-trityl W-band DEER distance measurements in proteins, which can be further exploited by designing orthogonal Gd(III)/ trityl labelling schemes.

Commonly, iron(ii) and copper(i) complexes bind dioxygen (O-2) and then activate O-2 through a reductive reaction pathway. There is, however, significant interest in low temperature oxygenation with O-2 without the use of a sacrificial reductant. Here, earth-abundant metal complexes (Fe-II, Co-II, Ni-II and Cu-II) coordinated by two different tetra-dentate mono-carbon bridged bis-phenanthroline ligands, (1,10-Phen)(2)-2,2-CR1R2, where R-1 = n-butyl and R-2 = n-butyl or H were synthesized. The structures all showed the expected metal complexation in the equatorial plane by the bridged bis-phenanthroline ligands. For R-1 = n-butyl; R-2 = H, where the ligand has a tertiary carbon bridging group, fast intramolecular oxygenation occurred at the pseudobenzylic position. Depending on the transition metal the main products formed were oxygen bridged dimers of the metal complexes (Co and Fe) or metal complexes with a carbonyl moiety at the bridging pseudobenzylic position as a result of C-R-1 bond cleavage (Ni and Cu). The different product assemblages are explained by different reaction pathways that are metal specific. For quaternary carbon bridged ligands, R-1 = R-2 = n-butyl, the complexes catalytically activated C-H bonds of cyclohexene under catalytic conditions, showing higher effective turnover numbers at low catalyst loading. The reactivity observed is commensurate with a room temperature autooxidation reaction although the initiation of the free radical reaction is metal specific. In general labelling studies with O-18(2), UV-vis and EPR spectroscopy as well as cyclic voltammetry measurements led to a conclusion that the reaction pathways involved both C-H bond activation and O-2 activation.

We introduce the first series of enantiopure twistacene radical cations, which form reversibly upon chemical or electrochemical oxidation. Their vis-NIR chiroptical properties (Cotton effect and anisotropy factor) increase systematically with the backbone twist. The hyperfine constants observed by EPR demonstrate significant spin delocalization even for large backbone twist angles.

Dynamic nuclear polarization (DNP) for the enhancement of the NMR signals of specific metabolites has recently found applications in the context of magnetic resonance imaging (MRI). Currently, DNP signal enhancement is implemented in clinical systems through the use of exogenous stable organic free radicals, known as polarization agents (PAs), mixed in a solution with the metabolite of interest. These PAs are medically undesirable and thus must be filtered out prior to patient injection - a task that involves considerable technical complexity and consumes valuable time during which the polarization decays. Here, we aim to demonstrate DNP enhancements large enough for clinical relevance using a process free of exogenous PAs. This is achieved by processing (soft grinding) the metabolite in its solid form and subsequently exposing it to plasma in a dilute atmosphere to produce chemically-unstable free radicals (herein referred to as electrical-discharge-induced radicals EDIRs) within the powder. These samples are then subjected to the normal DNP procedure of microwave irradiation while placed under a high static magnetic field, and their NMR signal is measured to quantify the enhancement of the protons' signal in the solid. Proton signal enhancements (measured as the ratio of the NMR signal with microwave irradiation to the NMR signal without microwave irradiation) of up to 150 are demonstrated in glucose. Upon fast dissolution, the free radicals are annihilated, leaving the sample in its original chemical composition (which is safe for clinical use) without any need for filtration and cumbersome quality control procedures. We thus conclude that EDIRs are found to be highly efficient in providing DNP enhancement levels that are on par with those achieved with the exogenous PAs, while being safe for clinical use. This opens up the possibility of applying our method to clinical scenarios with minimal risks and lower costs per procedure.

We study GaAs/AlGaAs devices hosting a two-dimensional electron gas and coated with a monolayer of chiral organic molecules. We observe clear signatures of room-temperature magnetism, which is induced in these systems by applying a gate voltage. We explain this phenomenon as a consequence of the spin-polarized charges that are injected into the semiconductor through the chiral molecules. The orientation of the magnetic moment can be manipulated by low gate voltages, with a switching rate in the megahertz range. Thus, our devices implement an efficient, electric field-controlled magnetization, which has long been desired for their technical prospects.

