Simultaneous Electron Spin Resonance Research Articles

Large cross-effect dynamic nuclear polarisation enhancements with kilowatt inverting chirped pulses at 94 GHz

Dynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei by applying resonant microwave radiation, and has been widely used to improve the sensitivity of nuclear magnetic resonance (NMR). Here we demonstrate new levels of performance for static cross-effect proton DNP using high peak power chirped inversion pulses at 94 GHz to create a strong polarisation gradient across the inhomogeneously broadened line of the mono-radical 4-amino TEMPO. Enhancements of up to 340 are achieved at an average power of a few hundred mW, with fast build-up times (3 s). Experiments are performed using a home-built wideband kW pulsed electron paramagnetic resonance (EPR) spectrometer operating at 94 GHz, integrated with an NMR detection system. Simultaneous DNP and EPR characterisation of other mono-radicals and biradicals, as a function of temperature, leads to additional insights into limiting relaxation mechanisms and give further motivation for the development of wideband pulsed amplifiers for DNP at higher frequencies.

Design Considerations of a Dual Mode X-Band EPR Resonator for Rapid In-Situ Microwave Heating

This paper describes the design considerations for a dual mode X-band continuous wave (CW) Electron Paramagnetic Resonance (EPR) cavity, for simultaneous EPR measurement and microwave heating of the same sample. An elliptical cavity geometry is chosen to split the degeneracy of the TM110 mode, allowing for a well resolved EPR signal with the TM110,a and TM110,b modes resonating at around 10 GHz and 9.5 GHz, respectively, the latter of which is used for EPR measurements. This geometry has the benefit that the TM010 mode used for microwave heating resonates at 6.1 GHz, below the cut off frequency of the X-band waveguide used for the EPR channel, providing effective isolation between the heating and EPR channels. The use of a pair of 9 µm thick copper clad laminates as the flat cavity walls allows for sufficient penetration of the modulation field (Bmod) into the cavity, as well as maintaining a high cavity Q factor (> 5700) for sensitive EPR measurements. Locating the heating port at an angle of 135° to the EPR port provides additional space for easier coupling adjustment and for larger sample access to be accommodated. The associated decrease of EPR signal strength is fully compensated for by using a 7.2 GHz low pass filter on the heating port. EPR spectra using 1.6 mm and 4.0 mm sample tubes are shown at room temperature (298 K) and 318 K for a standard Cu(acac)2 solution, demonstrating the effectiveness of this dual-mode EPR cavity for microwave heating during EPR detection.

Development of Millimeter Wave Fabry-Pérot Resonator for Simultaneous Electron-Spin and Nuclear Magnetic Resonance Measurement

We report a Fabry-Perot resonator with spherical and flat mirrors to allow simultaneous electron-spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements that could be used for double magnetic resonance (DoMR). In order to perform simultaneous ESR and NMR measurements, the flat mirror must reflect millimeter wavelength electromagnetic waves and the resonator must have a high Q value (Q > 3000) for ESR frequencies, while the mirror must simultaneously let NMR frequencies pass through. This requirement can be achieved by exploiting the difference of skin depth for the two frequencies, since skin depth is inversely proportional to the square root of the frequency. In consideration of the skin depth, the optimum conditions for conducting ESR and NMR using a gold thin film are explored by examining the relation between the Q value and the film thickness. A flat mirror with a gold thin film was fabricated by sputtering gold on an epoxy plate. We also installed a Helmholtz radio frequency coil for NMR and tested the system both at room and low temperatures with an optimally thick gold film. As a result, signals were obtained at 0.18 K for ESR and at 1.3 K for NMR. A flat-mirrored resonator with a thin gold film surface is an effective way to locate NMR coils closer to the sample being examined with DoMR.

