Electron spin resonance line width

An alternate approach to the study of fast reactions is by means of competition methods. Here the rate of a process is measured separately in systems where chemical reaction can and cannot occur. If the process is perturbed by the reaction, the difference in the rates is a measure of the rate of reaction. For example, an unstable species may be formed photochemically by irradiation and its lifetime determined. The experiment is then altered by adding a substance which can react with the unstable species it will thus have a shorter lifetime from which the rate of the chemical reaction can be deduced. To be a useful technique the chemical and physical processes must proceed at roughly similar rates which limits the applicability of this approach, as is emphasized in Fig. 4.1. Other competition experiments (and the corresponding observables) have been made using polarography (current), nuclear magnetic resonance (line width), electron spin resonance (line width), and diffusion (diffusion coefficient). 

The sample heated at 605 °C showed the maximum activity for O/P conversion, and this sample also showed the maximum area under the electron spin resonance line and the minimum width, which means the maximum amount of localized paramagnetic centres. The samples heated at higher temperatures showed enhanced activities for H/D exchange and decreasing activities for O/P conversion. The ESR lines were broadened, which indicates exchange interactions between the paramagnetic centres. 

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... 

There are many more solvent effects on spectroscopic quantities, that cannot be even briefly discussed here, and more specialized works on solvent effects should be consulted. These solvent effects include effects on the line shape and particularly line width of the nuclear magnetic resonance signals and their spin-spin coupling constants, solvent effects on electron spin resonance (ESR) spectra, on circular dichroism (CD) and optical rotatory dispersion (ORD), on vibrational line shapes in both the infrared and the UV/visible spectral ranges, among others. 

We report an electron spin resonance (ESR) study on a C60 anion and a metal (M) encapsulated in fullerene (C ) (a metallofullerene M C ). The anisotropy components of the g-factor of Cg0 were determined accurately from the analysis of angular-dependent ESR spectra of single crystal Cg0 salt. The evaluation of the g-factor was performed according to the classification of symmetry of the C60 geometry. It was found out from the evaluation that the molecular structure of Cg0 should he distorted to lower symmetry, C2h or C,. The variety of ESR spectra of metallofullerenes of La C s was obtained in terms of a g-factor, a hyperfine coupling constant, and a line width. In the case of the isomer I of La C80 and the isomer II of La C84, an abnormally large line width was measured. The molecular structure with high symmetry would reflect on the specific spin dynamics. 

Free Radicals in Macerals. Electron spin resonance (ESR) has been used to study carbon free radicals in coals, and to some extent, separated macerals. The technique provides information on radical density and the environment of the radicals. The resonance position, termed the g-value, is dependent on the structure of the molecule which contains the free electron. The line width is also sensitive to the environment of the unpaired electron. In an early study, Kroger (71) reported that the spin concentration varied between maceral groups with liptinite < vitrinite inertinite. For this limited set of samples the spin concentration increases with rank for liptinites and vitrinites and decreases for the micrinite samples. On the other hand, van Krevelen (72) found the same general results except... 

EPR measurements were first performed on wurtzite GaN in 1993 by Carlos and co-workers [2-4] and on cubic GaN by Fanciulli and co-workers at about the same time [5], The primary resonance in the wurtzite films is slightly anisotropic (gy = 1.9510 and gi = 1.9483) with a width 0.5 mT at 4.2 K and generally acknowledged to be due to a band of delocalised effective mass (EM) donor electrons. The average g value is consistent with the expectations of a 5-band k.p analysis and is also similar to that obtained by Fanciulli [5] for a much broader line (—10 mT) in their conduction electron spin resonance experiments on zincblende films. With this exception all of the work discussed in this Datareview is on the wurtzite phase. 

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

Fig. 25. Electron spin resonance spectra of NO2/AI2O3 adsorbates at different NO2 coverages. The bottom trace was recorded after 0.5 ML coverage had been annealed at 105 K. The inset shows the line widths as a function of temperature [121].

