FHF species

Finally, the impact of the ionic stmcture is fleshed out by comparison of (FHF) with the corresponding radical species, (FHF) . Thus, with one electron less, in the (FHF) species, the triple ionic structure is replaced by the F H "F stmctures which loses at least half of the electrostatic stabilization, and therefore, rises above the HL curves. This loss has a tremendous impact on the reaction profile, and the > 40 kcal/mol energy well of (FHF) becomes an (FHF) transition state ca. 18 kcal/mol [21,79] above the reactants. For the same reason, it is expected, therefore, that (XHX) species will generally be transition states for the hydrogen abstraction process with a barrier significantly larger than the corresponding proton transfer process via the (XHX) species. Experimental data show that this is indeed the case [80]. 

Pimentel employed this three-center, four-electron (3c/4e) MO model to discuss the bonding in triiodide (I3-), bifluoride (FHF-), and other prototypical hypervalent species. In triiodide and other trihalides, for example, the relevant AOs are the (pa, Pb, Pc) orbitals along the bonding axis,... 

Tables 3.28-3.30 summarize the geometry, binding energies, and NBO/NRT descriptors for a variety of linear triatomic XYZ- species. These include representatives of the p-p-p orbital motif (such as symmetric trihalides [X3-, X = F, Cl, Br] and the mixed chlorofluorides [FFCl-, C1FC1-, FC1F-, I CICI-]) as well as the p-s-p (FHF-) and s-s-s (HLiH-, HJ) orbital motifs. (Examples of transition metal species manifesting the s-d-s and p-d-p motifs will be considered in Section 4.10.)...

Because FHF- epitomizes the limit of strong hydrogen bonding in a particularly simple geometrical form, let us examine some further aspects of its potential-energy surface. The triatomic species can generally be described in terms of three variables,... 

What makes FHF an attractive species to investigate by nmr spectroscopy is that it consists of three nuceli each with spin 1/2 bonded directly. Also, the proton of the strong hydrogen bond should have an unusual chemical shift. Early work failed to detect the expected F doublet and H triplets (e.g. Soriano et al., 1969), and it was not until the importance of the solvent was appreciated that coupling was observed (Fujiwara and Martin, 1971, 1974a,b). Suitable media were found to be the dipolar aprotic solvents acetonitrile, nitromethane and dimethylformamide. 

On the basis of analysis of typical byproduct spectra W. Dmowski postulated a mechanism for the reaction [139]. In aHF as solvent the SF, species is generated in a solvolytic equilibrium. The strongly electrophilic SF, ion adds to the carbonyl oxygen, making the a-carbon atom highly electrophilic. A fluorine atom is then transferred intramolecularly to the carbon and sulfonyl fluoride is expelled. The resulting resonance-stabilized (by r-donation from fluorine lone electron pairs) a-fluoro carbenium ion adds fluoride from ambient (FHF) ions (Scheme 2.64). 

The approach used so far can be applied to other linear species—snch as CO2, N3, and BeH2—to consider how molecular orbitals can be constructed on the basis of interactions of group orbitals with central atom orbitals. However, we also need a method to understand the bonding in more complex molecules. We will first illustrate this approach using carbon dioxide, another linear molecule with a more complicated molecular orbital description than FHF . The following stepwise approach permits examination of more complex molecules ... 

A simple and provocative example of such strange association complexes is provided by the bifluoride ion (FHF ). This species can be formulated perfectly well as the Lewis-compliant HF molecule and F fluoride anion. 

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