Hyperfine splitting in ESR

ESR spectra of radicals show some similarities with the NMF spectra of spin-paired molecules. The ESR g value corresponds to the NMR chemical shift. Hyperfine splitting in ESR caused by electron-nuclear spin interaction , have an NMR counterpart in the splitting of resonances by miteraclions between two nuclear spins. 

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

Work of several types has been carried out. As long ago as 1970 Strom and Norton studied electron-withdrawal by substituents containing Group IV and Group VI elements through their effect on ESR methyl hyperfine splitting in l-phenyl-1,2-propanesemidiones 39. 

Several general observations can be made from the spectra and the calculated ESR parameters. First, the Na+- smectites possess narrower resonance line widths than the Ca +-smectites, with the exception of the saponite. Since Ca +-smectites, unlike Na+ -smectites, do not disperse into individual platelets in aqueous suspension, the Na+-smectite films formed by drying suspensions onto a smooth flat surface have the silicate surfaces more perfectly oriented in the plane of the film. As a result, less angular variation of the z-axis of Cu + relative to the plane of the film would narrow the spectra. One can see evidence of hyperfine splitting in the gj component of the Na+-smectite spectra, but not in the Ca +-smectite spectra. Saponite, unlike the other smectites, has very similar spectral linewidths for the Na+ and Ca + form (Figure 16). Since this Na+-saponite sample does not disperse completely in water (Table II), the alignment of Na+-saponite platelets in the clay film may be no better than that of the Ca +-saponite. 

Finally, bisgalvinoxyl, 11, is a stable triplet.89 Its triplet ESR spectrum displays no appreciable hyperfine splittings, in contrast to the monoradical spectrum which is a quintet due to splitting by four equivalent meta protons. Therefore this biradical is a true triplet with reasonably strong spin coupling. 

Let us briefly mention here that proton hyperfine splittings have been observed in the high-field ESR spectra of the naphthalene triplet (33,40) in single crystals. Recently Grivet (43) has succeeded in observing hyperfine splittings in the intermediate field (AMg=2) line of the triplet of partially deuterated naphthalene in rigid solution with a conventional spectrometer. 

Cobalt analogues of biferrocene and bis(fulvalene)diiron have not been investigated as extensively as the iron compounds, despite the information on electronic interactions which could be gained from observing cobalt hyperfine splittings by ESR spectroscopy. The bis(fulvalene)dicobalt dication (M = Co) can be reduced in two successive one-electron steps at mild potentials ( = -0.07 V and -0.95 V), and [Co2(tj, t7 -CjoHg)Cp2], [1] " (M = Co), is reduced at -0.53 V and -0.88 V (3i) the preparation of the monocation [3]" (M = Co) has been briefly reported (32, 33). 

The alkylidyne-capped clusters [Co3(CO)9(ju-CR)] (73), also with one less valence electron than 72, are easily reduced to anion radicals 177-180). The reversibility of the redox couple was verified by a variety of voltammetric methods 178) that the values (e.g., R = F, = -0.28 V R = Me, = -0.43 V vs Ag/AgCI), and cobalt hyperfine splittings in the ESR spectra of the anions (As9co 36 G)... 

Dimerization of thiopyranyl yields a product, 4,4 -bithiopyranyl, which may be dehydrogenated to the dication bithiopyrylium. This dication is reducible, by uptake of one electron, to give 4,4 -bithiopyryliumyl (162). The reduction has been effected chemically for 162 ° and electrochemically for 162 and its 2,2, 6,6 -tetramethyl and -tetraphenyl derivatives. Electronic absorption spectra were recorded in the latter work while ESR spectra were measured in the former. The hyperfine splittings in 162 are in gauss. 

Nitro-radical anions have proved to be especially accessible to ESR study [180-183], The large nitrogen hyperfine splitting in the radical anion often leads to partly or completely resolved hyperfine structure (Fig. 12) that can be analyzed directly or by computer simulation. In the case of the nitro compound furylfuramide (AF-2), the cis-trans isomerization catalyzed by nitro reductases appears to proceed via the radical anion ESR spectra of both cis and trans radical anions have been resolved during reduction of the cis parent compound. 

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