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Liquid crystalline physical gels which consist of a chiral racemic nitroxide radical compound (±)-1showing a nematic phase and a diamagnetic, chiral organic gelator (R,R)-2were prepared to investigate the effects of the gelator on the “positive magneto-LC effects” observed in the nematic phase of (±)-1and the molecular alignment of (±)-1in the resulting nematic gels. With the increasing amount of the gelator which decreased the domain size, the molecular reorientation by a magnetic field (0.34 T) was considerably restricted in the nematic gel phase and the intermolecular magnetic interactions (positive magneto-LC effects) increased in the fine and random nematic polydomain structure formed by self-assembled fibers of organic gelator molecules. [ABSTRACT FROM AUTHOR]

The reduction technique of 2,2,5,5-tetramethylpyrroline compounds to pyrrolidines on sodium amalgam preformed under the electrolysis of sodium hydroxide solution is described.

In this review on advanced biomolecular EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR regarding the measurement of interactions and dynamics of large molecules embedded in fluid-solution or solid-state environments. Our focus is on the characterization of protein structure, dynamics and interactions, using sophisticated EPR spectroscopy methods. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy to new horizons reaching millimeter and sub-millimeter wavelengths and 15 T Zeeman fields. Expanding traditional applications to paramagnetic systems, spin-labeling of biomolecules has become a mainstream multifrequency approach in EPR spectroscopy. In the high-frequency/high-field EPR region, sub-micromolar concentrations of nitroxide spin-labeled molecules are now sufficient to characterize reaction intermediates of complex biomolecular processes. This offers promising analytical applications in biochemistry and molecular biology where sample material is often difficult to prepare in sufficient concentration for NMR characterization. For multifrequency EPR experiments on frozen solutions typical sample volumes are of the order of 250 μL (S-band), 150 μL (X-band), 10 μL (Q-band) and 1 μL (W-band). These are orders of magnitude smaller than the sample volumes required for modern liquid- or solid-state NMR spectroscopy. An important additional advantage of EPR over NMR is the ability to detect and characterize even short-lived paramagnetic reaction intermediates (down to a lifetime of a few ns). Electron⁻nuclear and electron⁻electron double-resonance techniques such as electron⁻nuclear double resonance (ENDOR), ELDOR-detected NMR, PELDOR (DEER) further improve the spectroscopic selectivity for the various magnetic interactions and their evolution in the frequency and time domains. PELDOR techniques applied to frozen-solution samples of doubly spin-labeled proteins allow for molecular distance measurements ranging up to about 100 Å. For disordered frozen-solution samples high-field EPR spectroscopy allows greatly improved orientational selection of the molecules within the laboratory axes reference system by means of the anisotropic electron Zeeman interaction. Single-crystal resolution is approached at the canonical g-tensor orientations—even for molecules with very small g-anisotropies. Unique structural, functional, and dynamic information about molecular systems is thus revealed that can hardly be obtained by other analytical techniques. On the other hand, the limitation to systems with unpaired electrons means that EPR is less widely used than NMR. However, this limitation also means that EPR offers greater specificity, since ordinary chemical solvents and matrices do not give rise to EPR in contrast to NMR spectra. Thus, multifrequency EPR spectroscopy plays an important role in better understanding paramagnetic species such as organic and inorganic radicals, transition metal complexes as found in many catalysts or metalloenzymes, transient species such as light-generated spin-correlated radical pairs and triplets occurring in protein complexes of photosynthetic reaction centers, electron-transfer relays, etc. Special attention is drawn to high-field EPR experiments on photosynthetic reaction centers embedded in specific sugar matrices that enable organisms to survive extreme dryness and heat stress by adopting an anhydrobiotic state. After a more general overview on methods and applications of advanced multifrequency EPR spectroscopy, a few representative examples are reviewed to some detail in two Case Studies: (I) High-field ELDOR-detected NMR (EDNMR) as a general method for electron⁻nuclear hyperfine spectroscopy of nitroxide radical and transition metal containing systems; (II) High-field ENDOR and EDNMR studies of the Oxygen Evolving Complex (OEC) in Photosystem II, which performs water oxidation in photosynthesis, i.e., the light-driven splitting of water into its elemental constituents, which is one of the most important chemical reactions on Earth.

We synthesized 1-ethylimidazolyl-substituted nitronyl nitroxides, i.e., 2-(1-ethylimidazol-4-yl)- (L) and 2-(1-ethylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole 3-oxide-1-oxyl (L). The stable radical L is an ethyl analog of 2-(1-methylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole 3-oxide-1-oxyl (L) described earlier, the reaction of which with Cu(hfac) (hfac is 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) leads to the formation of the [Cu(hfac)(L)] jumping crystals. The reaction of Cu(hfac) with L with reagent ratios 1: 2 and 1: 1 yields heterospin complexes [Cu(hfac)(L)] and [Cu(hfac)L], respectively. X-ray diffraction study of the mononuclear complex [Cu(hfac)(L)] determined that the compound has a packing similar to that of jumping crystals studied earlier, with the only difference being that the O...O contacts between neigh- boring nitroxide groups were found to be 0.3-0.5 Å longer than in [Cu(hfac)(L)]. As a result of the lengthening of these contacts, [Cu(hfac)(L)] crystals lack chemomechanical activi- ty. We found that when cooling crystals of binuclear complex [Cu(hfac)L] below 50 K, the antiferromagnetic exchange between unpaired electrons of the >N-•O groups of neighboring molecules leads to the full spin-pairing of the nitroxides, with only the Cu ions contributing to the residual paramagnetism of the compound. [ABSTRACT FROM AUTHOR]

