Solvent Effects on Electron Spin Resonance Spectra

4 Solvent Effects on Electron Spin Resonance Spectra 

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]. 

An illustrative example of a strongly solvent-dependent ESR speetrum is given in Fig. 6-10. It shows low-field portions of the ESR spectra of the stable, distillable ( ) 4-(methoxycarbonyl)-l-methylpyridinyl radieal, measured in three solvents of increasing polarity [381]. The different patterns of the ESR lines observed in different media arise from the solvent influence on the spin distribution, as measured by the HFS constants fl( H) and fl( N). This change in spin distribution results from solvent-induced polarization of the earbonyl group, whieh is eoplanar and conjugated with the pyridinyl ring. 

Whereas solvent effects on line-widths and g-factors have been studied only in a small number of eases, there are numerous data on the solvent dependence of the HFS 

The solvent dependence of gf-factors has been studied for 1,4-benzosemiquinone anion radieal [191], di-t-butyl aminyloxide [212], 2,2,6,6-tetramethyl-4-piperidone-l-oxyl [216], and nitrosyl protoporphyrin derivatives [371]. Apparently, changes in distribution of the unpaired eleetron and changes in spin-orbit coupling in going from one solvent to another are refleeted in the gf-factor ehanges. A dramatic line width alteration has been observed for the nitrobenzene anion radieal in hexamethylphosphorie triamide upon addition of methanol [205], 

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