We report a C−C bond-forming reaction between benzyl alcohols and alkynes in the presence of a catalytic amount of KO ^t Bu to form α-alkylated ketones in which the C=O group is located on the side derived from the alcohol. The reaction proceeds under thermal conditions (125 °C) and produces no waste, making the reaction highly atom efficient, environmentally benign, and sustainable. Based on our mechanistic investigations, we propose that the reaction proceeds through radical pathways.

A method for implementing a secret sharing scheme at the molecular level is presented. By creating molecular code generators that are self-assembled from several molecular components, we established a means for distributing distinct code-activating elements among several participants. In this way, an authorization code can only be generated when all the participants are present, which ensures that highly secured systems cannot be operated by unauthorized individuals or disloyal users. Additional layers of protection result from the ability to program the security code by replacing one or several molecular components and by subjecting the system to distinct chemical inputs.

The magnetic properties of undoped, bulk CeO2 are not fully understood. In contrast to nanocrystalline ceria that exhibits paramagnetism attributed to Ce3+ at grain surfaces, bulk ceria is weakly paramagnetic, despite the absence of magnetic ions. In the present work, the magnetic susceptibility of bulk ceria ceramics doped with Lu3+, which has neither spin nor orbital angular momentum, was measured in order to assess the relative contributions of the crystal lattice, residual Ce3+ and oxygen vacancies to the overall bulk magnetization. We observed a magnetic response consisting of two parts: temperature independent (5-300 K) magnetic susceptibility, and Curie-Weiss paramagnetism. The temperature independent susceptibility decreases linearly with Lu content, and becomes diamagnetic at 30 mol% Lu. The Curie-Weiss magnetism visible at low temperatures was identified as resulting from a few ppm of Fe contaminant. However, Fe contamination does not contribute to the temperature independent paramagnetism. No contribution from Ce3+ could be detected. The fact that the magnetization decreases with Lu content, even though the concentration of oxygen vacancies, and the lattice defects associated with them, increases, indicates that neither is coupled to the magnetic field. Weak, temperature-independent paramagnetism in non-metals is usually attributed to a second order, Van Vleck-type magnetization. However, Van Vleck paramagnetism requires that the population of the first excited state be constant within the range of temperatures investigated. We discuss possible modifications of the large band gap electronic structure of undoped ceria which could account for our observations.

The electrochemical reduction of CO2 has been extensively investigated in recent years, with the expectation that a detailed mechanistic understanding could achieve the goal of finding a stable catalyst with high turnover frequencies and low reduction potentials. In the catalytic cycle of the carbon dioxide hydrogenase enzyme, it has been suggested that the reduced metal center reacts with CO2 to form a carboxylate intermediate that is stabilized by hydrogen bonding using a histidine moiety in the second coordination sphere. Using the well-known fac-Re(I)bipyridine(CO)(3)Cl complex as a starting point, the bipyridine ligand was modified in the second coordination sphere with a thiourea tether that is known to form hydrogen bonds with carbonyl moieties. The resulting Re(I) catalyst was an excellent electrocatalyst for the selective reduction of CO2 to CO, with a turnover frequency of 3040 s(-1). The binding of CO2 to the thiourea tether was observable by H-1 NMR, and NOE experiments showed that the hydrogen atoms of the thiourea group were labile. Further experiments indicated that the thiourea moiety is also a local proton source and addition of an external proton source actually inhibits catalysis. The absence of a kinetic isotope effect was explained through DFT calculations that showed that the proton invariably jumps to the nearest CO2 oxygen atom to form a metal-carboxylic acid without going through any minimum or transition state. EPR and NMR spectroscopies were used to identify the various reduced intermediates. Thus, the thiourea tether in the second coordination sphere can bind CO2, stabilize carboxylic acid reaction intermediates, and directly act as a local proton source, leading to a significantly more active catalyst.

The selective oxidation of light hydrocarbons and their valorization with only dioxygen (O-2) are important transformations toward development of efficient chemical processes. Monooxygenase enzymes can catalyze selective aerobic reactions under reducing and protic conditions. The translation of such enzymatic pathways to the practical electrocatalytic oxidation of light, gaseous hydrocarbons, using O-2 as sole oxidant is now reported. An iron tungsten oxide inorganic molecular catalyst with a capsular structure {Fe30W72} stabilized inside by sulfate/bisulfate anions provides a protic environment where three iron atoms are located at each of the pores of the capsule leading to a unique and potent active site for the oxidation reactions. Under mild electrochemical conditions, 1.8 V, in water at room temperature, using O-2 from air, we demonstrate the low-pressure (1-2 bar) hydroxylation of alkanes, notably ethane to acetic acid, and the ozone like cleavage of the carbon carbon double bonds of alkenes. Typical turnover frequencies were 300-400 min(-1). Initial mechanistic studies support a reaction through a very active iron-oxo species.