The electronic structure of free radicals and Cu(II) complexes with the nitronyl nitroxide radical and aminonitrone is studied by X-ray photoelectron spectroscopy (XPS). N1s XPS spectra of nitrogen atoms of Cu(II) complexes with aminonitrones confirm the diamagnetic nature of the ligands. The binding energies of the main peak and the intense satellite structure in the Cu2 p spectra of the complexes under study correspond to the Cu(II) state. The structure of the satellite in the Cu2 p spectrum depends on the environment of the metal ion. [ABSTRACT FROM AUTHOR]

Abstract: Electron paramagnetic resonance spectra of nitroxide radicals are reporters of molecular tumbling correlation times. The concepts of electron paramagnetic resonance and molecular tumbling are demonstrated by examination of spectra of the radical tempol (4‐hydroxy‐2,2,6,6‐tetramethylpiperidin‐1‐oxyl) in water:glycerol mixtures. Analysis of the spectral line shapes with the Kivelson model and computer simulation with EasySpin are discussed. Values of the tumbling correlation times obtained by the two methods are shown to be in good agreement. Comparison of the experimental tumbling correlation times with values calculated with the Stokes‐Einstein model indicates a slip coefficient of 0.066, which is consistent with other reports for nitroxides in various solvents. [ABSTRACT FROM AUTHOR]

The angular dependences of concentration broadening of EPR spectra for nitroxide spin probes in liquid crystals were experimentally measured. The obvious angular dependence of the broadening found for oriented smectic liquid crystal HOPDOB proves the paired localization of the probe molecules. The numerical calculation of the angular dependence taking into account the magnetic dipolar and spin exchange interactions have been used for quantitative determination of position of probes in the pairs. The probable localization of the probes in the smectic layer is discussed. [ABSTRACT FROM AUTHOR]

Purpose Electron paramagnetic resonance (EPR) imaging in the spectral-spatial domain with application of soluble paramagnetic probes provides an opportunity for spatially resolved functional measurements of living objects. The purpose of this study was to develop EPR methods for visualization of oxygenation and acidosis of ischemic myocardium. Methods EPR oxygen measurements were performed using isotopically substituted 2H,15N-dicarboxyproxyl. The radical has an EPR line width of 320 mG and oxygen-induced line broadening of 0.53 mG/mm Hg, providing oxygen sensitivity down to 5 μM. pH measurements were performed using previously developed pH-sensitive imidazoline nitroxide. The radical has an EPR spectrum with pH-dependable hyperfine splitting, pK = 6.6, providing pH sensitivity of approximately 0.05 U in the physiological range. Results EPR imaging of isolated and perfused rat hearts was performed in the two-dimensional + spectral domain. The spatial resolution of the measurements was about 1.4 mm. Marked tissue hypoxia was observed in the ischemic area of the heart after occlusion of the left anterior descending coronary artery. Tissue oxygenation was partly restored upon reperfusion. EPR mapping of myocardial pH indicated acidosis of the ischemic area down to pH 6.7-6.8. Conclusion This study demonstrates the capability of low-field EPR and the nitroxide spin probes for mapping of myocardial oxygenation and pH. The developed approaches might be used for noninvasive investigation of microenvironment on living objects. Magn Reson Med 76:350-358, 2016. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

We have synthesized a racemic and enantiomerically enriched all-organic paramagnetic nitroxide radical (NR) liquid crystalline (LC) compound, which contains a chiral NR unit in the mesogen core and a methyl di(ethylene glycol) chain as one of their terminal units. These new compounds showed remarkable substituent effects of the methyl di(ethylene glycol) chain on the phase transition behaviors and hydrophilic properties in contrast to structural analogs of the previous reported NR-LC compounds containing alkoxy chains. [ABSTRACT FROM AUTHOR]

5-Formylpyrrolyl-substituted nitronyl and imino nitroxide radicals HL and HL were synthesized. Their solid phases are formed by packing pairs of the molecules. In the {HL...HL} pairs, the dominant interaction is the ferromagnetic exchange with J/kB = 8.8 K (Hamiltonian $$H = 2J\left( {\overrightarrow {{s_1}} \overrightarrow {{s_2}} } \right)$$ ). The ferromagnetic exchange occurs also in the heterospin molecules [Ni(L)], [Cu(L)], and [Ni(L)(MeOH)]. In the complexes [Ni(L)] and [Cu(L)], a small change in the mutual orientation of the coordinated ligands has a considerable effect on the value and the sign of the energy of exchange interactions between the unpaired electrons of the metal ion and paramagnetic ligands. [ABSTRACT FROM AUTHOR]

Salts 1 and 2 that combine the [W6O19]2- Lindqvist anion with the cationicnitronyl nitroxide (NN) free radicals p-MepyNN+ and nBu3NCH2NN+ , respectively, havebeen synthesized and their structural and magnetic properties have been studied.