Synthesis, Electronic Structures, and Electrochemistry of 3‐Triarylphosphoraniminato‐1,3,5‐trithia‐2,4,6,8‐tetrazocines: Detection of Trithiatetrazocinyl Radical Anions

AbstractReaction of the bicyclic sulfur‐nitrogen heterocycles RC6H4CN5S3 (R=4‐CH3O, 4‐CH3, 4‐H, 4‐Cl, 4‐CF3, 3‐CF3) with PR’3 (R′=C6H5 or 4‐CH3OC6H4) produces 3‐phosphoraniminato‐7‐aryl‐1,3,5‐trithia‐2,4,6,8‐tetrazocines RC6H4CN4S3NPR’3. In all cases, only the endo‐isomers were isolated and characterized by 1H, 31P, and 19F NMR as well as UV spectroscopy and X‐ray crystallography. RB3LYP/6‐311+G(d,p)// RB3LYP/6‐31G(d,p) computations were undertaken to help explain the atom‐exact syntheses via intermediates previously detected spectroscopically. Three reasonable intermediates have been computed as stationary points with energies consistent with the observed reaction path. Cyclic and square‐wave voltammetry studies of RC6H4CN4S3NPR’3, using a glassy carbon working electrode in CH2Cl2 with 0.4 M [nBu4N][PF6], reveal two IRR reduction processes at RT in all cases at approximately −1.9 and −2.2 V, respectively, and one IRR oxidation process at approximately +1.0 V (versus Fc+/0). [RC6H4CN4S3NPR’3]−. radical anions (R’=4‐CH3OC6H4, R=4‐CH3O, 4‐H, 4‐CF3) were detected at −50 °C in CH2Cl2 by using in situ electrolysis and simultaneous electron paramagnetic resonance spectroscopy: [4‐RC6H4CN4S3NP(4‐CH3OC6H4)3]−., estimated a(31P)=0.078 mT, a(14N1)=0.031 mT, a(14N2,3)=0.256 mT, a(14N4,5)=0.341 mT. The signals decay rapidly, but after the electrolysis is ended, a persistent EPR signal is always obtained with EPR parameters that match known 4‐aryl‐1,2,3,5‐dithiadiazolyls, that is, a(N)=0.51 mT and g=2.0105, which are consistent with the production of [4‐CF3‐C6H4CN2S2]..

Spectroelectrochemistry: ESR of Paramagnetic Intermediates in the Electron Transfer Series [Cr(bpy)3]n (n = 3+, 2+, 1+, 0, 1-)

A new method of cyclic voltammetry with simultaneous ESR spectra multiple recording (CV-MRESR) has been proposed and implemented. Along with cyclic voltammetry (CV) and electron spin resonance (ESR), the combined method involving cyclic voltammetry and ESR (CV-ESR) has been used for the study of electron transfer in a series of complexes [Cr(bpy)3]n (n=3+, 2+, 1+, 0, 1-).

Experimental setup for investigation of nanoclusters at cryogenic temperatures by electron spin resonance and optical spectroscopies.

We present the design and performance of an experimental setup for simultaneous electron spin resonance (ESR) and optical studies of nanoclusters with stabilized free radicals at cryogenic temperatures. A gas mixture of impurities and helium after passing through a RF discharge for dissociation of molecules is directed onto the surface of superfluid helium to form the nanoclusters of impurities. A specially designed ESR cavity operated in the TE011 mode allows optical access to the sample. The cavity is incorporated into a homemade insert which is placed inside a variable temperature insert of a Janis (4)He cryostat. The temperature range for sample investigation is 1.25-300 K. A Bruker EPR 300E and Andor 500i optical spectrograph incorporated with a Newton EMCCD camera are used for ESR and optical registration, respectively. The current experimental system makes it possible to study the ESR and optical spectra of impurity-helium condensates simultaneously. The setup allows a broad range of research at low temperatures including optically detected magnetic resonance, studies of chemical processes of the active species produced by photolysis in solid matrices, and investigations of nanoclusters produced by laser ablation in superfluid helium.

