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•High-field (36 T) EPR spectrometer.•High-resolution (<1 mT) THz EPR spectroscopy.•Resolved BDPA g-tensor.•Gd spin label g-anisotropy.
Electron Paramagnetic Resonance (EPR) is a ...powerful technique to study materials and biological samples on an atomic scale. High-field EPR in particular enables extracting very small g-anisotropies in organic radicals and half-filled 3d and 4f metal ions such as MnII (3d5) or GdIII (4f7), and resolving EPR signals from unpaired spins with very close g-values, both of which provide high-resolution details of the local atomic environment. Before the recent commissioning of the high-homogeneity Series Connected Hybrid magnet (SCH, superconducting + resistive) at the National High Magnetic Field Laboratory (NHMFL), the highest-field, high-resolution EPR spectrometer available was limited to 25 T using a purely resistive “Keck” magnet at the NHMFL. Herein, we report the first EPR experiments performed using the SCH magnet capable of reaching the field of 36 T, corresponding to an EPR frequency of 1 THz for g = 2. The magnet’s intrinsic homogeneity (25 ppm, that is 0.9 mT at 36 T over 1 cm diameter, 1 cm length cylinder) was previously established by NMR. We characterized the magnet’s temporal stability (5 ppm, which is 0.2 mT at 36 T over one-minute, the typical acquisition time) using 2,2-diphenyl-1-picrylhydrazyl (DPPH). This high resolution enables resolving the weak g-anisotropy of 1,3-bis(diphenylene)-2-phenylallyl (BDPA), Δg = 2.5 × 10–4 obtained from measurements at 932 GHz and 33 T. Subsequently, we recorded EPR spectra at multiple frequencies for two GdIII complexes with potential applications as spin labels. We demonstrated a significant reduction in line broadening in GdDTPA, attributed to second order zero field splitting, and a resolution enhancement of g-tensor anisotropy for GdsTPATCN-SL.
Aprotic lithium–oxygen (Li–O2) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility ...caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen (1Δg) is formed upon Li2O2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time‐ and voltage‐resolved in operando EPR spectroscopy in a purpose‐built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long‐overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li–O2 cells.
The missing link: Evidence found for the evolution of substantial amounts of highly reactive singlet oxygen at potentials above 3.5 V during the charging of Li–O2 cells (see scheme) might complete our understanding of electrolyte degradation and carbon corrosion reactions hampering the development of reversible lithium–air batteries. Measures to eliminate 1O2‐induced side reactions could thus improve the cycle life of such batteries.
This work confirms the presence of a large facet‐dependent photocatalytic activity of Cu2O crystals through sparse deposition of gold particles on Cu2O cubes, octahedra, and rhombic dodecahedra. ...Au‐decorated Cu2O rhombic dodecahedra and octahedra showed greatly enhanced photodegradation rates of methyl orange resulting from a better separation of the photogenerated electrons and holes, with the rhombic dodecahedra giving the best efficiency. Au–Cu2O core–shell rhombic dodecahedra also displayed a better photocatalytic activity than pristine rhombic dodecahedra. However, Au‐deposited Cu2O cubes, pristine cubes, and Au‐deposited small nanocubes bound by entirely {100} facets are all photocatalytically inactive. X‐ray photoelectron spectra (XPS) showed identical copper peak positions for these Au‐decorated crystals. Remarkably, electron paramagnetic resonance (EPR) measurements indicated a higher production of hydroxyl radicals for the photoirradiated Cu2O rhombic dodecahedra than for the octahedra, but no radicals were produced from photoirradiated Cu2O cubes. The Cu2O {100} face may present a high energy barrier through its large band edge bending and/or electrostatic repulsion, preventing charge carriers from reaching to this surface. The conventional photocatalysis model fails in this case. The facet‐dependent photocatalytic differences should be observable in other semiconductor systems whenever a photoinduced charge‐transfer process occurs across an interface.
Structure is the key: Whereas Au‐decorated Cu2O octahedra and rhombic dodecahedra (RD) show enhanced photocatalytic activities compared to their pristine particles, small and large Au‐decorated Cu2O cubes remain inactive. EPR measurements indicate a higher production of hydroxyl radicals for Cu2O rhombic dodecahedra than for octahedra upon photoirradiation, but no radicals are produced from photoirradiated Cu2O cubes (see figure).
•Increasing temperature shifts melanoidins towards high molecular weights.•Formation of conjugated double bond systems (elimination of water and oxidation)•Radical structures are formed and IR ...signals broaden inhomogeneously.•Incorporation of roughly 0.5 mol amino compound per mole carbohydrate.•Formation of homogeneous polymers from low molecular weight compounds.
The aim of this study was to identify specific chemical bonds and characteristic structures in melanoidins formed from d-glucose and l-alanine between 130 and 200 °C. The results might be used to control the type and amount of melanoidin produced during food processing. For this purpose, complementary techniques, such as FTIR, NMR, EPR, and MALDI-ToF, were employed. At 160 °C color, solubility and UV/Vis absorption change characteristically and consequently, structural transformations could be observed in FTIR and NMR spectra. For example, sharp signals of N-H, C-N, and C-H oscillations in the l-alanine spectrum are prone to inhomogeneous broadening in melanoidins prepared above 150 °C. These changes are caused due to formation of heterogeneous macromolecular structures and occur during condensation reactions that lead to an increasing loss of water from the melanoidins with increasing temperatures. Additionally, MALDI-ToF-MS indicates the polymerization of glyoxal/glyoxylic acid and EPR shows the formation of radical structures.