Counterions. 1. Sodium-23 Alkali metal MIR is a sensitive >robe of the immediate chemical environment and mobility of alkali metal ions in aqueous and nonaqueous solvents (7, 8). The chemical shifts of alkali metal nuclei will respond to" electronic changes only in the immediate environment of the cation since alkali metals rarely participate in covalent bonding (7). All alkali metal nuclei have spins greater than 1/2 and hence have quadrupole moments. The interaction of these moments with electric field gradients, produced by asymmetries in the electronic environment, is modulated by translation and rotational diffusive motions in the liquid. It is via this relaxation mechanism that the resonance line width is a sensitive probe of ionic mobility. 

The preparation and the crystal structures of single crystals of alkali aromatic ion pairs are discussed. A close relationship was found between the ion-pair structures in solution and in the solid state. The physical properties of the pseudo-two-dimensional magnetic alkali biphenyl crystals are reviewed. The effect of spin diffusion manifests itself clearly in the line width and line shape of the exchange-narrowed electron spin resonance (ESR) line. The monoanions of cycloocta-tetraene, produced by X-ray irradiation, rotate rapidly about their eightfold axes. At 20 K this rotation is frozen, and an alternating spin density distribution is found around the ring. The equilibrium position of the monoanion is rotated by 22.5 compared with the equilibrium position of the dianion. 

An additional argument in favor of the presence of intercalation is the temperature dependence of the line width of the electron spin resonance signal. Although the starting metaanthracite shows a derivative extremum line width of 12.5 1.0 gauss (G) that is independent of temperature, the C8K product shows a line width that depends upon temperature ... 

The total number of unpaired electrons is proportional to the area under the spin-resonance line. The sensitivity of detection is increased if the absorption line is narrow and decreases to zero when it is so broad as to be indistinguishable against the background noise. For spin resonances whose width is of the order of 1 gauss at half-height, the practical limit of detection is about 10 mole of unpaired spins at 10,000 Me. The sensitivity increases with increase in frequency, being proportional to the square of the latter. On the other hand, dielectric losses which lower the sensitivity are smaller at the lower frequencies. Usually, the intensity of absorption increases with decrease in temperature, as would be expected from the Curie-Weiss law. 

It does not seem necessary or advisable to describe once again the basic principles of electron spin resonance, since there are now available a substantial number of reviews and books dealing with these matters. Introductory articles have been written by Carrington S and Atherton S and there exist several text books S " s which treat the subject on a more quantitative level. Electron spin resonance papers have been regularly reviewed in the Chemical Society of London s Annual Reports and these articles may be used as guides to the more significant aspects of current developments in the field. Three features of an electron spin resonance spectrum are of interest the hyperfine splitting constants of any nuclei with non-zero spin in the molecule, the g-factor of the radical, and the widths of the various lines in the spectrum. 

Line-width effects in electron spin resonance spectra have been the subject of three recent reviews " and so there seems little point in further detailed repetition of the principles at this time. Basically, line-width effects will be observed when the Hamiltonian describing the spin systems contains time-dependent elements having frequency components comparable to frequency separations in the spectrum. The mechanisms... 

In Fig. 4.15.1 we reproduce the electron spin resonance spectrum of monoprotonated / -benzosemiquinone in tetrahydrofuran at — 63°C, a spectrum which shows most of the features discussed above. The spectrum has been analysed in terms of four hyperfine splitting constants, as shown in Fig. 4.15.1, since the asymmetric disposition of the —OH group causes the protons meta to the site of protonation to be non-equivalent. The four splitting constants are readily obtained by measuring distances from the first line of the spectrum the g-value may of course be obtained by simultaneous measurement of microwave frequency and magnetic field at the centre of the spectrum. Readers unfamiliar with line-width effects may care to compute the expected relative intensities of the lines and compare the results with the experimental amplitudes of the first derivative trace. In such a presentation the peak-to-peak amplitude, for a Lorentzian line, is proportional to the reciprocal of the square... 

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