Superhyperfine Coupling Research Articles

Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals.

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120K when subjected to a sub-bandgap (405nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO.

Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various MnII PMHs have been proposed as intermediates in catalysis, but isolated MnII PMHs are limited to dimeric high-spin MnII structures with bridging hydrides. In this paper, a series of the first low-spin monomeric MnII PMH complexes are generated by chemical oxidation of their MnI analogues. This series is of the type trans-[MnH(L)(dmpe)2]+/0 where the trans ligand L is PMe3, C2H4, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the MnII hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe3, the complex is the first example of an isolated monomeric MnII hydride complex. In contrast, when L is C2H4 or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)3]+, accompanied by ethane and ethylene, whereas the latter eliminated H2, generating [Mn(MeCN)(CO)(dmpe)2]+ or a mixture of products including [Mn(κ1-PF6)(CO)(dmpe)2], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe3)(dmpe)2]+ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. MnII-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe3) to 47 kcal/mol (L = CO).

Optically Detected Magnetic Resonance Spectroscopy of Cu-Doped CdSe/CdS and CuInS2 Colloidal Quantum Dots.

Copper-doped II-VI and copper-based I-III-VI2 colloidal quantum dots (CQDs) have been at the forefront of interest in nanocrystals over the past decade, attributable to their optically activated copper states. However, the related recombination mechanisms are still unclear. The current work elaborates on recombination processes in such materials by following the spin properties of copper-doped CdSe/CdS (Cu@CdSe/CdS) and of CuInS2 and CuInS2/(CdS, ZnS) core/shell CQDs using continuous-wave and time-resolved optically detected magnetic resonance (ODMR) spectroscopy. The Cu@CdSe/CdS ODMR showed two distinct resonances with different g factors and spin relaxation times. The best fit by a spin Hamiltonian simulation suggests that emission comes from recombination of a delocalized electron at the conduction band edge with a hole trapped in a Cu2+ site with a weak exchange coupling between the two spins. The ODMR spectra of CuInS2 CQDs (with and without shells) differ significantly from those of the copper-doped II-VI CQDs. They are comprised of a primary resonance accompanied by another resonance at half-field, with a strong correlation between the two, indicating the involvement of a triplet exciton and hence stronger electron-hole exchange coupling than in the doped core/shell CQDs. The spin Hamiltonian simulation shows that the hole is again associated with a photogenerated Cu2+ site. The electron resides near this Cu2+ site, and its ODMR spectrum shows contributions from superhyperfine coupling to neighboring indium atoms. These observations are consistent with the occurrence of a self-trapped exciton associated with the copper site. The results presented here support models under debate for over a decade and help define the magneto-optical properties of these important materials.

Ab-initio calculations of shallow dopant qubits in silicon from pseudopotential and all-electron mixed approach

Obtaining an accurate first-principle description of the electronic properties of dopant qubits is critical for engineering and optimizing high-performance quantum computing. However, density functional theory (DFT) has had limited success in providing a full quantitative description of these dopants due to their large wavefunction extent. Here, we build on recent advances in DFT to evaluate phosphorus dopants in silicon on a lattice comprised of 4096 atoms with hybrid functionals on a pseudopotential and all-electron mixed approach. Remarkable agreement is achieved with experimental measurements including: the electron-nuclear hyperfine coupling (115.5 MHz) and its electric field response (−2.65 × 10−3 μm2/V2), the binding energy (46.07 meV), excited valley-orbital energies of 1sT2 (37.22 meV) and 1sE (35.87 meV) states, and super-hyperfine couplings of the proximal shells of the silicon lattice. This quantitative description of spin and orbital properties of phosphorus dopant simultaneously from a single theoretical framework will help as a predictive tool for the design of qubits.

Hyperfine Decoupling of ESR Spectra Using Wavelet Transform.