Simultaneous Imaging of an Enantiomer Pair by Electron Paramagnetic Resonance Using Isotopic Nitrogen Labeling

This Article describes the simultaneous imaging of chiral nitroxyl radicals using electron paramagnetic resonance (EPR). Chiral nitroxyl radicals could be simultaneously visualized with the labeling of isotopic nitrogen. Chiral nitroxyl radicals, hydroxylmethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl, were visualized using the method of simultaneous EPR imaging, which refers to the visualization of two kinds of molecules with unpaired electrons in a single image scan. EPR spectra of a racemic mixture of chiral nitroxyl radicals and those of the respective R and S configurations confirmed labeling by isotopic nitrogen. (1)H nuclear magnetic resonance (NMR) imaging and simultaneous imaging of solutions of chiral nitroxyl radicals were performed. The advantages and limitations of simultaneous imaging using EPR are also discussed. Simultaneous imaging with chiral-labeled nitroxyl radicals is a new application of EPR imaging and may be useful for biological studies involving biologically active chiral molecules.

Probing the paramagnetic interactions between the unpaired electronic spins of carbon atoms and the nuclear spins of hydrogen molecules with Raman spectroscopy

Carbon materials typically have a high density of unpaired electronic spins but the exact nature of the defect sites that give rise to their magnetic properties are not yet well understood. In this work, the paramagnetic interactions between the unpaired electronic spins of carbon atoms and the nuclear spins of hydrogen molecules were probed with Raman spectroscopy by monitoring the relative population of H2 rotational states. For H2, the symmetries of nuclear spin and rotational wave functions are correlated. Because of the weak interactions between H2 nuclear spins, the transitions between odd and even rotational states are normally hindered. The magnetic field generated by unpaired electronic spins relaxes the selection rules and promotes transitions between H2 rotational levels of different symmetry. This affects the rotational levels' relaxation kinetics toward equilibrium and makes H2 molecules useful to study unpaired electrons in paramagnetic materials. It is suggested that simultaneous electron paramagnetic resonance and Raman measurements on carbon materials interacting with hydrogen molecules could result in a better understanding of the nature of paramagnetic defects in carbon materials, which could have a substantial impact on Li‐ion batteries or for understanding the graphene electronic properties. Copyright © 2011 John Wiley & Sons, Ltd.

A High-Conversion-Factor, Double-Resonance Structure for High-Field Dynamic Nuclear Polarization

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W1/2 for the microwave conversion factor and 0.8 mT/W1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state 1H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about −16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure.

Charge Carrier Transport and Optical Properties of Poly[N-methyl(aniline)

Simultaneous electron spin resonance (ESR) and UV−vis−NIR measurements have been carried out during oxidation of a poly[N-methyl(aniline)] (PNMA) film, which was prepared electrochemically in aqueous solutions containing 1 M trifluoroacetic acid. In situ conductance measurements were also performed during the electrochemical doping of PNMA. The combination of the different experimental techniques allows a simultaneous elucidation of optical and electrochemical properties of the conducting polymer film. Although there are a lot of predictions and discussions on the free charge carriers of poly(aniline) (PANI) and its derivatives, there is no direct evidence of the correlation between polarons and conductance for PANI and its derivatives during doping. This article provides the correlation between the spin and spinless species and the conductance of PNMA during the electrochemical doping in acidic aqueous solutions. PNMA differs from other conjugated polymers (e.g. polypyrrole) in that the results from ESR and in situ conductance are consistent. At higher doping (oxidation) levels, the conductance decreases with the decrease in the ESR signal, indicating that there is no spinless bipolaronic conducting state in PNMA. Polaron pairs are supposed to be responsible for these decreases.

