Experimental Electron Paramagnetic Resonance Spectra Research Articles

Exploiting Atomic Control to Show When Atoms Become Molecules

AbstractPrecision‐placed atom qubits in silicon offer a unique means to confine electrons and control their spins with extreme accuracy, which can be leveraged to construct powerful quantum computers. To date atom qubits in silicon have been successfully realized using electrons hosted either on a single phosphorus atom or on a multi‐donor quantum dot. Here, a novel molecular regime is explored in which electrons are bound to two donor dots separated by ≈8 nm in a natural silicon substrate. The molecular state, provided by these spatially separated donors, is used to study with exquisite precision the impact of confinement potential on the electronic and spin properties of qubits. Unique spin filling measurements, performed on up to five electrons, confirm how electrons are shared between both sites of the molecule, forming hybridized molecular states. The precise atomic locations of the donor atoms in the silicon lattice are determined by combining the experimental electron spin resonance spectra and the state‐of‐the‐art atomistic modeling of multi‐electron wave‐functions in presence of realistic electric fields. The donor molecule studied in this work exhibits excellent qubit properties and addresses the impact that the confinement potential has, at the atomic scale, on the desired properties of electron spin qubits.

Free radical mechanism of toxic organic compound formations from o-chlorophenol.

Organic free radical intermediates are pivotal to our understanding of toxic chemicals formation from chlorophenols that widely exist in thermal processes. However, in most cases, multiple free radical intermediates exist and produce complex spectra that are hard to deconvolute. Identification of free radical intermediates is the current difficulty for detailed formation mechanisms of toxic products from chlorophenols. In this study, a universal bottom-up method was developed to identify the organic free radical intermediates. Candidate organic free radicals were firstly speculated according to the critical parameters obtained from experimental electron paramagnetic resonance (EPR) spectra and the calculated bond dissociation energies of precursors. Their theoretical spectra were then used retrospectively to justify the accordance with the experimental EPR spectra. Identification of the organic free radicals provides straightforward evidence for the formation pathways of pollutants from chlorophenol. Internal factors influencing formation of radical intermediates and the toxic products were also studied, including the ortho effect of the precursor, spin densities of the organic free radical intermediates, and steric hindrance effects of the molecular intermediates. In combination of the experimental results and theoretical calculations, detailed formation mechanisms of toxic pollutants intermediating by organic free radicals from thermal oxidation of chlorophenol were strongly evidenced.

Investigations on the role of doped 3d transition ions in Li2B4O7 glasses: Local structures, spectroscopic properties and optical basicities

Glasses with doped transition metal ions (TMs) have shown specific spectroscopic properties, but the micro-mechanism is hard to be obtained due to the amorphous structure. Based on the high order perturbation formulae of electron paramagnetic resonance (EPR) parameters, the local environment of 3d ions (V4+, Cr3+, Mn2+ and Cu2+) in Li2B4O7 glasses is studied to explain the experimental EPR spectra. The spectroscopic properties of Li2B4O7 glasses are closely correlated with the doped 3d ions, which can be reasonably predicted from the transition bands of divided energy orbitals. The crucial optical basicities of Li2B4O7 glasses doped with different TMs are also investigated, which are consistent with the above calculation results with respect to the local environment. The results show that TMs doping can be an effective way to modify the intrinsic properties of glasses.

EPR Study on the Oxidative Degradation of Phenyl Sulfonates, Constituents of Aromatic Hydrocarbon-Based Proton-Exchange Fuel Cell Membranes.

Sulfonated aromatic hydrocarbon-based ionomers are potential constituents of next-generation polymer electrolyte fuel cells (PEFCs). Widespread application is currently limited due to their susceptibility to radical-initiated oxidative degradation that, among other intermediates, involves the formation of highly reactive aromatic cation radicals. The intermediates undergo chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups, all leading to performance loss and eventual membrane failure. Laser flash photolysis experiments indicated that cation radicals can also be formed via direct electron ejection. We aim to establish the major degradation pathway of proton-exchange membranes (PEMs). To this end, we irradiated aqueous solutions of phenyl sulfonate-type model compounds with a Xe arc lamp, thus generating radicals. The radicals were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the formed adducts were observed by electron paramagnetic resonance (EPR). The formed DMPO spin adducts were assigned and relative adduct concentrations were quantified by simulation of the experimental EPR spectra. Through the formation of the DMPO/•SO3 - adduct, we established that desulfonation dominates for monoaromatic phenyl sulfonates. We observed that diaryl ether sulfonates readily undergo homolytic C-O scission that produces DMPO/•aryl adducts. Our results support the notion that polyphenylene sulfonates are the most stable against oxidative attack and effectively transfer electrons from DMPO, forming DMPO/•OH. Our findings help to identify durable moieties that can be used as building blocks in the development of next-generation PEMs.