A new series of all-organic liquid-crystalline (LC) chiral radical compounds,trans-1-alkoxyphenyl-4-[(4-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl)-benzylideneamino]benzenes (1) with various alkyl chains, containing a chiral nitroxide unit in the mesogen core were synthesized and their LC properties were fully characterized. The enantiomerically enriched compounds (2S,5S)-1showed N*, TGBA*, SmA* and SmC* phases, while the racemates (±)-1exhibited N, SmA and SmC phases. The phase transition behavior and the ferroelectric properties of the SmC* phase of (2S,5S)-1differed from those of previously reported analogues with an ester group as a substitute for the imino group of (2S,5S)-1. Some of the (2S,5S)-1series showed a temperature-dependent spontaneous polarizationPS(θ) inversion. We discuss the origin of this difference in terms of their molecular structures optimized by molecular orbital calculations. The phase transition behavior supports the hypothesis that the large ϕ, which is defined as the angle between the molecular long axis (one of the principal axes of inertia) and the direction of the dipole moment in each molecule, results in the stabilization of the SmA* (SmA) phase. [ABSTRACT FROM AUTHOR]

We have synthesized a new series of all-organic paramagnetic liquid crystalline (LC) compounds, which include a five-membered ring nitroxide radical (NR) moiety in the mesogen core and fluorine atoms as terminal substituents. These new compounds are the structural analogs of the previously reported NR-LC compounds, and showed remarkable fluorine substituent effects. The introducing fluorine atom into terminal unit stabilizes nematic phase. However, an excess of fluorination leads to the destabilization of nematic phase. [ABSTRACT FROM AUTHOR]

Within the developing interface between free radicals and supramolecular chemistry, nitroxides play a special role because they are stable and easy to functionalize. This Focus Review summarizes recent progress in the design and synthesis of paramagnetic host-guest complexes characterized by the presence of a nitroxide label covalently linked to the organic macrocycle. Examples include the synthesis of open-shell, mechanically interlocked molecules (MIMs), such as rotaxanes, in which the dumbbell, the wheel, or both are tagged with nitroxide radicals, and how electron spin resonance (ESR) spectroscopy methods have been used for detecting and identifying the assemblies and for clarifying their structure and properties. [ABSTRACT FROM AUTHOR]

Two cobalt(II) radical complexes: [Co(NIT4-py)2(PNB)2] ( 1) and [Co(im4-py)2(PMB)2(H2O)2] ( 2) [NIT4-py = 2-(4′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, im4-py = 2-(4′-pyridyl)-4,4,5,5-tetramethylimidazole-1-oxyl, PNB = p-nitrobenzoic anion, PMB = p-methylbenzoic anion] were synthesized and characterized. X-ray diffraction analysis shows that complex 1 is four coordinated with tetrahedral arrangement, where the CoII ion is located in the central site of the tetrahedron, two radicals and two PNB ligands are trans-coordinated to the CoII ion. Complex 2 exhibits a six coordinated distorted octahedron, in which two im4-py radicals, two PMB ligands, and two solvent molecules act as terminal ligands. The magnetic behaviors of complexes 1 and 2 were investigated. Complex 1 shows ferromagnetic interactions between the CoII ion and the NIT4-py radical, whereas antiferromagnetic coupling dominates between the im4-py radical and the CoII ion in complex 2. The magnetic properties can be explained by the structures and a magnetic exchange mechanism. [ABSTRACT FROM AUTHOR]

Intrinsic p -type conductivity was observed in an otherwise aliphatic epoxy backbone through derivatization with stable radicals. The conductive potential of this combined radical-epoxy material was further investigated by prospecting for energy storage systems. This work identifies macroradical epoxy composites with 10% (w/w) conductive carbon as viable devices in supercapacitor application. In cross-linking, the appropriate selection of the diamine spacer has a profound impact on the curing and the resulting conformation of the polymer. The short-chain diamine spacer 1,2-diaminoethane exhibits the best supercapacitor performance yielding a maximum specific capacitance of 195 F g−1 at 1.0 A g−1. The specific capacitance remains at 99% at 1.0 A g−1 after repeatedly charging and discharging 10,000 times. The observed cycling stability is rare for materials exhibiting pseudocapacitance. This better performance than a reference conventional epoxy composite is attributed to the overall contributions of the electric double layer and pseudocapacitance effect with the effective utilization of the nanoscale porous polymeric structure. Furthermore, the pseudocapacitance is significantly enhanced by using the short-chain diamine spacer due to the closer radical-to-radical interactions and the excellent dispersibility of the added conductive particles. [Display omitted] • Macroradical epoxies have promising energy-storage prospects. • Macroradical epoxy composite electrodes are viable supercapacitors. • Superior cycling performance is observed, rare for pseudocapacitors. • Rheology and small-angle x-ray scattering correlate with electrode chemistry. • Magnets used during cross-linking are valuable in distinguishing morphology. [ABSTRACT FROM AUTHOR]

This work presents an experimental and theoretical study to address the chemical reactivity of series of nitroxide radicals. For that purpose two physicochemical properties: the half-wave potential and the hyperfine coupling constants of the nitrogen nuclei, were analyzed. Experimental values are compared with electronic structure calculations at the BHandHLYP/6-311++G(2d,2p) level. E 1/2 values were in good agreement with the adiabatic ionization potential when including the solvent effects by the Cramer and Truhlar Solvation Model. Preeliminar experimental electron spin deslocalization studies suggest that structural hindrance plays an important role in their deslocatization mechanism. [ABSTRACT FROM AUTHOR]