The objective of spectral analysis is to resolve and extract relevant features from experimental data in an optimal fashion. In continuous-wave (cw) electron spin resonance (ESR) spectroscopy, both values of a paramagnetic center and hyperfine splitting caused by its interaction with neighboring magnetic nuclei in a molecule provide important structural and electronic information. However, in the presence of - and/or -anisotropy and/or large number of resonance lines, spectral analysis becomes highly challenging. Either high-resolution experimental techniques are employed to resolve the spectra in those cases or a range of suitable ESR frequencies are used in combination with simulations to identify the corresponding and values. In this work, we present a wavelet transform technique in resolving both simulated and experimental cW-ESR spectra by separating the hyperfine and super-hyperfine components. We exploit the multiresolution property of wavelet transforms that allow the separation of distinct features of a spectrum based on simultaneous analysis of spectrum and its varying frequency. We retain the wavelet components that stored the hyperfine and/or super-hyperfine features, while eliminating the wavelet components representing the remaining spectrum. We tested the method on simulated cases of metal-ligand adducts at L-, S-, and X-band frequencies, and showed that extracted values, hyperfine and super-hyperfine coupling constants from simulated spectra, were in excellent agreement with the values of those parameters used in the simulations. For the experimental case of a copper(II) complex with distorted octahedral geometry, the method was able to extract and hyperfine coupling constant values, and revealed features that were buried in the overlapped spectra.

Synthesis and Magneto‐Structural Study of Distorted Octahedral CoII Complexes with Picolinamide and Azide (μ1,1)‐Bridge

AbstractTwo compounds [Co2(pa)2(μ1,1‐N3)2]n (1) and [Co4(pa)6(μ1,1‐N3)6] (2), (pa=picolinamide) were obtained and studied by electronic and magnetic spectroscopies. 1 is a polymer and 2 is a tetramer of mixed‐valence CoII/III. The N3− anions act as bridges between cobalt ions in the μ1,1 coordination mode. UV‐Vis spectrum analysis showed the octahedral distortion of the compounds. ESR solution spectra revealed a low‐spin ground state, s=1/2 ( ) for CoII. The energy gap was J> between levels, to , of 2; consequently, its transition would not be observed. 1 showed zero‐field splitting of the excited level with D∼0.31 cm−1. Hyperfine (59Co, I=7/2) and superhyperfine (14N, I=1) couplings were observed. Magnetic susceptibility studies (3–300 K) revealed antiferromagnetic exchange interactions in 1 and 2 with exchange coupling constants, J1=−108 cm−1 and J2=−20 cm−1, g=2 and s= , for the Co(2)‐Co(2) magnetic exchange pathways.

Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation.

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O2-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O2-evolving rate constants, indicating that the O2-releasing step is the rate-determining step in the O2 evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Selective Optical Addressing of Nuclear Spins through Superhyperfine Interaction in Rare-Earth Doped Solids.

In Er^{3+}:Y_{2}SiO_{5}, we demonstrate the selective optical addressing of the ^{89}Y^{3+} nuclear spins through their superhyperfine coupling with the Er^{3+} electronic spins possessing large Landé g factors. We experimentally probe the electron-nuclear spin mixing with photon echo techniques and validate our model. The site-selective optical addressing of the Y^{3+} nuclear spins is designed by adjusting the magnetic field strength and orientation. This constitutes an important step towards the realization of long-lived solid-state qubits optically addressed by telecom photons.