Mn environment in doped SrTiO3 revealed by first-principles calculation of hyperfine splittings

In order to take advantage of the strong correlation between geometric changes and electronic properties, better understanding of the structure and properties of perovskites doped with paramagnetic transition metal ions is required. Computational characterization of these dopants and calculated hyperfine values provide a guide that can ultimately aid in the interpretation of experimentally obtained electron paramagnetic resonance (EPR) spectra. In this study, we perform ab initio calculation of the hyperfine splitting parameter for Sr-substituted Mn2+ in SrTiO3 in various geometries in order to assign experimentally reported EPR peaks to exact dopant structures. Additionally, we calculate the hyperfine parameters for Mn4+ and Mn2+∕4+ with adjacent oxygen vacancy, which remain to be assigned peaks in experimental EPR spectra. Calculation of Ti-substituted Mn is largely hindered by the highly correlated electronic structure; however, it is shown that reasonable hyperfine values for Ti-site defects can be obtained by tuning functional parameters. Overall, this study demonstrates that calculating the hyperfine splitting for a transition metal dopant in a complex oxide is feasible and can provide a fingerprint for different geometries in equivalently defected systems.

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry.

Quantitative determination of the molecular orientation distribution function in samples of liquid crystals with a complex director geometry was performed using the numerical simulation of electron paramagnetic resonance (EPR) spectra of the spin probes in a liquid-crystalline medium. To achieve the quantitative agreement of experimental and simulated EPR spectra, the hierarchy of the orientation order was explicitly taken into account, namely, the local ordering of liquid crystal molecules by the mean-field potential of surrounding molecules, and the partial disordering of local directors within the sample. The samples under study are planar and twist LC cells with liquid crystal 8CB cooled from the nematic into the smectic A phase in the magnetic field. The presence of the magnetic field perpendicular to the cell director leads to distortion of the orientation of the liquid crystal in the cell. The spin probe technique was successfully employed for the reliable measurement of orientation distribution functions of the low nonorthorhombic symmetry. Orientation order parameters up to 12th rank were measured, including nonaxial and nonorthorhombic order parameters. It is shown that the presence of several contradicting aligning forces leads to the tilt of the preferential director toward the direction, which is a compromise between the orienting forces.

Products of Prolonged Autoxidation of Simple Dihydric Phenols in the Presence of Copper(II) Ions – An Electron Spin Resonance Study

Electron spin resonance (ESR) spectroscopy was used for characterizing the products obtained by prolonged autoxidation of simple dihydric phenols (hydroquinone, catechol, and 4-methylcatechol) in the presence of copper(II) ions. Room temperature ESR spectra revealed that both paramagnetic copper(II) ions and organic radicals are present in obtained autoxidation products similarly to the humic acid complexed copper(II) ions. The ratio of organic radical signal intensity to the copper(II) ion signal intensity suggests that the smallest amount of copper(II) ions is incorporated in the hydroquinone autoxidation product while the highest amount of copper(II) ions is incorporated in the autoxidation product of catechol. Satisfactory computer simulations of experimental ESR spectra were obtained by considering only one type of copper(II) ion binding site for hydroquinone autoxidation product and two distinct types of copper(II) ion binding sites for catechol and 4-methylcatechol autoxidation products. Parameters obtained by the computer simulation of ESR spectra indicated prevalent ionic bonding of copper(II) ions in polymeric matrices with tetrahedral distortion at copper(II) ion binding sites and negligible exchange interactions between them. Products obtained by the hydroquinone and catechol autoxidation have more similar characteristics in comparison to the product obtained by the 4-methylcatechol autoxidation where more expressed ionic bonding of copper(II) ions, and smaller tetrahedral distortion are present. Due to the dipolar interactions of oxygen-centered organic radicals in autoxidation products with paramagnetic copper(II) ions, their ESR linewidths are larger and g-values smaller in comparison to the values found in humic acids from various soil types.

Porous Pyrene Organic Cage with Unusual Absorption Bathochromic-Shift Enables Visible Light Photocatalysis

Porous Pyrene Organic Cage with Unusual Absorption Bathochromic-Shift Enables Visible Light Photocatalysis

•BMPO-OOH Spin-Adduct as a Model for Study of Decomposition of Organic Hydroperoxides and the Effects of Sulfide/Selenite Derivatives. An EPR Spin-Trapping Approach.