Abstract: The article presents results of a study of TEMPO-labeled polymer coated superparamagnetic iron(II,III) oxide nanoparticles using both Electron Paramagnetic Resonance (EPR) spectroscopy and Electron Paramagnetic Resonance imaging technique (EPRI). The X-band (9.4 GHz) EPR spectroscopy was used to investigate the behavior of TEMPO-labeled polymer coated magnetite nanoparticles in different conditions (temperature and orientation in magnetic field). The broad line, which comes from the core of Fe3O4 nanoparticles, shows anisotropy. This signal broadens with decreasing temperature, its intensity increases with increasing temperature and the g factor decreases with increasing temperature. The shape of the signal from nitroxide radical strongly depends on temperature. When temperature is higher than 200 K, a narrow triplet appears, but when it is lower than 200 K the signal consists of broad asymmetric lines. Analysis of the signal allowed characterization of the motion of the spin label attached to nanoparticles. Values of anisotropy parameter ɛ and rotational correlation time τ c were calculated for TEMPO in the fast rotation regime. The ability of TEMPO-labeled PEG coated magnetite nanoparticles to diffuse within the hydrogel medium was also investigated. The EPR imaging of nanoparticles diffusion in hydrogel was made at room temperature using an EPR L-band (1 GHz) spectrometer. EPRI has been proved effective for evaluation of changes in the spatial distribution of nanoparticles in the sample. [Copyright &y& Elsevier]

Stable free radicals have a variety of applications. Particularly, the aminoxyl group is frequently used in spin-labeling experiments. Nitroxides may also exhibit intriguing chiral and magnetic properties, and can be of interest for studies of molecular parity violation. We present results for three different groups of persistent nitroxide radicals: i) acyclic: dimethylaminoxyl (Me2NO), bis(trifluoromethyl)aminoxyl (CF3)2NO, and di-tert-butyl nitroxyl [(Me3C)2NO]; ii) cyclic: aziridine-N-oxyl, azetidine-N-oxyl, pyrrolidine-N-oxyl and piperidine-N-oxyl; and iii) imino nitroxides. We used density functional and ab initio (MP2, coupled cluster) methods to obtain insight into the underlying chemistry. The molecular structures, harmonic vibrational frequencies, inversion barriers, and hyperfine coupling constants are reported. The cyclic aziridine-N-oxyl exhibits a considerable inversion barrier of ?hc 3500 cm–1 compared to only ?hc 500 cm–1 for the other examples. Stable imino nitroxides are theoretically characterized for the first time in our work. We discuss the possibilities that some of the chiral derivatives may be dominated by molecular parity violation in their dynamics.

The synthesis, structures, and magnetochemical data for the heterospin chain polymer complexes Cu(hfac)L and Cu(hfac)L·0.5Solv, where hfac is the hexafluoroacetylacetonate anion, L and L are 2-(1-allyl-1 H-pyrazol-4-yl)- and 2-(1-butyl-1 H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole-3-oxide-1-oxyls, respectively, Solv is isopropylbenzene or tert-butylbenzene, were described. These polycrystalline solids were found to exhibit similar magnetic anomalies. Thus, the effective magnetic moment first decreases and then increases with temperature, resulting in the specific minimum in the curve μ( T). The study of the magneto-structural correlations showed that the appearance of the minimum is attributed to different factors. For Cu(hfac)L, this is the phase transition accompanied by the structural rearrangement of the exchange cluster >N-·O-Cu-O·-N<, which leads to a change in the energy of the exchange interaction between the unpaired electrons of the paramagnetic centers. By contrast, for Cu(hfac)L·0.5PrPh and Cu(hfac)L·0.5BuPh, the appearance of the minimum is a consequence of the coexistence of exchange interaction energies with opposite signs, while the structure of the solid phase remains unchanged. [ABSTRACT FROM AUTHOR]

A method was developed for the synthesis of a nitronyl nitroxide containing cyclopentane substituents in positions 4 and 5 of the imidazoline ring, viz., 2-(3-pyridyl)-4,5-bis(spiropentyl)-4,5-dihydro-1 H-imidazole-3-oxide-1-oxyl (L). The reaction of Cu hexafluoroacetylacetonate with L affords different products depending on the reaction conditions: mononuclear [Cu(hfac)(L)], binuclear [Cu(hfac)L], tetranuclear {[Cu(hfac)](L)}, or chain polymer {[Cu(hfac)](L)}. Temperature changes induce structural transformations accompanied by a change in the spin state in exchange clusters in the solid [Cu(hfac)L] and {[Cu(hfac)](L)}. [ABSTRACT FROM AUTHOR]