Raman and EPR spectroscopic studies of chromium-doped diamond-like carbon films

The diamond-like carbon (Cr-free DLC) and chromium-doped DLC (Cr doped-DLC) films prepared by a hybrid technology using a combination of pulsed laser deposition and magnetron sputtering were studied by Raman and electron paramagnetic resonance (EPR) spectroscopy. Raman spectra are dominated by the well-known carbon (D/G) band. The intensity ratio of the D/G bands was found to increase with the increase of the Cr content. Careful elimination of the substrate-related (SiO2) peaks enabled us to identify new weak features attributable to the presence of the CCr bonds. Two EPR signals were observed in Cr-free and Cr-doped DLC films. The EPR signal at g = 2.0025(3) was attributed to the sp2 coordinated carbon-related defect (CRD) and the second EPR signal at g = 2.0064 was assigned to the Pb0 interface defect which may appear due to the formation of the oxidized Si on the interface of the DLC film and SiO2 substrate. The observed temperature behavior of the CRD EPR signal g-factor in Cr free DLC films was described by the model of exchange coupled two spin systems (localized defects and conduction electrons) in the bottleneck regime. From the temperature dependence of the linewidth and integral intensity of the CRD EPR signal it was concluded that ferromagnetic ordering occurs in the spin system with exchange constant of J = 0.069 meV. The spin density of the CRD was found to be significantly decreased in Cr-doped DLC films, possibly due to the formation of the CCr bonds. The observed change of the CRD lineshape from Lorentzian in Cr free DLC films to Gaussian in Cr-doped DLC films indicates that there is a superhyperfine coupling between unpaired spins of the CRD and surrounding Cr nuclei.

1,2-CH Bond Activation of Pyridine across a Transient Titanium Alkylidene Radical and Re-Formation of the Ti═CHtBu Moiety

Reduction of the titanium alkylidene [(PNP)Ti═CHtBu(OTf)] (PNP– = N[2-PiPr2-4-methylphenyl]2–) with KC8 in the presence of pyridine results in formation of the transient titanium(III) alkylidene radical [(PNP)Ti═CHtBu)] (A) or the adduct [(PNP)Ti═CHtBu)(NC5H5)] (B), which activates the C–H bond of pyridine to form the titanium(III) pyridyl alkyl complex [(PNP)Ti(CH2tBu)(η2-NC5H4)] (1) in 64% yield as brown microcrystals. Low-temperature X-band EPR spectroscopy and single-crystal X-ray diffraction studies confirm the identity of 1 as a d1 metal-centric radical with superhyperfine coupling to one nitrogen atom and having a side-on pyridyl moiety, which results in formation of the two isomeric forms 1a,b. Oxidation of 1 with [FeCp*2][OTf] cleanly promotes α-hydrogen abstraction to re-form [(PNP)Ti═CHtBu(OTf)] with concurrent elimination of pyridine and FeCp*2. Re-formation of the alkylidene moiety most likely stems from an intermediate such as [(PNP)Ti(CH2tBu)(η2-NC5H4)(OTf)] (C).

Actinide covalency measured by EPR

Mounting evidence suggests that actinide bonds have notable covalent character, making their chemistry more akin to transition metals than lanthanides. That covalency can be directly measured using pulsed electron paramagnetic resonance spectroscopy, yielding some surprising results, reports a team from the University of Manchester (Nat. Chem. 2016, DOI: 10.1038/nchem.2692). So-called superhyperfine coupling of a metal’s unpaired electrons with ligand nuclei that have non-zero spin can be used to assess covalency, but signals couldn’t be resolved by older EPR techniques. Using newer pulsed methods, Floriana Tuna, Eric J. L. McInnes, David P. Mills, and coworkers studied thorium(III) and uranium(III) organometallic complexes with ligands derived from the cyclopentadienyl anion (Cp). Computational analysis of the Th and U complexes suggested that they had very similar, weakly covalent bonds. But the EPR measurements indicate that the complexes differ, with much more covalency in U-Cp bonds than in Th-Cp bon...

Well-Known Mediators of Selective Oxidation with Unknown Electronic Structure: Metal-Free Generation and EPR Study of Imide-N-oxyl Radicals.

