Fluorine hyperfine

Cyclopentadienyl radicals are likely to experience Jahn-Teller-type distortion from planarity. The pentafluorocyclopentadienyl radical C5F 5214 was obtained by photolytic dissociation of the chlorine atom of C5F5C1. The fluorine hyperfine structure (A1S0 = 16 G)... 

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)... 

An additional observation made by Sauer, Wang and Hinds [83] was the Stark effect which enabled them to determine the electric dipole moment to be 3.91 D. This value taken with the relatively small fluorine hyperfine interaction suggests that YbF has a largely ionic structure. 

Addition of F or a protic solvent (water, alcohol) causes an upfield shift of the resonance accompanied by line broadening with concommitant loss the silicon-fluorine hyperfine structure. 

Some AB a-Radicals. In a previous review (19) we showed how the fluorine hyperfine parameters (17, 23, 43, 44, 47) for various XF" o-radi-cals (Vr centers) depend upon the electronegativity of the other halogen atoms (X). Revised values for the parameters (79) have led us to present the data again (see Figure 1 and Table V). Also included are results assigned to KrF and XeF, although direct comparison is difficult because it is not clear what measure of electron-attracting power one needs to use. 

Another example of a hydrated salt which gives the stable radicals obtained from C—C bond breakage is sodium perfluorosuccinate hexa-hydrate. When irradiated at room temperature the principal product is OOC-CFa-CF-COO- (28) but at 77°K. the ESR spectrum of CF2COO is also seen (31). In this case the magnitude of the fluorine hyperfine splitting rules out the electron attachment products such as "OOC-CF2-CF COO2" since /3-fluorine splittings are much smaller. In a detailed study of the 77°K. irradiation products of succinic acid (8) only the radical-ion HOOC-CHjj-CHjj-COOH" was mentioned. More recently it is claimed (16) that some of the weak lines are from HOOC-CH CH2 , but HOOC-CH2 has not been reported. 

Fluorination of aromatics by XeF2 in the presence of HF is thought, contrary to earlier belief, not to involve the aromatic cation radical. However, a cation radical is thought to accept fluoride ion in reactions of XcF2 with easily oxidized aromatics (anisole) in the absence of HF (Anand etal., 1975). In this connection zinc tetraphenylporphyrin has been oxidized to its cation radical with xenon difluoride (Forman et al., 1971) although fluorine hyperfine splitting was found in the esr spectrum, reaction with fluoride ion did not occur. 

The EPR spectrum of NFJ has been measured both as a radiolysis product of NF4 salts and as an intermediate in their formation [1,2, 3]. NFJ salts were y-irradiated [1,2], or mixtures of NF3, F2, and a Lewis acid (e.g., AsFg) were UV-photolyzed at 77 K [1,3]. The spectra proved to be rather complex due to the presence of both nitrogen and fluorine hyperfine structures (hfs) and anisotropies which were different at low and high temperatures [1,2]. Final analysis [5] of a spectrum at 25 K [1] gave the following tensor components of the splitting factor g and hfs constants A(N) and A(F), in MHz, in the respective principal axis system ... 

TABLE I. An Estimation of the Unpaired Spin Population in Various Orbitals and Their Contributions to the Fluorine Hyperfine Splitting Tensor (in Gauss) of the CF2C00" Radical( ) at 300 K.(a)... 

Alpha - Fluorine Hyperfine Splitting Tensors (Gauss) for Fluorine Containing Radicals in Single Crystals... 

Figure 4. Temperature variation of the a, /3t, and P-2 fluorine hyperfine splittings of the 300°K radical with the magnetic field parallel to a, b, and c. The spectra become too complex to analyze where the curves are shown dashed. From Reference 12.

In the previous two examples, the g tensor variation with temperature and crystal orientation was not discussed since such a measurement has not been reported. Nevertheless, as expected for any radical undergoing internal motion, the g value did vary with temperature along selected orientations. Because the g anisotropy is small even for the rigid form of the radical, quite careful measurements are required if the data is to be used to identify the particular type of motion which occurs. This is usually not done as the fluorine hyperfine anisotropy variation is usually large and provides the most sensitive measure of the internal reorientation. 

Provided the unshifted centres of the transitions can be located the central-atom hyperfine interaction and value may be determined exactly from an iterative solution of the Breit-Rabi equations. Ligand fluorine hyperfine interactions should be determined from the highest-field transition available where spin-spin decoupling is essentially complete. 

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