Abstract: This manuscript elucidates the influence of special functional groups of organic xenobiotic chemicals on partly simultaneous molecular binding mechanisms. Organic xenobiotics are released as contaminants into the environment and are partially bound to soil constituents, such as humic substances (HS) and the mineral phase. The interaction of chemical contaminants with HS of soil was investigated using a labeling technique of soil samples with stable nitroxide radicals as model contaminants. These nitroxide radicals only differed in one functionality, a hydroxy or an amino functional group. Electron spin resonance (ESR) analyses of contaminated soil samples showed that the interaction was a multistage process that comprised an initial increase in the concentration of free organic radicals of soil influenced by contaminants with hydroxy or amino functionality, a decrease in the polarity of the molecular environment of soil, and a change in mobility of contaminant molecules. Towards the end of interaction, the concentration of free organic radicals usually decreased. Binding of contaminants to soil constituents via specific functionality was revealed detecting a change in their ESR spectra. Basing on the ESR analyses, a two-way exchange of protons'' electron pairs between contaminants and HS was hypothesized to interpret the beginning of interaction. [Copyright &y& Elsevier]

Some compounds containing mono- and polynitroxide radicals have been investigated and characterized by electron ionization (EI), chemical ionization (CI), and matrix-assisted laser desorption-ionization (MALDI) mass spectrometry. Some characteristics of the mass spectra of these radicals are demonstrated. It was found that, under CI and MALDI conditions, ions [M+ nH] are formed along with monoprotonated molecules [M+H], depending on the type of the radical n = 2-5. [ABSTRACT FROM AUTHOR]

Electron spin resonance spectroscopy and mass spectrometry are two analytical methods that are very rarely used in combination. In this paper, we will show that the methods complement one another in the example of the distribution of stable nitroxide radicals in human skin, including the spatial resolution of these distribution processes. There are many ESR investigations dealing with this subject, but unfortunately, they are all limited to the detection of paramagnetic species. The combination with MS allows the successful examination of the distribution profile of the main biotransformation product of the nitroxide radicals, the respective 'ESR-silent' hydroxylamines. In order to maintain the biological state of the sample material as far as possible, atmospheric pressure matrix-assisted laser desorption/ionization with ion trap detection has been used for the mass spectrometric investigations. The results validate the former findings of the strong reduction of stable free radicals by biological material; moreover, the diamagnetic species formed during these processes have been identified. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

To find the factors favorable for the appearance of chemomechanical activity of heterospin crystals, a series of new heterospin complexes were synthesized and characterized. It includes [Cu(tfac)L]·2CHCl, [Cu(tfac)L]·2EtOH, [[Cu(pfu)]L]·1/2CHCl, [Cu(pfh)L]·1/2CHCl, [Cu(piv)L]·2MeOH, [Co(piv)L], [ $$ Cu(hfac)_2 L^{CD_3 } _2 $$], [Cu(hfac)L]·CHCl, and [Cu(hfac)L] (L, $$ L^{CD_3 } $$, and L are N-methylimidazolyl-, N-trideuteriomethylimidazolyl-, and N-methyltriazolyl-substituted nitronyl nitroxides, respectively; tfac, hfac, pfu, pfh, and piv are the charged coordinated diamagnetic ligands 1,1,5,5-tetrafluoropentane-2,4-dionate, 1,1,1,5,5,5-hexafluoropentane-2,4-dionate, 1,1,1,2,2,3,3,4,4,8,8,9,9,10,10,11,11,11-octadecafluoroundecane-5,7-dionate, 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dionate, and 2,2-dimethylpropionate, respectively). The crystal and molecular structures of all compounds were determined. The results of the X-ray diffraction study of the complex [Ni(hfac)L] synthesized earlier are reported. In the solid state of the complexes [Cu(pfh)L] and [Co(piv)L], the paramagnetic ligands are cis-coordinated to the central atom in a monodentate fashion via the donor N atom of the imidazole ring. In the dinuclear complexes [[Cu(pfu)]L] and [Cu(hfac)L], the paramagnetic ligands are also in cis positions but act as bridges through coordination of the donor N atoms of the azole ring and the O atom of the nitronyl nitroxide moiety to different Cu ions. In the solid complexes $$ Cu(hfac)_2 L^{CD_3 } _2 $$ , [Cu(hfac)L], [Cu(tfac)L]·2CHCl, [Cu(tfac)L]·2EtOH, and [Cu(piv)L]·2MeOH, the nitronyl nitroxide radicals in the mononuclear heterospin molecules are in trans positions. The packing motif in the crystal structures of the complexes $$ Cu(hfac)_2 L^{CD_3 } _2 $$ , [Cu(tfac)L]·2CHCl, and [Cu(tfac)L]·2EtOH is the same as that in the previously studied complexes [M(hfac)L] exhibiting chemomechanical activity. Among the complexes under consideration, only crystals of $$ Cu(hfac)_2 L^{CD_3 } _2 $$ can exhibit chemomechanical activity, that is to make jumps upon heating or irradiation. The results of the present study suggest that the packing of the solid-state structure plays a key role in the generation of mechanical activity of the crystals. [ABSTRACT FROM AUTHOR]

A procedure was developed for the synthesis of the kinetically stable nitroxide 2-( N- tert-butyl- N-hydroxyamino)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole-3-oxide-1-oxyl containing the sterically hindered hydroxylamino group. The crystal and molecular structure of this compound was determined. The isolation of this nitroxide hydroxylamine in the individual state enabled a new procedure for the preparation of its oxidation product, viz., the corresponding nitroxide biradical. The results of quantum chemical calculations of the electron exchange in the biradical molecule are in good agreement with the experimental data on the triplet ground state of the molecule in the crystal obtained earlier. [ABSTRACT FROM AUTHOR]