Nitroxyl radicals are widely used in chemistry, materials sciences, and biology. Imide-N-oxyl radicals are subclass of unique nitroxyl radicals that proved to be useful catalysts and mediators of selective oxidation and CH-functionalization. An efficient metal-free method was developed for the generation of imide-N-oxyl radicals from N-hydroxyimides at room temperature by the reaction with (diacetoxyiodo)benzene. The method allows for the production of high concentrations of free radicals and provides high resolution of their EPR spectra exhibiting the superhyperfine structure from benzene ring protons distant from the radical center. An analysis of the spectra shows that, regardless of the electronic effects of the substituents in the benzene ring, the superhyperfine coupling constant of an unpaired electron with the distant protons at positions 4 and 5 of the aromatic system is substantially greater than that with the protons at positions 3 and 6 that are closer to the N-oxyl radical center. This is indicative of an unusual character of the spin density distribution of the unpaired electron in substituted phthalimide-N-oxyl radicals. Understanding of the nature of the electron density distribution in imide-N-oxyl radicals may be useful for the development of commercial mediators of oxidation based on N-hydroxyimides.

Decoherence and absorption of Er3+:KTiOPO4 (KTP) at 1.5 μm

We present results of laser absorption spectroscopy and two-pulse photon echo decoherence measurements on the lowest 4I15/2 to lowest 4I13/2 transition in Er3+: KTiOPO4 (KTP—potassium titanyl phosphate) for the optical transition located at 1537.238nm. This transition was found to have an inhomogeneous absorption linewidth of 950MHz and pronounced polarization dependence. Two-pulse photon echo decay measurements as a function of applied magnetic field strength at 1.9K revealed a narrow homogeneous linewidth of 2.5kHz at 0.2T that increased to 5.8kHz at 1.2T and then decreased to 1.6kHz at 4.5T. This behavior was successfully described by decoherence due to Er3+–Er3+ magnetic dipole interactions. Significant superhyperfine coupling of Er3+ spins to the nuclear moments of ions in the host lattice was observed, modulating the photon echo decay at low magnetic fields and limiting the effective homogenous linewidth at high fields. Combined with the well-established potential of KTP for fabrication of high-quality optical waveguides and integrated non-linear frequency conversion, our results suggest that Er3+:KTP is a promising material system for practical spectral hole burning, signal processing, and quantum information applications.

In situ oligomerization of 2-(thiophen-3-yl)acetate intercalated into Zn2Al layered double hydroxide

A layered double hydroxide (LDH) with cation composition Zn2Al was intercalated with 2-(thiophen-3-yl)acetate (3-TA) monomers. To achieve in situ polymerization and/or oligomerization of the intercalated monomers, soft thermal treatments were carried out, and subsequent hybrid LDH materials were analyzed by means of several characterization techniques using powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), 13C CP–MAS nuclear magnetic resonance (NMR), electron spin resonance (EPR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), inductively coupled plasma optical emission spectroscopy (ICP–OES), and elemental analysis. PXRD analysis suggested that the intercalated monomers formed a bilayer. Thermal treatment of the hybrid LDH assembly above 120°C provokes partially the breakdown of the layered structure, generating the phase zincite. EPR results indicated that vicinal monomers (oligomerization) were bound to each other after hydrothermal or thermal treatment, leading to a polaron response characteristic of electron conductivity localized on a restricted number of thiophene-based monomer segments. Localized unpaired electrons exist in the material and interact with the 27Al nuclei of the LDH layers by superhyperfine coupling. These unpaired electrons also interact with the surface of ZnO (O2− vacancies), formed during the thermal treatments.

Metallodithiolates as ligands to dinitrosyl iron complexes: toward the understanding of structures, equilibria, and spin coupling.

