Single bond, dissociation

If, for instance, the orthogonal nonbonding orbitals are AOs Xa and Xh located at the atoms A and B and prevented from interacting either by excessive separation, as in a dissociated single bond, or by symmetry, as in a twisted double bond—that is, if

= Xh—fh configurations given in Equations (4.1) and (4.2) are the VB structures which may be represented by the formulae 2-7. 

The feature that distinguishes intemrolecular interaction potentials from intramolecular ones is their relative strengtii. Most typical single bonds have a dissociation energy in the 150-500 kJ mol range but the strengdi of the interactions between small molecules, as characterized by the well depth, is in the 1-25 kJ mor range. 

In the MO-CI language, the correct dissociation of a single bond requires addition of a second doubly excited determinant to the wave function. The VB-CF wave function, on the other hand, dissociates smoothly to the correct limit, the VB orbitals simply reverting to their pure atomic shapes, and the overlap disappearing. 

Tables 2.3 and 2.4 list a selection of typical dissociation energies. The values given in Table 2.4 are average dissociation energies for a number of different molecules. For instance, the strength quoted for a C—O single bond is the average strength of such bonds in a selection of organic molecules, such as methanol (CH3—OH), ethanol (CH,CH2—OH), and dimethyl ether (CH,—O—Cl l5). The values should therefore be regarded as typical rather than as accurate values for a particular molecule.

FIGURE 2.16 The bond dissociation energies, in kilojoules per mole of nitrogen, oxygen, and fluorine molecules. Note how the bonds weaken in the change from a triple bond in N, to a single bond in F,... 

These assumptions are consistent with the very large bond strength of the BF bond in BF3, which is larger than that of any other single bond. It has a bond dissociation enthalpy... 

Table 10.1 Single-Bond Homolytic Dissociation Energies AH° at 25°C... 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

Let us consider a model case for a single bond dissociation process where there is no need to call for an intermediate Hamiltonian in the sense discussed above. The discussion presented in 2.8 applies. Here, some formal aspects are discussed. 

Nitrogen. Molecular nitrogen N2 has a dissociation energy of 950 kJ/mol, and the N-N triple bond is one of the strongest known chemical bond. Shock-wave experiments disclosed the possibility of N-N dissociation in condensed phases [224, 319-322]. From this an interest arose in the possible obtainment of arrays of N-N single bonds that could form in potentially energetic materials. Ab initio calculations of various kinds [323-327] showed that actually at high pressure... 

The dissociative mechanism can explain both facts in that the hydrogen removed in the first step may recombine with an isomeric form of the ally lie intermediate to yield the isomeric olefin. Apparently syn and anti 7T-allylic complexes [67, 68) retain their configurations unless each may be converted into a common a-bonded complex in which the nonterminal carbon atoms of the allyl group are connected by a single bond and the isomerization of the intermediate can be represented as in Fig. 11. However, the recombination of the hydrogen atom with the allylic intermediate must be faster than the rate at which it enters the surface pool of... 

---