ESR study of N-acyl-N-alkyl nitroxide radicals from photolysis of N-nitrosoamides

A new method of generation of N-acyl-N-alkyl nitroxide radicals by photolysis of N-nitrosoamides is described. ESR parameters are reported for 17 radicals of the general formula R? CO? N(R′)O^.; the high resolution obtained allows conformational analysis which shows a preferred trans conformation around the CO? N bond and hindered rotation around the N? R′ σ-bond.     

Millimetre-wave studies on single crystals of the organic radical 2-benzimidazolyl nitronyl nitroxide

We report temperature dependent millimetre-wave spectroscopic experiments on single crystals of the organic molecular magnet 2-benzimidazolyl nitronyl nitroxide, in the range 1.4–100 K. This organic radical shows quasi one-dimensional (1-d) ferromagnetic intermolecular interactions with the intrachain exchange coupling constant J/k_B = +22 K and interchain coupling constant J′/k_B = 0.24 K. The linewidth of the electron spin resonance at low temperatures was seen to be considerably broadened as compared to that at higher temperatures. This is consistent with the low dimensional behaviour of the linewidth predicted theoretically. We also see a shift in resonance field at low temperatures. We also present the results of angular dependence experiments carried out at temperatures of 1.4 and 50 K, and frequency dependence experiments at 1.4 and 50 K.     

The ESR study of reactivity of perfluoroacetyldiisopropylmethyl radical

The kinetics of hydrogen abstraction from hydrocarbons by the air-stable perfluoroacetyldiisopropylmethyl radical was studied by ESR, and a reaction mechanism was proposed. The degree of delocalization of the unpaired electron in the model C(5)F₃−C(1)·[C(2)(O(4))C(3)F₃]C(6)F₃ radical was calculated by the MNDO/PM3 method in the UHF approximation. For the conformation in which the CO group lies in the plane passing through the C(1), C(2), and C(5) atoms, the electron density on the O atom is 0.22. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1954–1957, November, 1997.     

ESR study on radical conversions in irradiated low-density polyethylene

ESR studies were carried out on radical conversions by thermal and photochemical mechanisms in low-density polyethylene irradiated mainly with electron beams at ?196°C both in vacuum and in the presence of CO. According to the spectral change, the following radical conversions were elucidated for the samples irradiated in vacuo [eqs. (1) and (1′)] and in the presence of CO [eqs. (2) and (2′)]. From the resemblance of the ESR spectrum observed after direct photolysis of polyethylene to that observed after photo-induced radical conversions of the allylic radicals, it is concluded that an eight-line ESR spectrum observed immediately after photolysis of polyethylene at ?196°C could be attributed more reasonably to the alkyl radicals ? CH₂CHCH₃ than to ? CH₂CH₂ and ? CH₂CHCH₂? .     

The triethylenediamine cation radical: An ESR study

Compared with normal and bridge-head aminium cations, the triethylenediamine cation is relatively stable. Its ESR spectrum shows that the two nitrogen atoms and all the protons are magnetically equivalent even at 77 K. This favours a symmetrical ground state rather than a dynamic equilibrium between classical, asymmetric structures.     

Helical stereochemistry and chiral apple halves

The stereorelationship in ensembles of polysectioned objects such as apples, cut viala coupe du roi, shows similarities to the repeat units for some helical multiplexes of parallel-chain polymers [e.g., the 4₂-duplex of polyriboadenylic acid (acid form), and for the 6₃-triplex of (13)--d-glucan]. These polymers exhibit 2n _n -helical axes, and contrary to most helical multiplexes, the constituentn-strands are readily separated from each other in the excised helical repeat unit.     

Azobenzene derivatives carrying a nitroxide radical

Several trans-azobenzene derivatives carrying a nitroxide (aminoxyl) radical (2a, 6a-12a) were prepared, and their photoisomerization reactions to the corresponding cis-isomers were investigated. Although no fruitful results could be obtained for the photoisomerizations of the derivatives with para-subsituents (9a-12a), the unsubstututed derivatives at the para-position (2a, 6a, 7a, 8a) were found to show photoisomerizations by irradiation to give the corresponding cis-isomers (2b, 6b, 7b, 8b), being isolated as relatively stable solid materials, and the change of the intermolecular magnetic interactions was apparently observed by the structural change for each photochromic couple.     

Temperature dependence of ESR spectra of nitroxide spin probe in glassy polymers

Temperature dependencies of ESR spectra of nitroxide spin probes in glassy polymers near and below glass transition temperature were examined in detail. Three temperature ranges, each characterized by specific changes in spectral shape, were defined. (i) In the low temperature range, ESR spectra of nitroxide radical in glassy polymer matrix weakly depend on temperature and remain qualitatively the same. (ii) In the intermediate temperature range, significant changes in the shape of spectra are observed. (iii) A new phenomenon was revealed near and below glass transition temperature: narrowing of linewidths occurs while the ratio of amplitudes of different components varies insignificantly. Analysis of molecular rotational mobility was carried out by means of commonly used empirical approaches. It was shown that widely used formulas and empirical approaches are not applicable for characterization of molecular mobility in glassy polymers. Mechanisms of rotational molecular movements in glassy polymers are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 563–575, 2009     

13C NMR study of ethers complexed with nitroxide radical

The contact shifts of carbon nuclei in five ethers complexed with 2,2,6,6-tetramethylpiperidine nitroxide radical were determined from the ¹³C NMR spectra and explained by collision interactions.     

13C NMR study of ethers complexed with nitroxide radical

The contact shifts of carbon nuclei in 5 ethers complexed with 2,2,6,6-tetramethylpiperidine nitroxide radical were determined from the ¹³C NMR spectra and explained by collision interactions.     

13C NMR study of alcohols complexed with nitroxide radical

The contact shifts of carbon nuclei in 8 alcohols complexed with 2,2,6,6-tetramethylpiperidine nitroxide radical were determined from the ¹³C NMR spectra and explained by hydrogen bonding and collision interactions.     

ESR lineshapes and kinetic behavior of nitroxide spin probes in micellar solutions

The ESR spectrum of the nitroxide radical 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) has been investigated in aqueous solutions of the following surfactants:n-octylammonium bromide, sodiumn-octylsulfonate,n-decylammonium bromide,n-docecyltrimethylammonium bromide, and sodiumn-dodecylsulfate. The spectra were recorded at 25° C as function of surfactant concentration in the ranges 0–0.5m for the C₁₀ and C₁₂ compounds and 0–1.0m for the others. Analysis of the spectra using computer simulation methods yields the hyperfine coupling constanta _H and the widthW _H of the partially resolved lines in the proton hyperfine structure. Changes in these variables with surfactant concentration are interpreted from medium effects on the magnetic properties of the radical, kinetics of radical-micelle interactions, and rotational dynamics of the nitroxide probe in these solutions.