Single bond introduced

The best Lewis-type representation of the bonding in OCF3 would therefore appear to be as in 4, even though the carbon atom does not obey the octet rule. This molecule can be considered to be a hypervalent molecule of carbon just like the hypervalent molecules of the period 3 elements, such as SFfi. We introduced the atom hypervalent in Chapter 2 and we discuss it in more detail in Chapter 9. But it is important to emphasize that the bonds are very polar. In short, OCF3 has one very polar CO double bond and three very polar CF single bonds. A serious limitation of Lewis structures is that they do not give any indication of the polarity of the bonds, and much of the discussion about the nature of the bonding in this molecule has resulted from a lack of appreciation of this limitation. 

The test set used for most comparisons in the present paper is Database/3 18), which was introduced elsewhere. It consists of 109 atomization energies (AEs), 44 forward and reverse reaction barrier heights (BHs) of 22 reactions, 13 electron affinities (EAs), and 13 ionization potentials (IPs). There are a total of 513 bonds among the 109 molecules used for AEs, where double or triple bonds are only counted as a single bond. Note that all ionization potentials and electron affinities are adiabatic (not vertical), i.e., the geometry is optimized for the ions... 

A new reference bond energy must be introduced at this point for the CO single bond of diethylether, selected as reference ... 

Although the mechanism of the transformation of single into double bonds has not been investigated in detail, a heteroatom attached to the single bond is necessary for an efficient introduction of the double bond. Trisubstituted pyrazo-lines 46 can be oxidized with (diacetoxyiodo)benzene 3 to the corresponding pyrazoles 47 in good yields [98]. Two double bonds can be introduced in easy accessible proline derivatives 48 [99] by an oxidative decarboxylation with [bis(trifluoroacetoxy)iodo]benzene 4 yielding tetrasubstituted pyrroles of type 49, Scheme 22 [100]. 

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