Electron spin resonance hyperfine splitting constants

Q 8-13 TT-Electron Densities and Electron Spin Resonance Hyperfine Splitting Constants ... 

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

The electron spin resonance (ESR) spectrum of the radical anion of 1,10-phenanthroline obtained by reduction of 1,10-phenanthroline with sodium has been measured, and hyperfine splitting constants were assigned.116... 

The N//-azaindole 40 with X = 6 is paramagnetic. Its electron spin resonance (ESR) spectrum (in DMSO) shows a characteristic triplet with a 1 1 1 intensity and the hyperfine splitting constant An = 16.04 Oe. 

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]. 

ENDOR = electron nuclear double resonance EPR = electron-paramagnetic resonance ESR = electron-spin resonance NMR = nuclear magnetic resonance MA = modulation amplitude SOFT = second-order perturbation theory s-o = spin-orbit zfs = zero-field splitting (for S > 1/2) D = uniaxial zfs E = rhombic zfs g =. g-factor with principal components g, gy, and g ge = free electron g-factor a = hyperfine splitting constant A = hyperfine coupling constant for a given nucleus N (nuclear spin / > 0). 

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. 

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... 

The foundations for almost all quantitative discussions of solvent effects upon electron spin resonance spectra were laid down by the theory of Gendell, Freed and FraenkeF (G.F.F. theory), who assumed that changes in hyperfine splitting constants were solely due to redistribution of a radical s spin density. This redistribution was assumed to accompany the formation of complexes between the radical and the solvent molecules. In formulating a model for the complexes, Gendell et al. restricted their attention to radicals containing polar substituents and postulated that each substituent was able to form a localised complex with one solvent molecule. Thus for a radical RX dissolved in a binary solvent mixture A B, two solvates of the radical are postulated to be in equilibrium with each other according to eqn. 4.16.1... 

It should be noted that there is an important distinction to be made between the hexamethylacetone-sodium ion-quartet and the situation described by the original G.F.F. theory. In the G.F.F. theory it was assumed that the solvent dependence of hyperfine splitting constants is to be attributed to modifications in spin density distributions, whereas for the ion-quartet the spin density distribution is the same in tetrahydro-furan and in methyltetrahydrofuran. The variation in with solvent must be due to variation in the geometry of the ion-quartet, which will in turn vary the efficiency of the mechanism whereby spin is transferred to the alkali metal nucleus. Thus, in this case, the solvent dependence is to be attributed to variations in Q rather than p. The situation is common in the study of ionic association through electron spin resonance spectroscopy and has thwarted many attempts at quantitative descriptions of the effect of solvation upon such association until the geometry of the ionic associate in solution is firmly established it is not too rewarding to discuss how the spectrum varies with change in solvent. 

In summary, while the G.F.F. theory would seem to describe, in a qualitative fashion the variation of electron spin resonance spectra with change in solvent composition, its quantitative success has been limited. In order to improve upon the theory it seems necessary to acquire more systematic experimental information on the solvent dependence of hyperfine splitting constants. 

An interesting extension of this work is provided by the electron spin resonance spectrum of 2-6 di- -butyl-4-methoxymethylphenoxyl (III), which has been recorded in several hydrocarbon solvents all having extremely low dielectric constants and dipole moments. The hyperfine splitting constant of the para-methylene substituent was found to... 

The presence of radicals was demonstrated (De Groot et al., 1973) using the radical trap, 2-methyl-2-nitrosopropanol. By studying the hyperfine splitting constants of the electron spin resonance (esr) spectra of a series of adduct radicals formed when linoleic acid or different deuterated analogues were used as the substrate, it was shown that the radical scavenger reacted at position 9 or 13 on linoleic acid. This is evidence that the linoleyl radical... 

---