The redox mechanism of polyaniline studied by simultaneous ESR–UV–vis spectroelectrochemistry

The electrochemical redox process to charge and discharge polyaniline layers is studied by simultaneous Electron Spin Resonance (ESR) and UV–vis spectroscopic measurements. For this purpose, optically transparent electrodes were applied under potential-controlled conditions. The use of a calibrated manganese ESR standard ensures the calculation of the absolute numbers of free spins in the polymer layer during electrochemical doping. A general procedure is given to analyse the simultaneously recorded electrochemical and spectroscopic data. The procedure proposed following the time dependence of charge, ESR absorption and the absorbance demonstrates that the electrochemical charging/discharging mechanism of polyaniline can be described on the base of three oxidation states. In this way, the separated UV–vis spectra and the time/potential dependence of each redox state can be shown. Only little changes are observed in the shape of the separated spectra using the polaron/bipolaron model in comparison to the polaron/ σ-dimer model. A square ladder scheme of the redox mechanism is discussed, considering neutral, polaronic and bipolaronic states and the protonation equilibria. Furthermore, experimental evidence for the Faradaic nature of the “capacitive” current plateau is presented.

Separation of the Ultraviolet−Visible Spectra of the Redox States of Conducting Polymers by Simultaneous Use of Electron-Spin Resonance and Ultraviolet−Visible Spectroscopy

The electrochemical oxidation of polyaniline (PAni) films on gold electrodes is studied by simultaneous electron spin resonance (ESR) and UV−vis spectroelectrochemistry. The technique permits the separation of the superimposed UV−vis spectra of three redox states: the protonated reduced polyaniline chains, the deprotonated polaronic state, and the bipolaronic state. The separated UV−vis spectra as well as the potential dependence of each redox state concentration enables a more precise determination of the energy levels and the stability of paramagnetic species.

Simultaneous EPR and Kinetic Study of Styrene Atom Transfer Radical Polymerization (ATRP)

To investigate the mechanism of the controlled/“living” radical polymerization system, electron paramagnetic resonance (EPR) spectroscopy was applied to the atom transfer radical polymerization (ATRP) of styrene. Although only copper(I) species was added to the system initially, EPR signals of a copper(II) species were clearly observed in the polymerization of styrene initiated by 1-phenylethyl bromide, 1-phenylethyl chloride, and benzyl bromide in toluene solution or in bulk. As the polymerization proceeded, the concentration of copper(II) increased gradually until a steady state was reached. Correlation between the time dependence of the concentration of copper(II) species and the kinetics of the polymerization is also discussed for various ATRP systems. Our results indicate that 4−6% of the initial copper(I) species is converted to copper(II) species in polymerization of styrene at 110 °C. Investigation of the ATRP systems which contain an excess amount of copper(0) powder with a given amount of copper(II) species was also performed by EPR spectroscopy. Concentration of copper(II) species was progressively reduced by a reaction with copper(0), but the reaction was slow enough not to interfere with the control of radical polymerization.

Tunable Phase Transitions in Conductive Cu(2,5-Dimethyl-Dicyanoquinonediimine)$\mathsf{_2}$ Radical Ion Salts

The conduction process of the copper salts of DCNQI and the origin of the phase transition inducable in different ways, is explained in this review by an admixture of the copper d states to the DCNQI pπ band. This admixture depends on structural processes which shift the charge transfer δ from the copper moiety to the DCNQI stack to larger values with reduced lattice dimensions. Reaching exactly δ = 4/3 (band filling degree ρ = 2/3), the trimerization in the copper stack leads to the disproportion of the mixed valence state of Cu by the localization of the d states as Cu 2+ . A charge density wave is formed, opening a gap at the Fermi level E F . These statements are acquired by selective deuteration, selective alloying, applying pressure and by simultaneous electron spin resonance (ESR) and conductivity experiments in the ESR equipment under pressure as a function of temperature. For all systems, when reaching critical values of the unit cell volume, phase transitions take place. We could establish a general phase diagram which is based on an effective pressure scale including internal (chemical) and external applied pressure.

Young Investigator Award presentation at the 13th Annual Meeting of the ESMRMB, September 1996, Prague. A proton-electron double-resonance imaging apparatus with simultaneous multiple electron paramagnetic resonance irradiation at 10 mT.