Lipid hydroperoxides play an important role in various pathophysiological processes. Therefore, a simple model for organic hydroperoxides could be helpful to monitor the biologic effects of endogenous and exogenous compounds. The electron paramagnetic resonance (EPR) spin-trapping technique is a useful method to study superoxide (O2•−) and hydroxyl radicals. The aim of our work was to use EPR with the spin trap 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO), which, by trapping O2•− produces relatively stable •BMPO-OOH spin-adduct, a valuable model for organic hydroperoxides. We used this experimental setup to investigate the effects of selected sulfur/selenium compounds on •BMPO-OOH and to evaluate the antioxidant potential of these compounds. Second, using the simulation of time-dependent individual BMPO adducts in the experimental EPR spectra, the ratio of •BMPO-OH/•BMPO-OOH—which is proportional to the transformation/decomposition of •BMPO-OOH—was evaluated. The order of potency of the studied compounds to alter •BMPO-OOH concentration estimated from the time-dependent •BMPO-OH/•BMPO-OOH ratio was as follows: Na2S4 > Na2S4/SeO32− > H2S/SeO32− > Na2S2 ~Na2S2/SeO32− ~H2S > SeO32− ~SeO42− ~control. In conclusion, the presented approach of the EPR measurement of the time-dependent ratio of •BMPO-OH/•BMPO-OOH could be useful to study the impact of compounds to influence the transformation of •BMPO-OOH.

Paramagnetic centers in graphite oxide according to EPR spectra and DFT calculations

The first attempt to determine experimentally based structures of paramagnetic centers in graphite oxide is presented. The attribution of satellite signals in electron paramagnetic resonance (EPR) spectra to forbidden transitions was proved by X- and Q-band experiments. Proton responsible for satellite lines is found to be able to undergo rapid isotopic exchange. Magnetic parameters of the paramagnetic centers were determined by numerical simulation of experimental EPR spectra. Density functional theory (DFT) was used for theoretical calculation of magnetic parameters for probable paramagnetic moieties. The comparison of DFT results with experimental values reveals the structures of oxygen-centered paramagnetic species in graphite oxide.

Spectroscopic properties of the V-doped borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra in borate glasses of Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions with added 0.5 and 1.0 mol.% V2O5, were investigated. The EPR spectroscopy clearly shows that the V impurity is incorporated into the network of studied glasses, mainly, as isolated vanadyl (VO2+) molecular complex centres. Spin Hamiltonian parameters of axial symmetry (g||, g⊥, A||, A⊥), dipolar hyperfine coupling (P) and Fermi contact interaction (K) constants of the VO2+ centres in octahedral sites with a tetragonal compression (C4v local symmetry) for all investigated glasses were determined from their experimental EPR spectra. Optical absorption bands, observed in the investigated glasses have been identified and interpreted. Crystal field parameters (Dq, Ds, Dt) and molecular orbitals (bonding) coefficients (β⁎2, επ⁎2) for VO2+ centres in the studied glasses were calculated and compared with referenced data for other V-doped borate glasses with similar compositions.

Advanced surface resonators for electron spin resonance of single microcrystals.

Electron spin resonance (ESR) spectroscopy of paramagnetic species in single crystals is a powerful tool for characterizing the latter's magnetic interaction parameters in detail. Conventional ESR systems are optimized for millimeter-size samples and make use of cavities and resonators that accommodate tubes and capillaries in the range 1-5 mm. Unfortunately, in the case of many interesting materials such as enzymes and inorganic catalytic materials (e.g., zeolites), single crystals can only be obtained in micron-scale sizes (1-200 µm). To boost ESR sensitivity and to enable experiments on microcrystals, the ESR resonator needs to be adapted to the size and shape of these specific samples. Here, we present a unique family of miniature surface resonators, known as "ParPar" resonators, whose mode volume and shape are optimized for such micron-scale single crystals. This approach significantly improves upon the samples' filling factor and thus enables the measurement of much smaller crystals than was previously possible. We present here the design of such resonators with a typical mode dimension of 20-50 µm, as well as details about their fabrication and testing methods. The devices' resonant mode(s) are characterized by ESR microimaging and compared to the theoretical calculations. Moreover, experimental ESR spectra of single microcrystals with typical sizes of ∼25-50 µm are presented. The measured spin sensitivity for the 50-µm resonator at cryogenic temperatures of 50 K is found to be ∼1.8 × 106 spins/G √Hz for a Cu-doped single crystal sample that is representative of many biological samples of relevance.

