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Introduction. Polymeric materials (PM) are increasingly used in various industries and agriculture. Under the action of UV light, PM are destroyed. UV stabilizers are used to protect PM from photodegradation. Their action is based on the absorption of the photoactive sunlight component or on the deactivation of excited molecules that have already absorbed a light quantum, as well as on the inhibition of dark light-induced reactions. The work objective is to provide a comparative analysis of compounds of the sterically hindered phenols (SHP) series and nitroxide radicals (NR) as the PM photodegradation inhibitors. Materials and Methods . Reagents of the “purum” grade, LDPE premium grade 15803-020 polyethylene film were used for the investigations. The stabilizer was applied to the film by dipping. Carbonyl groups in polyethylene were determined by IR spectroscopy. IR spectra were recorded on Varian-640 instrument. Research Results. 2,4,6-tri-tert-amylphenol (1), 2-methyl-4,6-di-tert-butylphenol (2), 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl (3), 3-carboxamido-2,2,5,5-tetramethylpyrrolin-1-oxyl (4) were tested as stabilizers for photo-oxidative degradation of polyethylene. It is known that the accumulation of carbonyl and hydroxyl groups is recorded in PM samples under irradiation in the process of photodegradation through the IR spectroscopy. The absorption band of the carbonyl group at 1720 cm - 1 appears in the IR spectra of oxidized polyethylene. The IR spectra analysis shows that the content of carbonyl groups in the check samples is significantly higher than in the samples treated by stabilizer solutions. Discussion and Conclusions . The experiments show that ni-troxide radicals of 3-carboxamido-2,2,5,5-tetramethylpyrrolin-1-oxyl series and 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl are the best photostabilizers of polyethylene. Moreover, there is no significant difference between the radicals of the 2,2,6,6-tetramethylpiperidine and 2,2,5,5-tetramethylpyrroline series. Sterically hindered phenols, under photodegradation, have a far smaller stabilizing effect, falling short of nitroxide radicals.

Four albumin-nitroxide conjugates were prepared and tested as metal-free organic radical contrast agents (ORCAs) for magnetic resonance imaging (MRI). Each human serum albumin (HSA) carrier bears multiple nitroxides conjugated via homocysteine thiolactones. These molecular conjugates retain important physical and biological properties of their HSA component, and the resistance of their nitroxide groups to bioreduction was retained or enhanced. The relaxivities are similar for these four conjugates and are much greater than those of their individual components: the HSA or the small nitroxide molecules. This new family of conjugates has excellent prospects for optimization as ORCAs.

Redox-active polymer networks based on stable nitroxide radicals are a very promising class of materials to be used in the so-called organic radical batteries. In order to obtain fast-charging and high power electrodes, however, excellent ionic conductivity inside the electrode material is required to allow easy diffusion of ions and fast redox reactions. In this contribution, we investigated redox-active poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) chains cross-linked through ionic liquid-like 1,2,3-triazolium groups. Different networks were prepared in which the amount of cross-linker and the counter-anion associated to the 1,2,3-triazolium group were varied. The ionic conductivities of the different polymer networks were first measured in the solid state by electrochemical impedance spectroscopy at different temperatures, and an increased ionic conductivity was measured when 1,2,3-triazolium groups were present in the network. The effects of the chemical nature of the counterions associated to the 1,2,3-triazolium groups and of the crosslinking density were then studied. The best ionic conductivities were obtained when bis (trifluoromethane)sulfonamide (TFSI) counter-anions were used, and when the crosslinking density of the TFSI-containing gel was higher. Finally, those ion-conducting gels were loaded with free LiTFSI and the transference number of lithium ions was accordingly measured. The good ionic conductivities and lithium ions transference numbers measured for the investigated redox-active gels make them ideal candidates for application as electrode materials for either organic radical batteries or pseudo-capacitors energy storage devices.