Spin-polarized electrons are injected from an electrochemical cell through a chiral self-assembled organic monolayer into a AlGaN/GaN device in which a shallow two-dimensional electron gas (2DEG) layer is formed. The injection is monitored by a microwave signal that indicates a coherent spin lifetime that exceeds 10 ms at room temperature. The signal was found to be magnetic field independent; however, it depends on the current of the injected electrons, on the length of the chiral molecules, and on the existence of 2DEG.

The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [(CoW12O40)-W-III](5-) as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR. Hydrogen was formed at the Pt cathode. The sum transformation is ArH+H2OArOH+H-2. Non-optimized reaction conditions showed a Faradaic efficiency of 75% and selective formation of the mono-oxidized product in a 35% yield. Decomposition of formic acid into CO2 and H-2 is a side-reaction.

Phenolic compounds are common constituents of atmospheric aerosols. They form by pyrolysis of lignin and by biodegradation of plant material and are commonly found in biomass burning plumes, resuspended soil dust, and in anthropogenic secondary organic aerosols (SOA). In this study, we show that reactions of Fe(III), a major constituent of mineral dust, with several phenolic compounds (guaiacol, catechol, syringol, o- and p-cresol) that are common in atmospheric aerosols, result in the formation of water insoluble light-absorbing compounds and reduced Fe(II). The study was conducted under acidic conditions (pH = 1-2), relevant for areas impacted by biomass burning, anthropogenic emissions, and mineral dust. The reaction products have been characterized using a high-performance liquid chromatography coupled to photodiode array and high resolution mass spectrometry detectors, UV-visible spectroscopy, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. The major identified chromophores are oligomers of the reaction precursors that efficiently absorb light between 300 and 500 nm. The amounts of oligomers vary significantly between the systems studied. The highest amount was observed for guaiacol and catechol, and the least were detected in the syringol experiments, suggesting that the oligomerization proceeds through carbon-carbon coupling preferred at para- and ortho- positions, coupled to the reduction of Fe(III) to Fe(II). The results suggest that aqueous-phase radical reactions of phenolic compounds may be an efficient source of light-absorbing atmospheric organic compounds (brown carbon) that play important roles in Earth's radiative forcing on global and regional scales and of quinones that can affect health.

Many applications of copper nanoparticles (Cu-NPs) have been suggested in recent years, although the potential for use of Cu-NPs in water treatment processes has received relatively little attention. This work highlights the preparation, characterization, and application of polyethylenimine capped copper nanoparticles for use in oxidative degradation of organic pollutants in aqueous solutions; atrazine was selected as a representative pollutant. A stable aqueous Cu-NP suspension was prepared, with polyethylenimine (PEI) as capping agent, under ambient conditions. The Cu:PEI ratio during Cu-NP synthesis has a significant influence on nanoparticle properties as well as on the degradation of atrazine. The synthesized Cu-NPs, which comprised a mixture of Cu-0 and Cu2O, induced rapid atrazine degradation (> 99 % in 1 h) and significantly superior performance over commercial nano-copper oxide powder. Mechanistic insight into the atrazine degradation, via electron spin resonance (ESR) measurements, demonstrated (i) that significant hydroxyl radicals were generated only in the presence of Cu-NPs, (ii) longevity of radical generation, and (iii) regeneration of hydroxide radicals. The efficiency of the Cu-NPs applied to oxidative degradation was further demonstrated on eight other representative organic water pollutants.