Metallodithiolate ligands are used to design heterobimetallic complexes by adduct formation through S-based reactivity. Such adducts of dinitrosyl iron were synthesized with two metalloligands, namely, Ni(bme-daco) and V≡O(bme-daco) (bme-daco = bismercaptoethane diazacyclooctane), and, for comparison, an N-heterocyclic carbene, namely, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (Imes), by cleavage of the (μ-I)2[Fe(NO)2]2 dimer of electronic configuration {Fe(NO)2}(9) (Enemark-Feltham notation). With Fe(NO)2I as Lewis acid acceptor, 1:1 adducts resulted for both the IMes·Fe(NO)2I, complex 2, and V≡O(bme-daco)·Fe(NO)2I, complex 4. The NiN2S2 demonstrated binding capability at both thiolates, with two Fe(NO)2I addenda positioned transoid across the NiN2S2 square plane, Ni(bme-daco)·2(Fe(NO)2I), complex 3. Enhanced binding ability was realized for the dianionic vanadyl dithiolate complex, [Et4N]2[V≡O(ema)], (ema = N,N'-ethylenebis(2-mercaptoacetamide)), which, unlike the neutral (V≡O)N2S2, demonstrated reactivity with the labile tungsten carbonyl complex, cis-W(CO)4(pip)2, (pip = piperidine), yielding [Et4N]2[V≡O(ema)W(CO)4], complex 1, whose ν(CO) IR values indicated the dianionic vanadyl metalloligand to be of similar donor ability to the neutral NiN2S2 ligands. The solid-state molecular structures of 1-4 were determined by X-ray diffraction analyses. Electron paramagnetic resonance (EPR) measurements characterize the {Fe(NO)2}(9) complexes in solution, illustrating superhyperfine coupling via the (127)I to the unpaired electron on iron for complex 2. The EPR characterizations of 3 [Ni(bme-daco)·2(Fe(NO)2I)] and 4 [V≡O(bme-daco)·Fe(NO)2I] indicate these complexes are EPR silent, likely due to strong coupling between paramagnetic centers. Within samples of complex 4, individual paramagnetic centers with localized superhyperfine coupling from the (51)V and (127)I are observed in a 3:1 ratio, respectively. However, spin quantitation reveals that these species represent a minor fraction (<10%) of the total complex and thus likely represent disassociated paramagnetic sites. Computational studies corroborated the EPR assignments as well as the experimentally observed stability/instability of the heterobimetallic DNIC complexes.

Insights into the oxygen-based ligand of the low pH component of the Cu(2+)-amyloid-β complex.

In spite of significant experimental effort dedicated to the study of Cu(2+) binding to the amyloid beta (Aβ) peptide, involved in Alzheimer's disease, the nature of the oxygen-based ligand in the low pH component of the Cu(2+)-Aβ(1-16) complex is still under debate. This study reports density-functional-theory-based calculations that explore the potential energy surface of Cu(2+) complexes including N and O ligands at the N-terminus of the Aβ peptide, with a focus on evaluating the role of Asp1 carboxylate in copper coordination. Model conformers including 3, 6, and 17 amino acids have been used to systematically study several aspects of the Cu(2+)-coordination such as the Asp1 side chain conformation, local peptide backbone geometry, electrostatic and/or hydrogen bond interactions, and number and availability of Cu(2+) ligands. Our results show that the Asp1 peptide carbonyl binds to Cu(2+) only if the coordination number is less than four. In contrast, if four ligands are available, the most stable structures include the Asp1 carboxylate in equatorial position instead of the Asp1 carbonyl group. The two lowest energy Cu(2+)-Aβ(1-17) models involve Asp1 COO(-), the N-terminus, and His6 and His14 as equatorial ligands, with either a carbonyl or a water molecule in the axial position. These models are in good agreement with experimental data reported for component I of the Cu(2+)-Aβ(1-16) complex, including EXAFS- and X-ray-derived Cu(2+)-ligand distances, Cu(2+) EPR parameters, and (14)N and (13)C superhyperfine couplings. Our results suggest that at low pH, Cu(2+)-Aβ species with Asp1 carboxylate equatorial coordination coexist with species coordinating the Asp1 carbonyl. Understanding the bonding mechanism in these species is relevant to gain a deeper insight on the molecular processes involving copper-amyloid-β complexes, such as aggregation and redox activity.

