ESR spectra, of free radicals

The electron spin resonance (ESR) spectra of free radicals obtained by electrolytic or microsomal reduction of several potential antiprotozoal 1,2,5-oxadiazoles were characterized and analyzed. Ab initio MO calculations were performed to obtain the optimized geometries, and the theoretical hyperfine constant was carried out using Zerner s intermediate neglect of differential overlap (ZINDO) semi-empirical methodology. DFT was used to rationalize the reduction potentials of these compounds <2003SAA69>. 

Fig. 2. ESR spectra of free-radical intermediates during the cobaloxime(II) catalyzed oxidation of 3,5-CatH2 by 02 in benzene. Reprinted with permission from Simandi,T. L. Simandi, L. I. React. Kin. Catal. Let. 1998, 65, 301.

The ESR spectra of free radicals arising from the action of hydroxyl radicals on several phosphorous compounds (trialkyl phosphate and dialkyl phosphonate) have been studied. In all cases, the radicals observed are due to the removal of a hydrogen atom from the carbon atom adjacent to the oxygen in the ester group (Ca-atoms) (289). 

Bullock, A. T., and L. H. Sutcliffe ESR-spectra of free radicals derived... 

ESR-spectra of free radicals in irradiated polymethyl-methacrylate. Trans. Faraday Soc. 54. 1133 (1958). 

Some phenomena, like the existence of a hyperfine structure of the ESR spectra of free radicals, which is due to coupling with the proton spins, are completely outside the frame of naive n considerations and should be explained in terms of more general theories. However, most physical properties of molecules are not seriously sensitive to correlation effects they can be understood in terms of the MO theory, and, in the particular case when the given molecule is planar, within the frame of the a—n separation. 

ESR spectra of free radicals in solution often show hyperfine structure due to several nuclei, particularly H, with the same coupling constant. The ESR spectrum of the methyl radical in Fig. 3.1 shows a line pattern with three equivalent protons (/ = Vi). The analysis by the diagram under the spectrum shows the construction of a stick pattern with an intensity ratio 1 3 3 1. 

Abstract ESR spectroscopic applications to polymer science are presented. ESR parameters used for the molecular and material characterization of polymer materials are reviewed. It is emphasized that ESR studies of the polymer science are particularly effective in three areas. (1) Intermediate species such as free radicals produced in chemical reactions of polymer materials can be directly detected. (2) The temperature dependent ESR spectra of free radicals trapped in the polymer matrices are very effective for the evaluation of molecular mobility (molecular motion) of polymer chains. (3) The mobility of electron, the structure of solitons, and the doping behavior in conduction polymers can be observed in detail in order to clarify the mechanism of conduction. 

Fig. 7.30 Change of the ESR spectra of free radicals trapped in UPEC by the heat treatment in vacuum at 361 K. The figure is adapted from [32] with permission from Elsevier...

If the interaction of an electron with a magnetic field were the only effect operative, then all ESR spectra of free radicals would consist of one line. When the nuclear spin quantum number / is nonzero a nuclear hyperfine interaction A is observed. When several equivalent nuclei are present (e.g., -CHs, -C.Hs), the number of lines in the spectrum is given by n,(2n-,li + ). where n is the number of magnetically equivalent nuclei /. 

Relation Between the Variation of ESR Spectra of Free Radicals and their... 

ESR spectra of free radicals trapped in polymer matrices reflect the physical properties of the matrix polymer, usually indicated by g-values, hyperfine coupling constants, and line widths of the components of the whole spectra. Effects of the crystalline field and quadrupole interaction are usually very small in the case of polymer radicals. The g-value is defined by the resonance condition ... 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

Table 3.2 ESR spectra of free radicals formed during UV irradiation of polyolefins...

Arzhankov SI, Poznyak AL (1974) ESR spectra of free radicals formed during low temperature photolysis of complex ions of iron(III)-citric acid. J Appl Speetrose 21 745-747... 

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