In this paper, we have studied the effect of molecular oxygen from aqueous solution with different oxygen concentrations, on nitroxide radical Tempo (2, 2, 6, 6-tetramethyl-piperidine-1-oxyl) used in a free state and encapsulated on liposomes by EPR spectroscopy. The dependence of the EPR spectra on partial oxygen pressure, pO2 was obtained by measuring the line width as a function of pO2. It was found that EPR line width of the paramagnetic probe and oxygen concentration surrounding the probe is given by a linear relation. [ABSTRACT FROM AUTHOR]

Chelate complexes of copper(II) and nickel(II) with anions of nitroxide radical ethyl-2-(1-oxyl-2,2,5,5-tetramethylimidazolidin-4-yliden)-3-oxopropanoate (HL) are synthesized. The compounds have molecular structure and zero magnetic dimensionality. CuL crystallizes as two polymorphs having mononuclear structure. NiL forms dimeric molecules by virtue of the bridging function of the aldehyde oxygen atoms. Magnetic properties of NiL are determined by antiferromagnetic exchange interactions between the Ni(II) ions (-48(1) K) propagated through the oxygen bridges [ABSTRACT FROM AUTHOR]

Abstract: Titanium dioxide nanopowders were synthesized mechanochemically using TiCl4 and (NH4)2CO3 (series TC) or TiCl4/(NH4)2CO3 and Na2SO4–Na2SO4·10H2O (series TCSM), as precursors and post-annealing in the temperature range 150–850°C. Crystal structure and crystallite size of the titania nanomaterials were characterized by X-ray diffraction, and the optical properties of aqueous suspensions at various TiO2 loadings were measured using UV/visible spectroscopy. The photoinduced activity was measured in situ by EPR spectroscopy applying a variety of experimental techniques (spin trapping, spin probes, 4-hydroxy-2,2,6,6-tetramethylpiperidine oxidation). In the series TC the presence of an NH4Cl salt matrix, which was removed by decomposition to gaseous byproducts in the post-annealing process, leads to the formation of anatase nanocrystals with optimal size for high photoactivity (23–50nm) at temperatures of 550 and 650°C, and these titania samples are the most active in the photoinduced production of reactive oxygen species. The results found for TCSM series confirmed that the presence of a sulfate salt matrix (Na2SO4) during mechanochemical and annealing processes significantly hindered TiO2 nanocrystallite growth, increased agglomerate formation in aqueous media and lowered the values of photonic efficiencies determined via paramagnetic species formation or termination. [Copyright &y& Elsevier]

In the present study we propose a new diagnostic methodology for non-invasive imaging of tissue red/ox activity in intact healthy and cancer-bearing mammalians, which allows a differentiation of cancer development from normal (healthy) condition. The method is based on red/ox cycle of cellpermeable nitroxide radicals and their MRI contrast properties, which makes them useful molecular sensors for tissue red/ox activity. The nitroxide radical (which is characterized by T1 contrast) participates in electron-transfer reactions with the intracellular reactive oxygen species and reducing equivalents with formation of non-contrast intermediate products, which is accompanied with MRI signal decay. The half-life of MRI signal decay ( ½) was used as a marker of tissue red/ox activity to the nitroxide probe. The experiments were conducted on healthy and cancer-bearing mice. The mice were under anesthesia during the MRI measurements. All measurements were conducted on 7 Tesla MRI. In healthy mice, the half-life of MRI signal decay in the selected regions of interest (ROI; brain and surrounding tissues) was considered as a reference steady-state value, which is indicative of tissue red/ox activity in norm. In cancer-bearing mice, the half-life of MRI signal decay in the same or similar ROI was markedly different from this reference value. The results demonstrated that the normal (healthy) tissues possessed a significantly higher reduction activity to the nitroxide probe in comparison with cancer tissue, which could be an appropriate diagnostic marker for carcinogenesis. [ABSTRACT FROM AUTHOR]

Paramagnetic effects offer a rich source of long-range structural restraints. Here we review current methods for site-specific tagging of proteins and oligonucleotides with paramagnetic molecules. The paramagnetic tags include nitroxide radicals and metal chelators. Particular emphasis is placed on tags suitable for site-specific and rigid attachment of lanthanide ions to macromolecules. [ABSTRACT FROM AUTHOR]

Abstract: The radical cation 2-(3-N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide (abbreviated as m-MPYNN) is successfully intercalated into the layered host structure CdPS3. The reaction proceeds via an ion exchange reaction from methanol solutions containing the radical iodide salt, leading to materials described by the compositional formula Cd1− x PS3(metaMPYNN)2 x . Detailed characterization of the intercalates by chemical analysis, X-ray powder diffraction, EPR, and NMR spectroscopy indicate that the maximum uptake corresponds to x=0.20; attempts to produce higher intercalated materials result in the formation of a side product with inferred composition CdI4(metaMPYNN)2. Magnetic susceptibility measurements indicate Curie-like behavior for x=0.13, while at higher intercalation levels antiferromagnetic coupling is observed. The magnetic properties may be linked to the orientation of the radical ions relative to the host layers, which is also found to depend on x. While for the low-intercalated regime (x=0.13) both a transversal intercalation with the N–O director pointing towards the layers, and a longitudinal orientation, with the long axis of the molecule pointing towards the layers are found, at larger x-values only the longitudinal orientation is present. 1H NMR chemical shifts indicate that the orientation greatly influences the molecular spin density distributions. Modern single and double resonance solid state NMR techniques have been introduced successfully to study the structural modifications of the host lattice and the details of the intermolecular guest/host interactions. Specifically the internuclear distance correlations extracted from selectively measured 1H–31P magnetic dipole–dipole couplings via 31P/1H rotational echo double resonance (REDOR) experiments allow important conclusions regarding the orientation of the guest species relative to the inorganic layers. [Copyright &y& Elsevier]

