High-field electron spin resonance

Figure 3. High resolution electron spin resonance spectrum of an irradiated, frozen suspension of actomyosin. Sample was irradiated to 50 kGy at —40°C and spectrum was recorded at —40°C. The central portion of the spectrum corresponding to a gain of XI shows the doublet feature with a shoulder on either side. At a gain of X10, an additional three lines are clearly discernible on both the high and low field portions of the spectrum.

The electron spin resonance of the nitroxalkylcorrinoids can be readily observed in aqueous solution at room temperature. Both the cobalamin and cobinamide show nitrogen hyperfine coupling constants of 17.2 gauss. A typical spectrum is shown in Fig. 20. The line widths for the low, intermediate, and high field peaks are 1.87, 1.87, and 2.20... 

Fig. 20. Electron spin resonance spectra of nitroxalkylcobalamin. (a) Spectrum before photolysis the high field line is broadened and therefore has a lower peak amplitude, (b) Expanded view of center line before photolysis showing no indication of additional hyperfine from methyl protons, (c) Spectrum of nitroxide photolysis product which has been freed from the cobalamin. (d) Expanded view of center line after photolysis now faintly showing proton hyperfine...

As with the nitroxalkylcobalamins (119) and cobinamides, the co-binamides in which nitroxide is coordinated show electron spin resonance spectra very similar to the spectrum of free nitroxide. The high field line is not broadened as much as in the spectrum of a nitroxalkyl-cobinamide. No hyperfine splitting from methyl protons in the 2 or 6 positions can be observed for the bound nitroxide. However, treatment of the coordinate spin labeled compounds with cyanide releases the nitroxide. When this happens, the proton hyperfine can be observed (Fig. 25). Thus treatment with cyanide simply displaces the nitroxide and a spectrum for free nitroxide is observed. 

The real part is the magnetic permeability whereas the imaginary part is the magnetic loss. These losses are quite different from hysteresis or eddy current losses, because they are induced by domain wall and electron-spin resonance. These materials should be placed at position of magnetic field maxima for optimum absorption of microwave energy. For transition metal oxides such as iron, nickel, and cobalt magnetic losses are high. These powders can, therefore, be used as lossy impurities or additives to induce losses within solids for which dielectric loss is too small. 

Instrumentation. Two electron spin resonance spectrometers have been used in the course of the work. The Southampton instrument was built in the laboratory, using a Varian magnet, and operated at 3 cm. wavelength. It employed an Hon rectangular cavity and obtained high sensitivity by magnetic field modulation at 100 kc.p.s. with a crystal detector phase sensitive detection... 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

Fig. 14. Electron spin resonance spectrum of a frozen solution of rubidium in HMPA, at high machine amplification. The full lines show the variation of resonant field position with A for ge = 1.99800, and a microwave frequency of 9.1735 GHz. The lines are anchored at the crossovers of the MG species (A = 251.3 G). Positions of the Mc, M , ME, Mg, Mh, and M, absorptions are indicated. Reprinted with permission from R. Catterall and P. P. Edwards, Journal of Physical Chemistry, 79, 3010 (1975). Copyright 1975 American Chemical Society.

Sx, Sy, and Sz are the three components of the spin operator, D and E are the anisotropy constants which were determined via high-frequency electron paramagnetic resonance (D/kB 0.275 K and E/kB 0.046 K [10]), and the last term of the Hamiltonian describes the Zeeman energy associated with an applied field H. 

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