A general way of analyzing EPR spectroscopy for a pair of magnetically equivalent lanthanide ions in crystal: A case study of BaY2F8:Yb3+ crystal

Electron paramagnetic resonance (EPR) is an important tool to study the complex interactions (e.g., exchange and magnetic dipole-dipole interactions) for a pair of lanthanide (Ln) ions in crystals. How to analyze these EPR spectra and obtain the strength of each interaction is a challenge for experimentalists. In this work, a general way of calculating the EPR lines for two magnetically equivalent Ln ions is given by us to solve this problem. In order to explain their EPR spectra and obtain exchange interaction parameters Ji (i = x, y, z) between them, we deduce the analytic formulas for computing the angular dependent EPR lines for such Ln pairs under the condition of weak coupling (|Ji| ≪ hv, where v is the microwave frequency in the EPR experiment) and set up the spin-Hamiltonian energy matrix that should be diagonalized to obtain these lines if intermediate (|Ji| ∼ hv) and strong (|Ji| > hv) couplings are encountered. To verify our method, the experimental EPR spectra for the Yb3+ doped BaY2F8 crystal are considered by us and the EPR lines from the isolated Yb3+ ion and Yb3+-Yb3+ pair with distance R equal to 0.371 nm are identified clearly. Moreover, exchange interaction parameters (Jx ≈ –0.04 cm–1, Jy ≈ –0.24 cm–1, and Jz ≈ –0.1 cm–1) for such a pair are also determined by our calculations. This case study demonstrates that the theoretical method given in this work would be useful and could be applied to understand interactions between Ln ions in crystals.

Influence of the Nature of the Ion Pairs on the Segmental Dynamics in Polyelectrolyte Complex Coacervate Phases

A spin label has been covalently attached to the weak polyanion poly(ethylene-alt-maleic acid) (P(E-alt-MA)) to study the segmental dynamics of the polyanion backbone in polyelectrolyte complex coacervate phases formed with oppositely charged polycations such as poly(diallyldimethylammonium chloride) (PDADMAC), linear poly(ethylenimine) (l-PEI) and poly(allylamine hydrochloride) (PAH) by continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy. The segmental rotational mobility of the spin-labeled polyanion has been determined by the simulation of the line shape of the experimental CW EPR spectra using the microscopic order/macroscopic disorder (MOMD) model of restricted rotational diffusion. The study has shown that the segmental mobility of the polyanion backbone in the coacervate phases decreases in the sequence PDADMAC > l-PEI > PAH. The nature of the ion pairs formed between the oppositely charged polyelectrolytes has a strong impact on the dynamics of the polyanion.

In Quest of the Alanine R3 Radical: Multivariate EPR Spectral Analyses of X-Irradiated Alanine in the Solid State.

The amino acid l-α-alanine is the most commonly used material for solid-state electron paramagnetic resonance (EPR) dosimetry, due to the formation of highly stable radicals upon irradiation, with yields proportional to the radiation dose. Two major alanine radical components designated R1 and R2 have previously been uniquely characterized from EPR and electron-nuclear double resonance (ENDOR) studies as well as from quantum chemical calculations. There is also convincing experimental evidence of a third minor radical component R3, and a tentative radical structure has been suggested, even though no well-defined spectral signature has been observed experimentally. In the present study, temperature dependent EPR spectra of X-ray irradiated polycrystalline alanine were analyzed using five multivariate methods in further attempts to understand the composite nature of the alanine dosimeter EPR spectrum. Principal component analysis (PCA), maximum likelihood common factor analysis (MLCFA), independent component analysis (ICA), self-modeling mixture analysis (SMA), and multivariate curve resolution (MCR) were used to extract pure radical spectra and their fractional contributions from the experimental EPR spectra. All methods yielded spectral estimates resembling the established R1 spectrum. Furthermore, SMA and MCR consistently predicted both the established R2 spectrum and the shape of the R3 spectrum. The predicted shape of the R3 spectrum corresponded well with the proposed tentative spectrum derived from spectrum simulations. Thus, results from two independent multivariate data analysis techniques strongly support the previous evidence that three radicals are indeed present in irradiated alanine samples.

Alkyne-Bridged Multi[Copper(II) Porphyrin] Structures: Nuances of Orbital Symmetry in Long-Range, Through-Bond Mediated, Isotropic Spin Exchange Interactions.

