Anisotropic fluorine hyperfine

The spectrum of the tetrafluoroethylene radical anion was first obtained by y-irradiation of a solution of C2F4 in tetramethylsilane at 77 K228. Above 120 K the spectra were isotropic and the satellite lines due to 13CF212CF2 were detected (13C = 49 G) at 83 K the spectrum was anisotropic and exhibited axial hyperfine interaction with four equivalent fluorines 19F, = 74.4 G, T/7= 135.9 G, Aiso = 94.9 G. These couplings were first inter-... 

The study of radicals trapped in single crystals of their precursors has shown that the hyperfine tensors for both a- and j9-fluorine atoms are much more anisotropic than the tensors for related a- and jS-hydrogen atoms. This feature is well illustrated for the a-atoms by the observations for monofluoroacetamide radical, XXIV, shown in Table 10. 

The anistropic hyperfine tensor components for the a-fluorine atom are 133, —60, and -72 G, respectively. The direction of the maximum principal element for the fluorine atom is parallel to the axis of the carbon Ip orbital and is almost axially symmetric with respect to the perpendicular axis, x in XXIV. Iwasaki (52b) and Kispert (54) have discussed these results and other observations for a-fluorine atoms. Iwasaki concludes that the large anisotropy and the large positive coupling constants (55) have a common origin in the delocalization of spin density by direct overlap, XXV. It is pertinent that theory suggests that the anisotropic components for a fluorine atom are 1080, —540, -540 G, respectively. The results for the acetamide radical, consequently. 

ESR spectra were difficult to identify because of the large anisotropic hyperfine coupling to the fluorine nuclei. These authors also examined the radio-chemical products using solid-state NMR and FTIR spectroscopies (334). On irradiation helow the melting temperature, there was no evidence of branch/cross-link formation. However, the yields of chain ends, donble bonds, and branch points on irradiation at 493 K were 3.6, 0.2, and 0.5, respectively. The structure of the branches could not be confirmed however, the respective yields of chain ends and branches indicate that scission dominates for this polymer. 

Hyperfine coupling of the unpaired electron with fluorine 3-atoms is also anisotropic with the minimum and maximum values observed at a = 0° and a = 90°, respectively. The isotropic HFC constants for fluorine atoms can be assessed by the formula... 

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