Hyperfine splitting constants solvent

Table 6-5. -Factors and isotropic nitrogen and oxygen hyperfine splitting constants, a( N) and a( 0), of 2,2,6,6-tetramethyl-4-piperidone-l-oxyl in ten solvents of increasing polarity [216],...

Because of the large number of lines in a spectrum, overlapping of lines can lead to difficulties though these have largely been removed by the availability of computer simulation facilities. Furthermore, if one is interested in solvent effects then one obviously chooses a solute whose spectrum is capable of rapid analysis. Having obtained the isotropic hyperfine splitting constants it is necessary to relate them to the electronic structure of the radical. 

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... 

Plots of d vs. X, according to eqn. 4.16.7, are given in Fig. 4.16.1 for various values of K. These plots represent the expected solvent-dependence of a hyperfine splitting constant for a system described by the G.F.F. theory. 

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 hyperfine splitting constant a, as a function of temperature, for biradicals I-III, dissolved in different ionic liquids, is given in Fig 5. One can see that changes of a with temperature for all biradicals in RTILs are smaller to those in various molecular solvents in the case of piperidine-l-oxyl ling radicals and biradicals (Kokorin, 2004 Kokorin et al., 2006 Tran et al., 2009). It means that the electrostatic interaction between cations and anions of the ionic liquids and paramagnetic >N-0 groups do not reveal any specific peculiarities in comparison with molecular organic solvents. 

Most of the alcohol radicals have hyperfine coupling constants slightly dependent on both temperatures and solvents. Ayscough and McClung (220) have also studied in detail the effect of temperature variation on proton hyperfine splitting of the biacetyl semidione radical in the range 250-400 K. Livingston and Zeldes (156) observed... 

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]. 

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