Spin and conformational dynamics in symmetric alkyne-bridged multi[copper(II) porphyrin] structures have been studied in toluene solution at variable temperature using steady-state electron paramagnetic resonance (EPR) spectroscopy. Comparison of the dimer EPR spectra to those of Cu porphyrin monomers shows evidence of an isotropic exchange interaction (Javg) in these biradicaloid structures, manifested by a significant line broadening in the dimer spectra. The extent line broadening depends on molecular structure and temperature, suggesting Javg is modulated by conformational dynamics that impact the torsional angle distribution between the porphyrin-porphyrin least-squares planes. Computational simulation of the experimental EPR spectra, using a developed algorithm for J modulation in flexible organic biradicals, supports this hypothesis. Comparison of ethyne and butadiyne alkyne bridges reveals remarkable sensitivity to orbital interactions between the spacer and the metal, reflected in measurements of Javg as a function of temperature. The results suggest orbital symmetry relationships may be more important than recognized in design of optimized molecular spintronic devices.

EPR spectral investigation of radiation-induced radicals of gallic acid.

In the present work, spectroscopic features of the radiation-induced radicals of gallic acid compounds were investigated using electron paramagnetic resonance (EPR) spectroscopy. While un-irradiated samples presented no EPR signal, irradiated samples exhibited an EPR spectrum consisting of an intense resonance line at the center and weak lines on both sides. Detailed microwave saturation investigations were carried out to determine the origin of the experimental EPR lines. It is concluded that the two side lines of the triplet satellite originate from forbidden "spin-flip" transitions. The spectroscopic and structural features of the radiation-induced radicals were determined using EPR spectrum fittings. The experimental EPR spectra of the two gallic acid compounds were consistent with the calculated EPR spectroscopic features of the proposed radicals. It is concluded that the most probable radicals are the cyclohexadienyl-type, [Formula: see text] radicals for both compounds.

Theoretical investigations of the optical spectra and EPR parameters for the isolated and pairs of trivalent ytterbium ions in Li6Y(BO3)3 crystal

The optical spectra and electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants A) for an isolated Yb3+ ion in Li6Y(BO3)3 (LYB) crystal are investigated first by the complete diagonalization method based on the superposition model. The obtained results are in reasonable agreement with the experimental ones. For a pair of coupled Yb3+ ions in crystals, the extensively used formulas to calculate EPR splitting lines for such pair cannot be directly adopted in the present study and thus we extend them to the case of arbitrarily directed vector R connecting the two Yb3+ ions in crystal. By these amended formulas, we find that in addition to the interacting Yb3+ pair with nearest distance R (=0.385nm), the Yb3+ pair with next-nearest distance R (=0.662nm) would also make contribution to experimental EPR spectra. This point which was not noticed in previous works would help us further understand the energy transfer scheme between two Yb3+ ions in crystals.

Monitoring the Tetraethyl Orthosilicate (TEOS)-Based Sol–Gel Process with Cu(II) Ions as a Spin Probe

The various stages of the tetraethyl orthosilicate-based sol–gel process were investigated using electron paramagnetic resonance (EPR) spectroscopy with Cu(II) cations as a spin probe. The latter were introduced to the starting reaction mixture in the form of various copper(II) salts containing anions of different basicity (CuCl2, Cu(ac)2, or CuSO4). At the various defined stages of the sol–gel process, the experimental EPR spectra, recorded at both ambient and liquid nitrogen temperature, were found to be a superimposition of three main types of individual subspectra (Γ 1, Γ 2, and Φ), which reflect the different local environment in which the Cu(II) ions were located. The spin Hamiltonian parameters of each individual subspectrum remained identical, within experimental error, throughout the various stages of the sol–gel process. In contrast, the relative proportion of the individual subspectra varied significantly as the sol–gel process proceeded, from which the liquid-state to solid-state transition could be monitored as it occurred in the sol–gel reaction medium. Identical results were obtained, irrespective of the nature of the copper(II) salt employed. The results demonstrate that the EPR method provides an effective means with which to monitor the sol-to-gel transition from the viscous, colloid suspension to the final viscoelastic gel.

Theoretical investigations of the spin Hamiltonian parameters for the CoCl(PPh3)3 molecules

The perturbation formulas of the spin Hamiltonian parameters (zero-field splitting and g factor g// and g⊥) are established for a 3d8 ion in trigonally distorted tetrahedra for the first time. In the theoretical treatments, the contributions from the Jahn–Teller effect, the ligand orbital and spin–orbit coupling interactions and configuration interactions are taken into account from the cluster approach in a uniform way. The above formulas are applied to the studies of the spin Hamiltonian parameters for the three CoCl(PPh3)3 molecules, and the experimental electron paramagnetic resonance spectra of all the molecules are satisfactorily explained. The significant compressions of the ligand tetrahedra with mixed chlorine and PPh3 groups around Co+ are analysed for three distinct CoCl(PPh3)3 compounds, characterised by the Cl–Co–P bond angles θ larger than the tetrahedral angle of 109.47°. The local trigonal distortions are discussed in view of the Jahn–Teller effect.