The valorization of alkanes is possible via carbon carbon coupling reactions. A series of dialkyl cobalt complexes RRCH2)(2)Co-III(bpy)(2)]ClO4 (R = H, Me, Et, and Ph) were reacted with the H5PV2Mo10O40 polyoxometalate as a catalyst, leading to a selective oxidative carbon carbon bond coupling reaction. The reaction is initiated by electron transfer from [(RCH2)(2)Co-III(bpy)(2)](+) to (H5PV2Mo10O40)-Mo-V to yield an intermediate [(RCH2)(2)Co-IV(bpy)(2)](2+)-(H5PVVMo10O40)-V-IV-Mo-V, as identified by a combination of EPR and X-ray photoelectron spectroscopy experiments. The reaction is catalytic with O-2 as terminal oxidant representing an aerobic C-C bond coupling reaction.

We demonstrated a heterogeneous and efficient photocatalytic method to reduce the bacterial content in water with reusable metallo-organic thin films. These functionalized assemblies constructed of a network of polypyridyl complexes generate reactive singlet oxygen upon visible light irradiation that induces lysis of bacterial cells. The singlet oxygen photosensitizers are robust and were successfully applied to surface water samples contaminated with both Gram-positive and -negative bacteria. The catalytic formation of singlet oxygen by our antibacterial coatings was confirmed chemically by reactions with both inorganic and organic compounds.

Chirp and shaped pulses have been recently shown to be highly advantageous for improving sensitivity in DEER (double electron–electron resonance, also called PELDOR) measurements due to their large excitation bandwidth. The implementation of such pulses for pulse EPR has become feasible due to the availability of arbitrary waveform generators (AWG) with high sampling rates to support pulse shaping for pulses with tens of nanoseconds duration. Here we present a setup for obtaining chirp pulses on our home-built W-band (95 GHz) spectrometer and demonstrate its performance on Gd(III)-Gd(III) and nitroxide-nitroxide DEER measurements. We carried out an extensive optimization procedure on two model systems, Gd(III)-PyMTA–spacer–Gd(III)-PyMTA (Gd-PyMTA ruler; zero-field splitting parameter (ZFS) D ∼ 1150 MHz) as well as nitroxide–spacer–nitroxide (nitroxide ruler) to evaluate the applicability of shaped pulses to Gd(III) complexes and nitroxides, which are two important classes of spin labels used in modern DEER/EPR experiments. We applied our findings to ubiquitin, doubly labeled with Gd-DOTA-monoamide (D ∼ 550 MHz) as a model for a system with a small ZFS. Our experiments were focused on the questions (i) what are the best conditions for positioning of the detection frequency, (ii) which pump pulse parameters (bandwidth, positioning in the spectrum, length) yield the best signal-to-noise ratio (SNR) improvements when compared to classical DEER, and (iii) how do the sample's spectral parameters influence the experiment. For the nitroxide ruler, we report an improvement of up to 1.9 in total SNR, while for the Gd-PyMTA ruler the improvement was 3.1–3.4 and for Gd-DOTA-monoamide labeled ubiquitin it was a factor of 1.8. Whereas for the Gd-PyMTA ruler the two setups pump on maximum and observe on maximum gave about the same improvement, for Gd-DOTA-monoamide a significant difference was found. In general the choice of the best set of parameters depends on the D parameter of the Gd(III) complex.

Singlet oxygen plays a role in cellular stress either by providing direct toxicity or through signaling to initiate death programs. It was therefore of interest to examine cell death, as occurs in Arabidopsis, due to differentially localized singlet oxygen photosensitizers. The photosensitizers rose bengal (RB) and acridine orange (AO) were localized to the plasmalemma and vacuole, respectively. Their photoactivation led to cell death as measured by ion leakage. Cell death could be inhibited by the singlet oxygen scavenger histidine in treatments with AO but not with RB. In the case of AO treatment, the vacuolar membrane was observed to disintegrate. Concomitantly, a complex was formed between a vacuolar cell-death protease, RESPONSIVE TO DESSICATION-21 and its cognate cytoplasmic protease inhibitor ATSERPIN1. In the case of RB treatment, the tonoplast remained intact and no complex was formed. Over-expression of AtSerpin1 repressed cell death, only under AO photodynamic treatment. Interestingly, acute water stress showed accumulation of singlet oxygen as determined by fluorescence of Singlet Oxygen Sensor Green, by electron paramagnetic resonance spectroscopy and the induction of singlet oxygen marker genes. Cell death by acute water stress was inhibited by the singlet oxygen scavenger histidine and was accompanied by vacuolar collapse and the appearance of serpin-protease complex. Over-expression of AtSerpin1 also attenuated cell death under this mode of cell stress. Thus, acute water stress damage shows parallels to vacuole-mediated cell death where the generation of singlet oxygen may play a role.