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•Hyperpolarized 89Y-EDTMP is a new pH sensitive Y3+-based NMR sensor.•This HP 89Y-based sensor has a chemical shift dispersion of 16 ppm at pH 5-9 range.•89Y-EDTMP is able to detect ...two distinct pH compartments (e.g. pH 7.3 and pH 6.8).
Yttrium (III) complexes are interesting due to the similarity of their chemistry with gadolinium complexes that are used as contrast agents in nuclear magnetic resonance (NMR) spectroscopy or imaging (MRI). While most of the paramagnetic Gd3+-based MRI contrast agents are T1 or T2 relaxation-based sensors such as Gd3+-complexes for zinc or pH detection, a number of diamagnetic Y3+-complexes rely on changes in the chemical shift for potential quantitative MRI in biological milieu. 89Y, however, is a challenging nucleus to work with in conventional NMR or MRI due to its inherently low sensitivity and relatively long T1 relaxation time. This insensitivity problem in 89Y-based complexes can be circumvented with the use of dissolution dynamic nuclear polarization (DNP) which allows for several thousand-fold enhancement of the NMR or MRI signal relative to thermal equilibrium signal. Herein, we report on the feasibility of using hyperpolarized 89Y-complexes with phosphonated open-chain ligands, 89Y-EDTMP and 89Y-DTPMP, as potential chemical shift-based pH NMR sensors. Our DNP-NMR data show that hyperpolarized 89Y-DTPMP has an apparent pKa ~ 7.01 with a 4 ppm-wide chemical shift dispersion with the signal disappearing at pH below 6.2. On the other hand, pH titration data on hyperpolarized 89Y-EDTMP show that it has an apparent pKa of pH 6.7 and a 16-ppm wide chemical shift dispersion at pH 5–9 range. In comparison, the previously reported hyperpolarized pH NMR sensor 89Y-DOTP has a pKa of 7.64 and ~ 10-ppm wide chemical shift dispersion at pH 4–9 range. Overall, our data suggest that hyperpolarized 89Y-EDTMP is better than hyperpolarized 89Y-DOTP in terms of pH sensing capability at the physiological range.
Abstract
Glycolic acid (GA), as important building block of biodegradable polymers, has been synthesized for the first time in excellent yields at room temperature by selective oxidation of ...1,3‐dihyroxyacetone (DHA) using a cheap supported Cu/Al
2
O
3
catalyst with single active Cu
II
species. By combining EPR spin‐trapping and operando ATR‐IR experiments, different mechanisms for the co‐synthesis of GA, formates, and formamides have been derived, in which
.
OH radicals formed from H
2
O
2
by a Fenton‐like reaction play a key role.
The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar ...EPR spectroscopy (PDS) in conjunction with site‐directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in‐cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide‐functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm, and shows high stability against reduction. Using this label, the guanine‐nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.
SLIM fit: A trityl label with a short linker and high reduction stability, called SLIM, was synthesized. It enables highly sensitive pulsed dipolar EPR measurements down to low nanomolar concentrations and yields narrow distance distributions. Its exemplary use in in‐cell measurements showed that YopO preferably adopts one of the two conformations found in vitro.
The search for main‐group element‐based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 ...element‐centered radicals mainly feature a V‐shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R3E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris‐amide ligand. The selective one‐electron reduction of R3E afforded the first stable tricoordinate pnictogen‐centered radical anion salts; the pnictogen atoms retain planar T‐shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N3E planes.
The selective one‐electron reduction of nontrigonal R3E compounds (E=P, As, Sb) afforded the first stable tricoordinate pnictogen‐centered radical anion salts; the pnictogen atoms retain a planar T‐shaped structure. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbital of the pnictogen atom, which is perpendicular to the N3E plane.
Glass-like carbons (GLCs) are a large group of disordered carbon materials which have applications in various fields, but not all their properties are well characterized. In particular, they have ...high concentration of paramagnetic centers (PCs) which are easily observed by electron paramagnetic resonance (EPR). However, an apparently simple EPR spectrum, together with its peculiarities caused by high electrical conductivity, make it difficult to identify the PCs and their origin remains unknown. To resolve these unsettled questions, in this work we use the vacuum Rabi splitting and Rabi oscillations which so far have not been applied to study these materials by the continuous wave (CW) and pulsed EPR. GLC produced by pyrolysis of phenol-formaldehyde resin at 1120 K was studied in the temperature range of 4.3–300 K. Using the Rabi splitting we found conditions for undistorted registration of CW EPR spectra and determined spin concentration (1×1020 spin/g). The temperature dependence of the EPR susceptibility shows the coexistence of Pauli paramagnetism due to conduction electrons and the Curie-type paramagnetism of thermally activated PCs with the activation energy of 5.7 cm-1. These PCs may be excited triplet states, the presence of which is revealed by Rabi oscillations in the pulsed EPR.
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•Glass-like carbon shows strong electron paramagnetism leading to vacuum Rabi splitting.•Pauli and thermally activated Curie-type paramagnetism coexist in glass-like carbon.•Rabi oscillations of fast relaxing centers in conducting carbon are studied.•Triplet states are revealed by Rabi oscillations in the pulsed EPR.
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