Isotopic spectra, electron spin resonance

Figure 16-17 Electron paramagnetic resonance spectrum of the Fe-S protein putidaredoxin in the natural form (32S) and with labile sulfur replaced by selenium isotopes. Well-developed shoulders are seen in the low-field end of the spectrum of the 77Se (spin = l/2)-containing protein. From Orme-Johnson et al.29S Courtesy of W. H. Orme-Johnson.

Seventeen-electron species have also been found to form complexes with noble gases. For example, the two paramagnetic radicals RrMn(00)5 and [KrFe(00)5]+ have been detected by EPR spectroscopy by Morton, Perutz, and co-workers following the y-radiolysis of HMn(00)5 and Fe(00)5 in laypton matrices at 77 and 20 K, respectively (37). Evidence for the interaction of Kr with the unpaired electron on the metal center came from the observation of hyperfine couphng with a single Kr nucleus in the EPR spectra of these species. As an example, the EPR spectrum obtained from y-radiolysis of HMn(CO)5 in a matrix of krypton enriched to 42% in the isotope Kr (I = ) is shown in Fig. 5. The spectrum shows the resonances of the Mn(CO)5 radical with characteristic decets of satellites due to hyperfine interaction between the unpaired spin on Mn and a Kr nucleus. 

Sometimes it is very difficult to determine unambiguously the precise structure of the spin adduct from the EPR signal obtained. Isotopic substitution EPR experiments are recommended in an attempt to identify the observed adducts. The strategy is that the unpaired electron in a radical interacts with the nucleus of the atom it orbits, and the spin of the nucleus determines the number of lines or peaks in the spectrum. For example, has a nuclear spin of j while has no spin. An unpaired electron, which is associated with atoms having no spin, will exhibit an EPR spectrum containing only a single line. The spin of the nucleus influences the resonance of the unpaired electron so that the EPR resonance splits into two or more lines. The number of EPR resonance observed is equal to 21 + 1, where I is the nuclear spin. A practical... 

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