Covalent bonds energetics

As mentioned in Chapter 2.1, the formation of radicals requires the homolytic cleavage of a covalent bond. Energetically such homolytic cleavage is particularly favorable in gas-phase reactions and for liquid-phase reactions in nonpolar solvents. In polar solvents, however, the energy contribution from the solvation of ionic species formed in heterolytic cleavage reverses the picture and heterolytic cleavage becomes more favorable. 

When carbon forms four covalent bonds with halogen atoms the second quantum level on the carbon is completely filled with electrons. Most of the reactions of the Group IV tetrahalides require initial donation by a Lewis base (p. 91) (e.g. water, ammonia) which attaches initially to the tetrahalide by donation of its electron pair. Hence, although the calculated free energy of a reaction may indicate that the reaction is energetically favourable, the reaction may still not proceed. Thus we find that the tetrahalides of carbon... 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

By contrast, anions have larger radii and tend to be more weakly hydrated. In addition, they are able to form relatively strong ionic/covalent bonds to the surface of the metal electrode and, as a result, frequently find it energetically feasible to shed their inner hydration sphere, or at least part of it, and adsorh directly on the surface. The plane formed hy the nuclei of anions directly adsorbed on the metal surface is termed the inner Helmholtz plane (IHP). 

NU(C) base atoms (5) The stereoselectivity of the BPDEs during intercalative covalent binding in kinked DNA and (6) The possible reorientation of the complex to yield an externally bound adduct. The energetics for each of these processes will be presented to identify the important steps that influence the binding of specific isomers. It will be shown that the orientation of each diastereoisomer of BPDE about specific base atoms in kinked receptor sites in the duplex DNA during covalent bond formation is the determining factor in stereoselectivity. 

In sihcon and oxygen compounds, Si has coordination IV or VI. The SiO/ tetrahedron is energetically quite stable, with mixed ionic-covalent bonds. 

The corresponding calculations on the endohedral and exohedral complexes of CgQ with H, F or the methyl radical as guest predict the same behavior [98]. In all cases the formation of endohedral covalent bonds is energetically unfavorable due to the analogous strain arguments, even if such reactive species as F atoms are exposed to the inner surface of Cjq. 

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