Synthesis, characterization, biological activity of binuclear Co(II), Cu(II) and mononuclear Ni(II) complexes of bulky multi-dentate thiosemicarbazide

The chelation behavior of 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarbonyl) bis (N-ethylhydrazine-1-carbothioamide) (H6ETS)(1) towards Co2+, Ni2+and Cu2+ have been studied. The spectral data revealed that the ligand acts as a bi- and/or mono-negative multi-dentate. The isolated Ni(II) and Cu(II) complexes are square-planar while the Co(II) is tetrahedral. EPR spectrum of Cu(II) complex confirmed simulated an axial spin-Hamiltonian exhibiting a four-line pattern with nitrogen super-hyperfine couplings originating from imine hydrazinic nitrogen atoms and possess a significant amount of tetrahedral distortion leading to a pseudo-square-planar geometry with unpaired electron has dx2-y2 ground state. Also, the thermal behavior and kinetic parameters were determined. Furthermore, the title compounds were investigated for their antibacterial activity using inhibition zone diameter and for DNA degradation, superoxide-scavenging activity as well as hydroxyl radicals that generated by the oxidation of cytochrome c in L-ascorbic acid/CuSO4–cytochrome c system.

Photogenerated Charge Carriers and Paramagnetic Species in (W,N)-Codoped TiO2 Photocatalysts under Visible-Light Irradiation: An EPR Study

(W,N)-codoped TiO2 has recently attracted interest due to substantial band gap narrowing in conjunction with good visible-light photocatalytic activities. A complete picture of the fundamental mechanism at the origin of their photoactivity is, however, far from being understood. We present an EPR study on (W,N)-codoped titanias by recording spectra in the dark and under 550 nm visible-light irradiation to identify the major species involved in the formation and migration of photogenerated charge carriers. Interstitial N–lattice O (formally NO2–) paramagnetic groups with and without a specific close range coupling interaction with a neighboring W, that is, [NiO]W● and [NiO]●, respectively, are observed when visible light excites electrons from diamagnetic intraband gap states located at ∼2.30 V versus RHE (formally NO3–) to the conductance band at around −0.05 V versus RHE. The [NiO]W● EPR signal, reported here for the first time, is characterized by a much stronger hyperfine interaction between the unpaired electron and the 14N nucleus than ordinary [NiO]● found in N-doped TiO2, leading to a much higher spin density on the N center. The overall signal also contains superhyperfine coupling of the unpaired electron with the 183W nuclide (natural abundance 14.31%), the only naturally occurring nuclide of W with a nonzero nuclear spin.

Moving difference (MDIFF) non-adiabatic rapid sweep (NARS) EPR of copper(II)

Non-adiabatic rapid sweep (NARS) EPR spectroscopy has been introduced for application to nitroxide-labeled biological samples (Kittell et al., 2011) [1]. Displays are pure absorption, and are built up by acquiring data in spectral segments that are concatenated. In this paper we extend the method to frozen solutions of copper-imidazole, a square planar copper complex with four in-plane nitrogen ligands. Pure absorption spectra are created from concatenation of 170 5-gauss segments spanning 850G at 1.9GHz. These spectra, however, are not directly useful since nitrogen superhyperfine couplings are barely visible. Application of the moving difference (MDIFF) algorithm to the digitized NARS pure absorption spectrum is used to produce spectra that are analogous to the first harmonic EPR. The signal intensity is about four times higher than when using conventional 100kHz field modulation, depending on line shape. MDIFF not only filters the spectrum, but also the noise, resulting in further improvement of the SNR for the same signal acquisition time. The MDIFF amplitude can be optimized retrospectively, different spectral regions can be examined at different amplitudes, and an amplitude can be used that is substantially greater than the upper limit of the field modulation amplitude of a conventional EPR spectrometer, which improves the signal-to-noise ratio of broad lines.

Three‐Coordinate Nickel(I) Complexes Stabilised by Six‐, Seven‐ and Eight‐Membered Ring N‐Heterocyclic Carbenes: Synthesis, EPR/DFT Studies and Catalytic Activity

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d z 2〉 and ∣3d x 2-y 2〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1>4≈6≫5).