This paper reports on dihydroxycoumarins as fluorescent probes suitable for the detection and determination of the nitroxide radical, namely 4-amino-TEMPO. Since 4-amino-TEMPO is used as a spin label for the detection of various radicals and damage caused by these species, its determination under physiological conditions might help us to understand the mechanism of the oxidative stress. Among different coumarins studied, only dihydroxy-substituted derivatives show high sensitivity, specificity, and selectivity for the nitroxide radical. In this assay, dihydroxy-substituted coumarins under the action of 4-amino-TEMPO show a very fast and significant increase in fluorescence intensity and lifetime. Among them 6,7-dihydroxycoumarin (esculetin) exhibits the strongest fluorescence enhancement (up to 40 times), with an estimated limit of detection equal to 16.7 nM&mdash, a significantly lower value when compared with UV-Vis or electron paramagnetic resonance (EPR) spectroscopy. The method is characterized by an easy procedure of sample preparation and very short time of analysis. The mechanism of the interaction between 6,7-dihydroxycoumarin and 4-amino-TEMPO has been examined with the use of a series of complementary techniques, such as steady-state and time-resolved fluorescence spectroscopy, UV-Vis spectroscopy, electron paramagnetic resonance spectroscopy, potentiometric titration, and high-performance liquid chromatography. It has been proven that the only route of the reaction in the system studied is a proton transfer from the molecule of esculetin to the amino group of the nitroxide. Biological studies performed on prostate cancer cells, breast cancer cells, and normal skin fibroblasts revealed significant anticancer properties of 6,7-dihydroxycoumarin, which caused a considerable decrease in the viability and number of cancer cells, and affected their morphology, contrary to normal fibroblasts. Furthermore, the experiment performed on prostate cancer cells showed that fluorescence emission of esculetin is closely related to intracellular pH&mdash, the higher pH, the higher observed fluorescence intensity (in accordance with a chemical experiment). On the other hand, the studies performed in different pH levels revealed that when pH of the solution increases, the observed fluorescence intensity enhancement under the action of 4-amino-TEMPO decreases (better sensing properties of esculetin towards the nitroxide in lower pH).

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.

A spin pH probe technique was used to study the influence of the channel diameter on the EPR spectra of pH-sensitive nitroxide radicals (NR) located in the channels of the mesoporous molecular sieves MCM-41 and SBA-15 with diameters ranging from 2.3 to 8.1 nm. From EPR spectra analysis and the results of the NR retention by the mesoporous molecular sieves upon washing with an aqueous KCl solution, the regularities of NR molecular location inside the channels were studied. The obtained dependence of the fraction of the radical molecules in the fast motional regime (with the rotational correlation times, τc = 2 × 10-11 s-9 × 10-11s) in the channels of the mesoporous molecular sieves as a function of pH indicates that both NR in the fast and slow motional regime (with τc = 8 × 10 -9s-7 × 10-10s) may be used for estimation of the solution acidity inside the channels and of the near-surface electrical potential. © 2013 Elsevier Inc. All rights reserved. This work was supported by the Research Program of the Ministry of the Education of the Russian Federation (2007-2011). E.G.K. thanks the Program of Cooperation “Mikhail Lomonosov” between the Ministry of Education of Russian Federation and the German Service of Academic Exchanges (DAAD). We are grateful to Dr. G. Chandrasekar for assistance in the synthesis of MMS and their characterization. Appendix A

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. © 2013 Elsevier B.V.

1125 X-ray structures of nitroxide free radicals presenting intermolecular hydrogen bonds have been reported in the Cambridge Structural Database. We will report in this paper a qualitative and quantitative analysis of these bonds. The observation in some plots of an excluded region was statistically analyzed using convex hull and kernel smooting methodologies. A theoretical study at the MP2 level with different basis has been carried out indicating that the nitronyl nitroxide radicals (five electrons) lie just in between nitroso compounds (four electrons) and amine N-oxides (six electrons) as far as hydrogen-bond basicity is concerned., This work has been supported by the Spanish Ministerio de Economía y Competitividad (Project No. CTQ2015-63997-C2-2-P) and Comunidad Autónoma de Madrid (Project FOTOCARBON, ref S2013/MIT-2841). Computer, storage and other resources from the CTI (CSIC) are gratefully acknowledged

We measured electron paramagnetic resonance (EPR) spectra of 14N- and 15N-labeled perdeuterated TEMPONE radicals in normal and supercooled water at various radical concentrations. By fitting the EPR spectra to spectral shape functions based on the modified Bloch equations, we obtained concentration dependences of EPR parameters of radicals at each measured temperature. From concentration dependences of the EPR parameters quantifying spin dephasing, coherence transfer, and hyperfine splitting, we determined linear concentration coefficients, whose values depend on the relative motion of radicals due to modulation of the Heisenberg spin exchange (HSE) and dipole-dipole (DD) interactions between them. We applied the continuous diffusion model for relative motion of radicals and we evaluated the diffusion coefficients of radicals from the concentration coefficients using the standard relations and the relations derived from kinetic equations for the spin evolution of interacting radical pair. It was found that the latter equations lead to the better agreement between the diffusion coefficients calculated from different concentration coefficients. The calculated diffusion coefficients of 14N- and 15N-labeled radicals show similar values, which is an expected result that supports the presented method. Upon lowering the temperature into the supercooled state, the calculated diffusion coefficients decrease slower than is predicted by the Stokes-Einstein relation and slower than the rotational diffusion coefficient. Similar effects were detected in NMR studies of the rotational and translational motion of water molecules in supercooled water.

