Hyperfine interactions fluorinated radicals

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 radical ions of CF2=CF2 well illustrate the fact that the geometry and the electronic structure of a radical cation can be markedly different from those of the corresponding radical anion. As mentioned above (CF2=CF2) adopts a chair structure 18 (C2h)30, while (CF2=CF2)+ produced by irradiation of a dilute solution of C2F4 in FCC13 at 77 K is characterized by ESR parameters (Table 28) which are consistent with a planar structure with the unpaired electron in a bonding n-orbital283,284. An extra splitting was attributed to hyperfine interaction with a fluorine of the matrix. 

In all three radicals the main information obtained concerned the 19F hyperfine interaction. The data are summarised in table 10.21. The electron spin density on the fluorine atom in CF is estimated to be 17.8%, and in SiF it is found to be close to 8%. It is probably even smaller in GeF but the data are limited because only the 2ni/2 component could be observed. 

It is perhaps a pleasant surprise that the X 2 Y 1 state of YbF conforms to a simple case (b) coupling scheme, so that each rotational level N is split by the spin-rotation interaction into states characterised by. 1 = N 1/2 each of these states is further split into a doublet by the fluorine hyperfine interaction, giving final states TV, S, J, /, F) as shown in figure 11.42. The effective Hamiltonian is therefore written in the familiar form (see, for example, our discussion of the CN radical in chapter 9)... 

These studies indicate that the radical observed is OJF% where x > 1. A pattern has been observed showing hyperfine interaction with one fluorine nucleus and one 170 nucleus. However, there may be more than one oxygen in the radical. 

The radicals present in the four binary OF compounds (OF2, 02F2, 03F2, and 04F2) may be 02F% 03F% and 04F However, radicals with the same hyperfine interaction with a fluorine nucleus (13 gauss) have been observed in all four binary OF compounds. It may be that the same paramagnetic species is present in each compound. 

This study has established that oxygen difluoride dissociates photo-lytically into a paramagnetic species in which there is a hyperfine interaction between the unpaired electron and one fluorine nucleus. The photolytic rate of formation of the radical species increased with temperature. The decay of the signal intensity in the absence of light after photolysis did not depend on temperature. The kinetics have been interpreted in terms of a photolytic formation scheme. The radical has been characterized by means of the EPR spectrum, but not identified. However, the characteristics of the spectrum show that the radical was Oa-F, rather than F ... 

The trivalent fluoro-radicals which we shall discuss are SF3 and its derivatives. The ESR spectra of CF3 and other perfluoro-alkyl radicals have been adequately treated elsewhere (34, 35). The radical SF3 was first detected in electron-irradiated SF ( ). Its spectrum indicates the presence of two, rather than three equivalent F nuclei a iq(2) = 54. 3 G, jq(l) = 40.4 G. This observation is consistent with the structure of SF3 shown in Fig. 3, in which the semi-occupied orbital is described as anantibonding combination of F p(2p) orbitals and the S(3s, Sp ) orbitals. The bonds to the apical fluorines are weaker, longer, and more polarizable, resulting in the somewhat larger hyperfine interaction of the apical nuclei. 

Fortunately, for this solvent, the electron-capture centres give very broad e.s.r. features at 77 K, and hence the spectra for S + cations are readily distinguished. We know of no instance in which S + cations are not formed provided the ionization potential of S is less than that of the solvent. There are two complicating factors, one is unimolecular break-down or rearrangement of the radical cations, and the other is weak complexation with a solvent molecule. The latter is readily detected because specific interaction with one chlorine or one fluorine nucleus occurs, and the resulting hyperfine features are usually well-defined. 

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