The activation of very strong C-H bonds, such as those found in benzene, is important also in the quest for new routes for its functionalization. Using the H₅PV₂Mo₁₀O₄₀ polyoxometalate as an electron transfer oxidant in >50% aqueous H₂SO₄ as solvent, the formation of a benzene radicaloid species at RT as probed by visible spectroscopy and by EPR spectroscopy recorded at X-band and W-band, including ELDOR-detected NMR, was verified. The viability of the ET oxidation of benzene is supported by DFT calculations, showing the reaction to be exergonic under these conditions. Furthermore, we show that in the presence of O₂, very selective hydroxylation to phenol took place.

Compelling evidence has been found for the formation and direct detection of the cyclopentazole anion (cyclo-N₅⁻) in solution. The anion was prepared from phenylpentazole in two steps: reduction by an alkali metal to form the phenylpentazole radical anion, followed by thermal dissociation to yield cyclo-N₅⁻. The reaction solution was analyzed by HPLC coupled with negative mode mass spectrometry. A signal with m/z 70 was eluted about 2.1 min after injection of the sample. Its identification as N₅was supported by single and double labeling with¹⁵N, which yielded signals at m/z=71 and 72, respectively, with identical retention times in the HPLC column. MS/MS analysis of the m/z=70 signal revealed a dissociation product with m/z=42, which can be assigned to N₃⁻. To our knowledge this is the first preparation of cyclo-N₅⁻in the bulk. The compound is indefinitely stable at temperatures below −40 °C, and has a half-life of a few minutes at room temperature.

Ornidazole of the 5-nitroimidazole drug family is used to treat protozoan and anaerobic bacterial infections via a mechanism that involves preactivation by reduction of the nitro group, and production of toxic derivatives and radicals. Metronidazole, another drug family member, has been suggested to affect photosynthesis by draining electrons from the electron carrier ferredoxin, thus inhibiting NADP+ reduction and stimulating radical and peroxide production. Here we show, however, that ornidazole inhibits photosynthesis via a different mechanism. While having a minute effect on the photosynthetic electron transport and oxygen photoreduction, ornidazole hinders the activity of two Calvin cycle enzymes, triose-phosphate isomerase (TPI) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Modeling of ornidazole's interaction with ferredoxin of the protozoan Trichomonas suggests efficient electron tunneling from the iron-sulfur cluster to the nitro group of the drug. A similar docking site of ornidazole at the plant-Type ferredoxin does not exist, and the best simulated alternative does not support such efficient tunneling. Notably, TPI was inhibited by ornidazole in the dark or when electron transport was blocked by dichloromethyl diphenylurea, indicating that this inhibition was unrelated to the electron transport machinery. Although TPI and GAPDH isoenzymes are involved in glycolysis and gluconeogenesis, ornidazole's effect on respiration of photoautotrophs is moderate, thus raising its value as an efficient inhibitor of photosynthesis. The scarcity of Calvin cycle inhibitors capable of penetrating cell membranes emphasizes on the value of ornidazole for studying the regulation of this cycle.

A unique mode of molecular oxygen activation, involving metal-ligand cooperation, is described. Ir pincer complexes [((BuPNP)-Bu-t)Ir(R)] (R = C6H5 (1), CH2COCH3 (2)) react with O-2 to form the dearomatized hydroxo complexes [((BuPNP)-Bu-t*)Ir(R)(OH)] ((t)BuPNP* = deprotonated (BuPNP)-Bu-t ligand), in a process which utilizes both O-atoms. Experimental evidence, including NMR, EPR, and mass analyses, indicates a binuclear mechanism involving an O-atom transfer by a peroxo intermediate.

The H₅PV₂Mo₁₀O₄₀ polyoxometalate catalysed the electron transfer oxidation of sulphite to yield a sulphite radical, SO₃^- that upon addition of O₂ yielded a peroxosulphate species efficient for the H₅PV₂Mo₁₀O₄₀ catalysed epoxidation of alkenes. The acidic polyoxometalate further catalysed hydrolysis of the epoxide to give vicinal diols in high yields.