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 beta-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 beta-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. (C) 2007 Elsevier Ltd. All rights reserved.

A series of lipid-functionalized nitroxides having a pyrroline nitroxide moiety linked either to a glycerol or to a steroid unit has been synthesized, and their inclusion inside phospholipid bilayers has been investigated by Electron Paramagnetic Resonance (EPR) spectroscopy. The antioxidant behavior of these nitroxides has been studied in azo-initiator induced lipid peroxidation by means of the Thiobarbituric Acid Reactive Species (TBARS) assay; a correlation with their penetration depth within the bilayer has been found. The possible mechanisms involved in the antioxidant action have been considered, discussed and alternative pathways have been suggested for the synthesized liponitroxides due to their different localization. The steroid derivative is limited to scavenging radicals that are generated in the aqueous phase, while the glycerolipids can also act as chain breaking antioxidants.

A nitroxide [60]fullerene adduct containing a pyrrolidine-1-oxyl group has been synthesized. Its orientational order in the nematic phase of the liquid crystal solvent 4,4′-azoxyanisole (PAA) has been measured from the variation of the EPR spectral parameters on passing from the isotropic to the nematic phase. Highly resolved EPR lines allow for precise evaluation of the shifts of the g, a N and a H values. Since the g and the hyperfine tensors are known, the order matrix could be obtained. This is compared with the one calculated with a theoretical model based on short range solute-solvent interactions, which predicts a considerable degree of orientation of the molecular axes, despite the almost spherical shape of the molecule. The agreement with experimental findings is quite good and it is further improved if a bent structure of the pyrrolidine ring is taken into account.

[60]fullerene bisadducts containing the C 2 ν -symmetrical 2,2,5,5-tetramethylpyrrolidine-1-oxyl moiety have been synthesized. Their EPR spectrum is typical of nitroxide biradicals with electron exchange interaction J of the same order of the nitrogen hyperfine coupling. From the EPR spectrum the absolute value of J was obtained, while the sign was measured by ENDOR. When the derivatives are photoexcited strong spin polarization is observed with A/E/A/E or E/A/E/A patterns, according to a positive or a negative sign of J . This behavior is accounted for by an intramolecular triplet–triplet interaction. The analogy with the polarization of transient spin correlated radical pairs is pointed out.

Lanthanide-induced enhancement of the longitudinal relaxation of nitroxide radicals in combination with orthogonal site-directed spin labeling is presented as a systematic distance measurement method intended for studies of bio-macromolecules and bio-macromolecular complexes. The approach is tested on a water-soluble protein (T4-lysozyme) for two different commercially available lanthanide labels, and complemented by previously reported data on a membrane-inserted polypeptide. Single temperature measurements are shown to be sufficient for reliable distance determination, with an upper measurable distance limit of about 5-6 nm. The extracted averaged distances represent the closest approach in Ln(III) -nitroxide distance distributions. Studies of conformational changes and of bio-macromolecule association-dissociation are proposed as possible application area of the relaxation-enhancement-based distance measurements.

The proton hyperfine tensors of five nitroxide radicals have been obtained by ENDOR spectroscopy in frozen solution. The spectra are interpreted by computing the dipolar hyperfine interaction and simulating the spectra. EPR spectra in solution of the same radicals have been simulated by taking into account the effects of the proton hyperfine tensors. We have been able to reproduce accurately the line broadening effects of the proton hyperfine structures inside each nitrogen hyperfine component and we have determined the correlation times for the rotational motion. In the case of the radical Tempol, our analysis allows discrimination between the effects due to the protons of the axial and equatorial methyl groups. On the basis of experimental evidence we can attribute the larger isotropic hyperfine coupling constant to the axial methyl protons. The possible use of the present results for interpreting the spectra of other nitroxide radicals is discussed.

We describe the synthesis and properties of a series of fulleropyrrolidines bearing stable nitroxide free radical moieties covalently linked to the [60]fullerene cage. The nitroxide electron spin functions as a weakly interacting probe for the investigation of radical anions and paramagnetic excited states of functionalized fullerenes. Electron transfer processes, spin coupling in polyradical adducts, and interactions between radical and fullerene excited triplet unpaired electrons have been studied through electron paramagnetic resonance. Photogenerated quartet and quintet high spin species, obtained from the spin coupling of the fullerene triplet with the nitroxide spins, have been observed for the first time at room temperature. To cite this article: C. Corvaja et al., C. R. Chimie 9 (2006).