Abstract: The efficiency of Overhauser dynamic nuclear polarization (DNP) depends on the local dynamics modulating the dipolar coupling between the two interacting spins. By attaching nitroxide based spin labels to molecules and by measuring the 1H DNP response of solvent water, information about the local hydration dynamics near the spin label can be obtained. However, there are two commonly used types of nitroxide ring structures; a pyrroline based and a piperidine based molecule. It is important to know when comparing different experiments, whether changes in DNP enhancements are due to changes in local hydration dynamics or because of the different spin label structures. In this study we investigate the key parameters affecting DNP signal enhancements for 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl, a 5-membered ring nitroxide radical, and for 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, a 6-membered ring nitroxide radical. Using X-Band DNP, field cycling relaxometry, and molecular dynamics simulations, we conclude that the key parameters affecting the DNP amplitude of the 1H signal of water to be equal when using either nitroxide. Thus, experiments measuring hydration dynamics using either type of spin labels may be compared. [Copyright &y& Elsevier]

The cation of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate disproportionates in acidic aqueous medium to give piperidinoxyl and/or 1-hydroxypiperidine, nitrone, and acetone. A mechanism of this reaction was proposed and the rate constants for its particular steps were calculated. [ABSTRACT FROM AUTHOR]

Parameters of exchange interactions in heterospin chain polymeric complexes of Cu(hfac)2 (hfac is hexafluoroacetylacetonate anion) with pyrazolyl-substituted nitronyl nitroxides LR (R = Me, Et, Pr, Bu) were estimated using quantum chemical computational methods. The magnetic properties of the considered chain polymeric complexes can be described within the framework of the model of isolated exchange clusters. Experimental data on the structural dynamics of chains polymeric with “head-to-tail” (R = Me) and “head-to-head” (R = Et, Pr, Bu) motifs are discussed in the context of the concept of gradual phase transitions. Based on the analysis performed, a hypothesis of microscopic mechanisms of magnetic transitions in crystals of this type of compounds was proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Silver terephthalate possesses significantly higher charge transfer and ion transport kinetics than sodium terephthalate. • Anthraquinone groups facilitate the molecular-level π-π stacking of PTMA with MWCNT, thereby enhancing redox kinetics. • Full organic battery with PTMA cathode and terephthalate anode provides a 2.8 V cell voltage. • All-organic battery delivers the specific capacity of 43 mAh g−1 based on both anode and cathode active materials. Fully organic batteries have been recognised as ideal alternatives to traditional metal-based energy storage devices. However, significant challenges remain in mitigating their low cell voltage and capacity. Here, we construct an all-organic battery based on TEMPO (i.e., 2,2,6,6-tetramethylpiperidyl-1-oxy) radical polymer cathode, poly(TEMPO methacrylate) (PTMA), and conjugated dicarboxylate anode such as silver terephthalate (Ag 2 TP). Chemical modification of PTMA with aromatic anthraquinone (AQ) results in P(TMA 0.95 -co-AQ 0.05) 1610 , in which AQ groups facilitate the molecular level binding to CNTs through non-covalent π-π interaction and enhance the charge and mass transport. This fully organic battery delivers a capacity of 182 mAh g−1 based on the anode at 0.5C with an output voltage of 2.8 V. Although the capacity needs further improvement, our work shows the promise of developing high-voltage, fully organic batteries with a judicious electrode design. [ABSTRACT FROM AUTHOR]

Abstract: The cationic nitronyl nitroxide free radical of the N-methylpyridinium type p-MepyNN+ has been combined with [Mo8O26]4− and Keggin [SiW12O40]4− polyanions to afford salts (p-MepyNN)4[Mo8O26]·DMSO (DMSO=dimethylsulfoxide) (1) and (p-MepyNN)4[SiW12O40]·6DMF (DMF=dimethylformamide) (2). Herein, their structural and magnetic properties are described. [Copyright &y& Elsevier]

Abstract: The group frequency of the N–O radical stretching vibration has received scant attention in the literature. The few existing treatments of the vibrational spectroscopy of nitroxides are incomplete at best and potentially misleading to workers in the field. To close this gap in the available knowledge, the existing literature on the vibrational spectra of nitroxide stable free radicals is critically reviewed with particular reference to the wavenumber position of the N–Ong vibration, ν(N–Ovidentiary bases for the assignment ν(N–Ound in many instances. Ab initio Density Field Theory calculations using a model chemistry of UB3LYP at the 6-311++G(d,p) level were performed to obtain a theoretical band position of ν(N–Oparison with the published data. Large discrepancies between the theoretical and experimental values were found for the radical 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxyl, which currently sets the lower limit of the accepted wavenumber range of ν(N–Ol as for the nitronyl and iminyl nitroxides. The wavenumber position of ν(N–Ond to occur in the range 1450–1420cm−1 for 5-membered cyclic nitroxides and 1395–1340cm−1 for 6-membered cyclic and acyclic nitroxides. In nitronyl nitroxides, the symmetric stretching vibration occurs in the region 1470cm−1, but coupling to other modes makes specific band assignments problematic for the nitronyl nitroxide group. [Copyright &y& Elsevier]