The detection of free radicals in vivo is very important for the study of many physiologic and pathologic conditions. Free radicals have been implicated in a number of diseases such as ischemia, inflammation, kidney damage, and cancer. Proton-electron double-resonance imaging (PEDRI) allows the indirect detection of free radicals via the Overhauser effect. Nitroxide free radicals used for in vivo PEDRI studies present spectra with two or three lines, but most PEDRI experiments performed to date have used only single-line electron paramagnetic resonance (EPR) irradiation. There is theoretical evidence that simultaneous irradiation of multiple EPR transitions could increase the maximum achievable PEDRI enhancement. From the experimental point of view, this requires the combined use of a suitable multiple-frequency EPR source and a multiple-tuned EPR resonator. A novel radiofrequency (RF) triple-tuned loop-gap resonator for use in PEDRI has recently been developed, and dynamic nuclear polarization (DNP) data were reported. In the present study we describe a new PEDRI apparatus, equipped with a triple-tuned resonator, that is suitable for simultaneous double- or triple-EPR irradiation of nitroxide free radicals. In particular, the details of the EPR hardware used to generate the two or three EPR frequencies are given, and PEDRI images obtained with simultaneous multiple EPR irradiation are shown. Moreover, DNP experimental results showing the increase of the enhancement as a function of the EPR power for single and simultaneous double EPR irradiation are presented. The main goal of this apparatus is to improve the sensitivity and/or to reduce EPR irradiation power in a PEDRI experiment. This is likely to be particularly important in future biologic applications of PEDRI where the applied power must be optimized to reduce sample heating.

A simultaneous electron paramagnetic resonance and optical absorption study of ultraviolet-induced defect centers in Ge-doped silica fibers

Simultaneous electron paramagnetic resonance (EPR) and optical absorption studies were carried out at room temperature in a Ge-doped silica fiber-optic waveguide following in situ ultraviolet (UV) irradiation with 254-nm (4.88-eV) photons. We have observed that the UV-induced EPR is directly correlated with the UV-induced optical loss in the energy range 1.77–3.0 eV. No UV-induced loss was observed in the near-infrared spectral region (0.7–1.38 eV). The problem of correlating the UV-induced loss with the optical bands of the various possible defect centers in Ge-doped silica is discussed.

Simultaneous EPR electrochemical measurements on polyfluorene in ambient temperature ionic liquids

Simultaneous electron paramagnetic resonance (EPR) and electrochemical measurements have been carried out on polyfluorene prepared by monomer oxidation and utilized in an ambient temperature ionic liquid consisting of 1-methyl-3-ethylimidazolium chloride-aluminum chloride. A considerable EPR signal is observed in both the reduced and oxidized states; the EPR signal achieves a maximum value coincident with the peak current during a cyclic voltammogram. Assuming that the process is two one-electron steps, initially forming a radical cation, the difference between the two E 0 values is estimated as less than 70 mV.

Simultaneous ESR and optical absorption measurements of K in solid Ar

An apparatus was developed for simulataneous electron spin resonance (ESR) and optical absorption experiments on atoms imbedded in rare gas matrices. In a first application the temperature dependence of ESR signals and of optical absorption spectra of K atoms in Ar matrices were studied. One of the numerous ESR lines could be unambiguously assigned to the so-called red triplet absorption due to their identical annealing behaviour at 12–14 K. The correspondence of other ESR lines to optical absorptions is discussed.

Cryostat for Combined EPR and Optical Studies

The design of a cryostat which allows simultaneous electron paramagnetic resonance studies and optical absorption measurements is given. Details of the microwave cavity, whose coupling can be adjusted from outside the cryostat, optical characteristics, and sample positioning mechanism are included. In addition, discussion of possible use of the cryostat for x irradiation or uv irradiation of samples at low temperature is given. Provisions for electron-nuclear double resonance measurements and temperature variation of the sample are also discussed.