Time-resolved electron paramagnetic resonance spectra of two single spin-labeled and two double spin-labeled C60 derivatives in frozen solution are recorded with pulsed laser excitation and 100 ns time resolution. Quartet and quintet excited species are detected which arise from the electron spin coupling of the triplet excited fullerene moiety with the unpaired spin(s) of the nitroxide label(s). Despite the similar molecular structure, in both series of single and double labeled derivatives a different behavior was found, which is due to substantial difference of the energy of exchange coupling.

Salts 1 and 2 that combine the [W 6 O 19 ] 2- Lindqvist anion with the cationic nitronyl nitroxide (NN) free radicals p -MepyNN + and n Bu 3 NCH 2 NN + , respectively, have been synthesized and their structural and magnetic properties have been studied. Keywords: Nitroxide radicals, polyoxometalates, magnetism. Introduction Ferromagnetism based on purely organic materials has raised a lot of interest during the last decade [1]. Most of the studies have been performed with nitronyl nitroxide (NN) radicals due to their persistence, stability and ease of functionalization [2]. Bulk ferromagnetism in these compounds arises from ferromagnetic interactions between open-shell molecules in the solid state. For a proper design of such materials, chemists need control over 1) the organization of molecules in the solid state and 2) the sign of magnetic interactions between neighbouring molecules. The first problem has been addressed by using the concepts of crystal engineering [3]. Thus, hydrogen bonding and other types of non-covalent interactions have been used to create specific crystal packings by design [4]. The second aspect is more delicate to control since the prediction of the type of interaction (ferro or antiferromagnetic) has not been solved to date. Usually, ferromagnetic interactions have been ascribed to intermolecular contacts between atoms carrying spin densities of opposite signs (McConnell I proposal) [5]. This model is an easy-to-use tool, since spin densities can be obtained experimentally (NMR or polarized neutron diffraction methods) in the solid state [6]. However, it has been shown that

The modulation induced by different solvents and by pH on the nitrogen isotropic hyperfine coupling consts. (AN) of 2,2,5,5-tetramethyl-3-carboxypyrrolidine (3-carboxy-PROXYL,CP) and 2,2,6,6-tetramethyl-4-carboxypiperidine (4-carboxy-TEMPO,CT) nitroxides was studied through ab initio quantum mech. computations and by EPR spectroscopy. The geometry of the neutral and deprotonated forms and the corresponding dissocn. consts. are computed by using the hybrid PBE0 functional. With these structures, accurate AN values were obtained by an integrated post-Hartree-Fock/PBE0 approach. Specific solvent effects are modeled by including a few solvent mols. (strongly bound to the solute), whereas bulk contributions are taken into account by the polarizable continuum model (PCM). Both EPR spectra and quantum mech. computations point out that AN increases with increasing solvent polarity and, esp., H-bond ability. The AN values obtained by the authors' computational model both for the neutral and deprotonated forms in aq. soln. are in good agreement with EPR values. However, the computed pKa values(4.97 and 5.25 for CP and CT, resp.) are significantly larger than the value reported in the literature for CP (3.4). New potentiometric and spectroscopic measurements lead to values (4 and 4.30 for CP and CT, resp.) in better agreement with quantum mech. computations and chem. intuition. Also the calcd. magnetic titrn. curves in aq. soln. are in quite good agreement with the authors' new EPR curves.

A series of biradicals consisting of a C60 linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C60, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C60 and nitroxide is shown to depend on the nature of the nitroxide addend.

An ENDOR study on several nitroxide radicals included in polycrystalline and glassy matrices is reported. A calculation program has been developed to simulate ENDOR spectra of radicals in disordered matrices. The shape of the computed spectra can be changed from crystal-like to powder-like type through the intermediate cases. The principal values of the AH hyperfine tensors have been obtained for tempone, tempol, tempyo and di-t-butyl-nitroxide. The influence of the host matrix on radical conformations is also discussed.

Indolinone (1), benzimidazole (2) and quinoline (3) nitroxide radicals have the nitroxide function in a conjugated position with respect to the π system. Their electron nuclear double resonance (ENDOR) spectra in a [2H8] toluene glassy matrix have been detected and the principal values and the orientation of the proton hyperfine coupling tensors have been obtained by comparison with computer-simulated spectra. Typical features of ENDOR spectroscopy in frozen solution, and in particular its more general feasibility with respect to use in solution, are discussed.