Fluorine splittings

For a-substitution, we can compare, for instance, a for the p-fluoro-nitrobenzene radical-anion with a. for the nitrobenzene radical-anion in aqueous solution the ratio is 7-95 3-40, i.e. 2-3 1 (Kolker and Waters, 1964) for the o-fluoro and p-fluoro-substituted phenoxy radicals the corresponding ratios are 2-6 (Stone and Waters, 1964a). There are probably two contributions to the observed fluorine-splitting (Carrington et al., 1965) a spin-polarization term, as for C— H, giving negative spin density at the fluorine nucleus, and a spin-polarization of the fluorine Is and 2s electrons which arises from spin density in a 2p orbital on fluorine (C—— F) and results in a positive spin density at the... 

Few j8-fluorine splittings have been reported for radicals in solution. Smith et al. (1966) used a flow technique to examine CFs.CHOH and found to be 31-8 G. Scheidler and Bolton (1966) found that % for the nitroxide (CF8)2NO is temperature-dependent and suggested that there is a direct 1,3-interaction between fluorine and nitrogen involving overlap of the fluorine p orbitals and the nitrogen orbital (see also Strom and Bluhm, 1966). 

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. 

As one last note, it is also possible to deduce the complete structure of the peroxide radical as given in Figure 9 by knowing the direction cosines of the fluorine splittings for the CF2CONH2 structure. 

McFeely and co-workers used soft x-ray photoelectron spectroscopy (SXPS) to measure the changes in binding energies of Si(2p) levels after slight exposure to fluorine atoms via dissociative chemisoriDtion of XeF2 [39]. Using synclirotron radiation at 130 eV as the source enabled extreme surface sensitivity. Since this level is split into a... 

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

Although poly(vinyl fluoride) resembles PVC in its low water absorption, resistance to hydrolysis, insolubility in common solvents at room temperature and a tendency to split off hydrogen halides at elevated temperatures, it has a much greater tendency to crystallise. This is because the fluorine atom (c.f. the chlorine atom) is sufficiently small to allow molecules to pack in the same way as polythene. 

Fluorine spectra of two fluoroethylenes, both from the Japan Halon compilation [19], are shown m Figures 4 and 5. The splitting pattern of fluoroethene (doublet of doublets of doublets) is clarified by using a branching display above the peaks, from which coupling constants can be measured easily. The AA XX spectrum of 1,1-difluoroethene is also shown. 

P-F 153 pm). However, the F nmr spectrum, as recorded down to — 100°C, shows only a single fluorine resonance peak (split into a doublet by P- F coupling) implying that on this longer time scale (milliseconds, as distinct from instantaneous for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed pseudorotation by R. S. Berry (1960) indeed, PF5 was the first compound to show this effect. The proposed mechanism is illustrated in Fig. 12.13 and is discussed more fully in ref. 91 the barrier to notation has been calculated as 16 2kJmol". ( ... 

Thus when the degree of fluorination increased, the selectivity for the fluorination reaction decreased. The dehydrofluorination reaction required the rupture of the C-F bond of the CF3CH2CI molecule while the fluorination reaction involved the rupture of the C-Cl bond. The C-F bond being harder to split than the C-Cl bond [11], the dehydrofluorination reaction require stronger adsorption sites than the fluorination reaction. 

Tertiary alkyl fluorides exhibit an additional downfield shift of about +25 ppm, which is also very sensitive to branching as seen in Scheme 3.6. The fluorine spectrum for /-butyl fluoride is shown in Fig. 3.8. The signal at -131 ppm is split into 10 peaks with a three-bond H—F coupling constant of 21 Hz. 

The fluorine substituent at the 2-position of a trifluorovinyl group is much more highly shielded than the other two fluorines, and its presence gives rise to an enhanced split of the diastereotopic fluorines at the 1-position and enhanced coupling constants, both geminal and vicinal. 

The CF2 carbon at 154.16 ppm is essentially split into a triplet of doublets with an even larger (-281 Hz) one-bond F—C coupling constant, although the coupling constants for the Z and the E fluorines are slightly different, along with the doublet deriving from the smaller... 

Fluorine usually makes its presence felt in a fairly spectacular fashion, when it is present in a molecule. Once again, I = 1/2, so we only have two allowed states to worry about. Unlike 13C however, fluorine has only one isotope, 19F, and as this of course, has 100 % natural abundance, we see the whole proton signal split, instead of a couple of tiny satellites on either side of our signals ... 

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