Fluorine semi-occupied orbital

Semi-occupied Orbital of Inorganic Fluorine Radicals... 

The free radicals we shall discuss in this article have in common the basic structure of their semi-occupied orbitals. In its simplest form this semi-occupied molecular orbital is the linear combination F(2p) -M(ns) - F(2p), in which the fluorine 2p orbitals point towards the central atom (M) and are antibonding with respect to the M(ns) contribution (Fig. 3, top left). Also illustrated in Figure 3 are the prototypes of some of the fluorine-containing free radicals we shall be discussing. These examples illustrate the different ways linear F-M-F arrays can be combined to form molecular orbitals of various symmetries. In the case of the radicals SF3 and PF4 a single F-M-F array is used, but their low symmetries permit central-atom 3pg and apical fluorine Zp orbitals (z is the two-fold axis) also to be incorporated. 

For radicals of higher symmetry, such as CIF4 (D4h) and SF5 (C4v) two mutually perpendicular F-M-F arrays are utilized in the formulation ofthe semi-occupied orbital. Again, the lower symmetry of SF5 permits a contribution from sulfur 3pz and fluorine Zp atomic orbitals. 

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. 

This effect is probably due to an increased sulfur 3p contribution to the semi-occupied molecular orbital as the electronegativity of RO increases. An increased sulfur Sp contribution will cause increased polari zation of the S-F bond, resulting in a decrease in the F hyperfine interaction. It is not clear, however, why the effect is transmitted to the trans fluorine nucleus only, and it is a pity that no data are yet available on the 33s hyperfine interaction and hence on changes in the sulphur 3s contribution to the semi-occupied molecular orbital. 

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