Abstract: Gd(III)-containing dendrimers are promising contrast agents for magnetic resonance imaging (MRI). An important issue in the effectiveness and toxicity of a Gd(III) based MRI contrast agent is knowledge of the relative locations and concentrations of Gd(III) in dendrimer drug delivery hosts. In order to provide experimental information on this issue, we have investigated the electron paramagnetic resonance (EPR) of a stable Gd(III) complex with diethylenetriaminepentaacetic acid (DTPA) in various polyammidoamine (PAMAM) dendrimers as a function of dendrimer generation (G2, G4, and G6), dendrimer core (ethylenediamine = EDA, and cystamine = cys), and dendrimer surface functionality ( , 5-oxo-3-pyrrolidinecarboxylic acid methyl ester = pyr, and tris(hydroxymethyl) methylamine = tris). The dendrimer systems were investigated in the presence and absence of paramagnetic probes, that is, Cu(II) and nitroxide radicals (4-(trimethylammonium and dodecyl-dimethylammonium) 2,2,6,6-tetramethylpiperidine 1-oxyl bromide = CAT1 and CAT12, respectively). The analysis of the EPR spectra revealed anisotropic locations of Gd-DTPA inside the dendrimer. Computer analysis of the EPR spectra of the probes identified the interactions of the Gd-dendrimers with ions and organic molecules. The interaction between the probes and the dendrimer internal and external surface depends on the type of core, the composition of the external surface and the generation of the dendrimer. The negatively charged Gd-DTPA complex attracts the positively charged species and this provokes spin–spin interactions between Gd and the probes, which increases with a decrease in generation, mainly from G6 to G4, and with an increase in both the Gd-dendrimer concentration and the probe concentration. The cys core increases the internal volume and decreases the packing of the branches. [Copyright &y& Elsevier]

A total of 20 components were identified in Allium ursinum volatile oil (AUVO) by GC–MS, and 10 of them are found for the first time in this plant species. The antioxidant capacity of AUVO was examined by β-carotene–linoleic acid bleaching, lipid peroxidation by Fenton reaction, EPR spin-probing assay using stable nitroxide radicals, DPPH·, and ABTS·+ scavenging assays. Reaction mechanism of the volatile oil components with nitroxide radicals, based on IR spectra analysis, is proposed. AUVO demonstrated poor scavenging ability against DPPH· and ABTS·+ comparing to synthetic antioxidants BHT and trolox, while in β-carotene–linoleic acid system AUVO showed an effect comparable to those for BHT. AUVO was also capable to scavenge stable nitroxide radicals such as water-soluble Tempone, and 7-DS and 12-DS, incorporated into the liposome membrane. Finally, AUVO increased membrane fluidity, which could be an important feature for further in vivo investigation of some disorders, such as hypertension and atherosclerosis. [Copyright &y& Elsevier]

Abstract: High resolution holographic polymer dispersed liquid crystal gratings are photo-induced periodic structures commonly affected by shrinkage. This phenomenon represents a problem in several application fields such as in high density optical storage. In this work we observed that, by directly monitoring the blue-shift of the reflection peak of the periodic structure, an evident shrinkage reduction occurs using the nitroxide radical Tinuvin770-N-yloxy [(bis(2,2,6,6-tetramethyl-piperidine-1-yloxy-4-yl)-sebacate] as inhibitor of polymerization. This compound is a stable hindered amine light stabilizer-derived free radical, which plays a significant role in the photo-polymerization dynamics. [Copyright &y& Elsevier]

Syntheses of a nitroxide radical 2-(1-oxyl-2,2,5,5-tetramethyl-imidazolin-4-yliden)-3-oxo-5-phenylpentannitrile $$(HL^{CH_2 CH_2 Ph} )$$ and its copper(II) complex are reported. The coordination compound is characterized by the single crystal X-ray diffraction analysis ( a = 11.647(2) Å, b = 13.269(3) Å, c = 13.844(3) Å, α = 61.83(2)°, β = 88.13(2)°, γ = 74.12(2)°, space group $$P\bar 1$$ ). The $$Cu(L^{CH_2 CH_2 Ph} )_2 (H_2 O)$$ complex has a molecular structure. [ABSTRACT FROM AUTHOR]

Abstract: The stable radicals derived from different compounds were detected in process of styrene autopolymerization. The nitroxide radicals are produced from nitrosocompound, hindered hydroxylamine, nitrophenols and nitroanisoles. The phenoxyl radicals are formed from quinine methides, and naphtoxyl radicals are generated from 2-nitro-1-naphtol. The radicals are identified, the kinetics of their formation and follow-up evolution are studied. These radicals can participate in process of living radical polymerization as the mediators and can effect significantly on kinetics of polymerization and structure of the resulting polymer. [Copyright &y& Elsevier]