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In this work, we explored by means of electron paramagnetic resonance (EPR) spectroscopy the accessibility of a series of spin probes, covering a scale of molecular weights in the range of 200–60,000 Da, in a variety of hydrogels: covalent network, ionotropic, interpenetrating polymer network (IPN) and semi-IPN. The covalent gel network consists of polyethylene or polypropylene chains linked via isocyanate groups with cyclodextrin, and the ionotropic gel is generated by alginate in the presence of Ca2+ ions, whereas semi-IPN and IPN gel networks are generated in a solution of alginate and chitosan by adding crosslinking agents, Ca2+ for alginate and glutaraldehyde for chitosan. It was observed that the size of the diffusing species determines the ability of the gel to uptake them. Low molecular weight compounds can diffuse into the gel, but when the size of the probes increases, the gel cannot uptake them. Spin-labelled Pluronic F127 cannot be encapsulated by any covalent gel, whereas spin-labelled albumin can diffuse in alginate gels and in most of the IPN networks. The EPR spectra also evidenced the specific interactions of spin probes inside hydrogels. The results suggest that EPR spectroscopy can be an alternate method to evaluate the mesh size of gel systems and to provide information on local interactions inside gels. [ABSTRACT FROM AUTHOR]

A new nitronyl‐nitroxide ligand (H2L) featuring alcoholic and phenolic groups has been synthesised starting from 2‐hydroxy‐3‐(hydroxymethyl)‐5‐methylbenzaldehyde (A). Using this ligand, three 2p–4 f binuclear complexes have been obtained and crystallographically characterized: [Ln2(HL)2(hfac)4], (Ln=Gd 1, Tb 2, Dy 3). Apart from the desired compounds, two by‐products have been isolated as a molecular alloy containing an intermediate bearing hydroxylamino and N‐oxide groups, i. e. (1‐hydroxy‐2‐(2‐hydroxy‐3‐(hydroxymethyl)‐5‐methylphenyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole 3‐oxide, B, and the imino‐nitroxide derivative, C. This system was characterized by single crystal X‐ray diffraction, UV‐VIS and EPR spectroscopy. During the synthesis of compounds 1–3, two other complexes have been obtained and characterized: [Ln(C)(hfac)3] (4, Ln=Tb) and [Ln2(A−H)2(hfac)4] (5, Ln=Tb), where (A−H) results from the deprotonation of the phenolic group in A. The synthetic procedure was optimized in order to obtain pure 1–3 complexes. The cryomagnetic properties of compounds 1–3 have been investigated. Continuous‐wave (CW) electron paramagnetic resonance (EPR) measurements were carried out at X‐ and Q‐band for compound 1. Time‐dependent experiments were performed at three magnetic fields. Phase memory times, Tm, were found to be about 1000 ns at 6 K. [ABSTRACT FROM AUTHOR]

Two mononuclear complexes [(Et3NH)[M(hfac)2L] (M = Ni, 1; Zn, 2) have been synthesized using a nitronyl-nitroxide radical substituted nitrophenol, that is 2-(2-hydroxy-3-methoxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, HL, as a proligand. The crystal structures of 1 and 2 have been solved and indicate an octahedral coordination geometry of the metal ions. The magnetic behavior for 1 is characterized by a strong antiferromagnetic metal–radical interaction (J = −351 ± 1 cm−1; H = −JSNiSRad). This exchange interaction was rationalized by DFT calculations. The EPR spectra recorded in both solution and solid state at 120 K confirm the S = ½ ground state for 1. [ABSTRACT FROM AUTHOR]

Nitroxide-based radicals, having the advantage of firm chemical stability, are usable as probes in the detection of nanoscale details in the chemical environment of various multi-component systems, based on subtle variations in their electron paramagnetic resonance spectra. We propose a systematic walk through the vast area of problems and inquires that are implied by the rationalization of solvent effects on the spectral parameters, by first-principle methods of structural chemistry. Our approach consists of using state-of-the-art procedures, like Density Functional Theory (DFT), on properly designed systems, kept at the border of idealization and chemical realism. Thus, we investigate the case of real solvent molecules intervening in different configurations between two radical molecules, in comparison with radicals taken in vacuum or having the solvent that is treated by surrogate models, such as polarization continuum approximation. In this work, we selected the dichloromethane as solvent and the prototype radicals abbreviated TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl) oxyl). In another branch of the work, we check the interaction of radicals with large toroidal molecules, β-cyclodextrin, and cucurbit[6]uril, modeling the interaction energy profile at encapsulation. The drawn synoptic view offers valuable rationales for understanding spectroscopy and energetics of nitroxide radicals in various environments, which are specific to soft chemistry. [ABSTRACT FROM AUTHOR]

Literature data are abundant in oxidations catalyzed by nitroxides free radicals, while hydrazyls free radicals are very seldom encountered. In this work we made a comparison between some nitroxides and hydrazyl free radicals towards the ability to selectively oxidize activated alcohols to the corresponding carbonyl derivatives. As nitroxides we used 2,2,6,6-tetramethylpirrolidin- N -oxide (TEMPO) and phtalimide- N -oxyl (PINO) free radicals, while as hydrazyls were used 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2- p -nitrophenyl-1-picrylhydrazyl (DN-DPPH). The differences in the physico-chemical properties between nitroxides and hydrazyls were evaluated by UV–vis, EPR and cyclic voltammetry. For DN-DPPH the X-ray crystal structure was resolved. It has been shown that a nitroxide radical is a far better catalyst in such aerobic oxidations comparatively with a hydrazyl one. The explanation of these results consists firstly in a different mechanism involved in the oxidation procedures and secondly in the variation of the oxidation potentials of nitroxides comparatively with hydrazyls. [ABSTRACT FROM AUTHOR]

[Display omitted] • Inclusion complexes formed by four β-phosphorylated nitroxides with cyclodextrins were evidenced by EPR spectroscopy. • Binding constants were evaluated from the ratio free/complexed β-phosphorylated nitroxide components in EPR spectra. • EPR parameters of β-phosphorylated nitroxides are highly sensitive to the size of the cyclodextrin cavity. • EPR parameters of β-phosphorylated nitroxides as analytical markers for the composition of cyclodextrin solutions. The isotropic hyperfine coupling constants corresponding to nitrogen and phosphorous atoms of β-phosphorylated nitroxides are sensitive to the polarity sensed by these probes, and to local interactions determining a change in the molecular conformation of the probe. The β-phosphorylated nitroxides selected for this study exhibit very different solvent effects. In this article are presented results regarding the interaction of two cyclic and two non-cyclic β-phosphorylated nitroxides with cyclodextrins of different cavity size. In order to find the minimum concentration of cyclodextrin that determines a change in the EPR spectrum of the β-phosphorylated nitroxides, series of cyclodextrin solutions were prepared. The binding constants were of the 10–102 orders of magnitude. The reversibility of the association process was evidenced by competitive experiments with 1-adamantanol. The sensitivity of β-phosphorylated nitroxides to the interaction with cyclodextrins as a function of cavity size and concentration is translated in EPR spectra with different patterns. Starting from this observation, we exemplified that it is possible to discriminate between solutions containing one cyclodextrin and a mixture of cyclodextrins. As such, β-phosphorylated nitroxides can be used as analytical markers to provide information on the composition of cyclodextrin solutions. [ABSTRACT FROM AUTHOR]