Proton Hyperfine Anisotropy

Rao and Symons49 studied the formation of radicals in y-radiolysis of dilute solutions of dimethyl sulfoxide in fluorotrichloromethane. By ESR studies they found the radical cation (CH3)2SOt whose ESR spectrum show considerable g anisotropy and small methyl proton hyperfine coupling. 

Branca et al. performed pioneering work in analyzing the room- and low-temperature CW-EPR and CW-ENDOR spectra of oxo-Cr(V) complexes of a number of 1,2-diols (1-3) in methanol.55 They revealed a clear anisotropy of the g and ClA tensors (Table I) and determined the full proton hyperfine tensors. They were able to identify the presence of two classes of protons in the chelated ethylene bridge of the complexes. For complex 1, a fast interconversion between these two classes was found at room temperature. 

Typical values for proton hyperfine shift anisotropy are hardly larger than 105 s 1, i.e. much smaller than x l for all practical cases. 

Lines 2 and 3 in fig. 3 exhibit a large relative hyperfine anisotropy. They may therefore be assigned to a-protons (directly bound to the TT-system) at the methine positions (a,6,6, see fig. 1). The theoretical patterns for the largest couplings of these positions (6 L and B M) predict, after averaging over the two sites, the same phase as the experimental patterns (minimum at 90 ). 

With these assignments at hand the analysis of the hyperfine shifts became possible. An Fe(III) in tetrahedral structures of iron-sulfur proteins has a high-spin electronic structure, with negligible magnetic anisotropy. The hyperfine shifts of the protons influenced by the Fe(III) are essentially Fermi contact in origin 21, 22). An Fe(II), on the other hand, has four unpaired electrons and there may be some magnetic anisotropy, giving rise to pseudo-contact shifts. In addition, there is a quintet state at a few hundred cm which may complicate the analysis of hyperfine shifts, but the main contribution to hyperfine shifts is still from the contact shifts 21, 22). 

FIGURE 10.4 Anisotropy averaging in the EPR of TEMPO as a function of temperature. The spectra are from a solution of 1 mM TEMPO in water/glycerol (10/90). The blow-up of the middle 14N (/ = 1) hyperfine line in the 90°C spectrum has been separately recorded on a more dilute sample (100 pM) to minimize dipolar broadening and, using a reduced modulation amplitude of 0.05 gauss, to minimize overmodulation. The multiline structure results from hyperfine interaction with several protons. 

In radical IV the a- and /3-protons are thought to give the same hyperfine splitting constants. The quintet spectrum was observed even in the oriented samples and, in accordance with the interpretation, showed no anisotropy. Until further evidence is found, the interpretation of the quintet spectrum must be regarded as inconclusive. 

The angle dependence of the spin soliton in randomly oriented ladder poly-diactylene has also been investigated79 by pulsed HFEPR at 94 GHz. The shape of the 0-anisotropy-resolved nutation spectrum was discussed on the basis of the EPR transition moments and the differences between spin relaxation times. Reliable assignments of hyperfine couplings to the p-protons (P-H) of the alkyl side chains were achieved with the support of W-band ENDOR measurements. No significant orientational dependence of the 7i and Ti processes was found in terms of the isotropy of the p-H-hyperfine interaction. 

Cochran et al. (1961) have ignored this complication, but have extended the treatment to the more complicated case of two equivalent a-protons in the radical RCH2. For a single a-proton, each component will have weak shoulders on either side of the central peak (Fig. 11). Since the anisotropy is roughly of the form + A, 0, —A, estimates of hyperfine splitting from experimental results will in general give only... 

The envelope curve depicted in Fig. 116 also applies to anisotropies caused by nuclei other than protons. As has been stressed already, such anisotropy generally arises from dipole-dipole interactions between electrons in p- or d-levels on the atom concerned. In that case, the hyper-fine coupling tensor should have axial symmetry, so that two of the three elements of the hyperfine tensor will be equal, and components similar to that in Fig. 106 will be detected. 

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