Bond energies carbon-nitrogen

This IS an unusually high rotational energy barrier for a single bond and indicates that the carbon-nitrogen bond has significant double bond character as the reso nance picture suggests... 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

CN is closely similar. The normal nitrogen atom, 2s22p2p2p S, can form three bonds, and more cannot be formed by an excited neutral atom (with five L electrons), so that there is no reason to expect excitation. But a normal carbon atom can form only a double bond, and an excited carbon atom, only 1.6 v. e. higher, can form a triple bond, which contributes about 3 v. e. more than a double bond to the bond energy. Hence we write... 

In formulating a set of bond-energy values we first calculated the energies of formation of molecules from experimental values of the heats of combustion of the compounds6 and thermochemical data pertaining to the products of combustion—carbon dioxide, water, nitrogen, etc. The same values for the latter quantities were used as previously.4... 

FIGURE 3. Rotational energy profile around the methine carbon-nitrogen bond of isopropyldimethylamine (IDMA) as calculated by MM2-87. Reprinted with permission from Reference 72. Copyright (1992) American Chemical Society... 

This appears quite likely cf. K. T. Finley and L. K. J. Tong, in The Chemistry of the Carbon-Nitrogen Double Bond (Ed. S. Patai), Wiley, London, 1970. Nonetheless, in that the question of species identification persists is suggestive that file two tautomers are relatively close in energy and so our analysis continues quite unfazed by this seeming ambiguity. 

Table 4.1 Average Bond Energies of Bonds Between Carbon Atoms and Between Nitrogen Atoms...

Example 12.4 Influence of the Environment on D i. Nitromethane is interesting to some people because it explodes. The reason is, of course, in the cleavage of the carbon-nitrogen bond. The monomer, compared to its trimer (taken as a model for the crystal), reveals that the C and N net charges change by A c — 8.7 and A n—1-1 me. respectively, on crystallization. Our bond energy formula and the appropriate parameters thus indicate that the crystalline environment reinforces the CN bond by 4.7 kcal/mol, which is significant at the local point of rupture, responsible for the reaction [251]. 

Alkane carbon atoms satisfy the charge-NMR shift correlation [Eq. (6.8)]. With the alkylamines, things could be different because of a possible extra effect due to the presence of the nitrogen atom a-carbons should perhaps be compared only among themselves, and so should the /3- and y-carbons. The S-carbons, in contrast, which are sufficiently separated from the nitrogen center, could probably be treated as if they were part of an alkane. This point has been examined as follows for the —C 2—H2—NH2 motif, focusing on the dissociation and intrinsic bond energies, Dc Cp and sc Cp, respectively. 

It is difficult to assess how close the two sets of results really are. The first one evidently depends on the precision of the thermochemical data that have been used, namely, AHf and ZPE + Hj-— Hq- Equation (6.8), on the other hand, is accurate. It is perhaps our best means for testing sp carbon charges an error of 1 me in the evaluation of one of the carbons translates into an error of 0.5 kcal/mol in bond energy. Now, the two sets are too close to warrant revision of the procedure, yet we cannot endorse it for more than it is an acceptable approximation. For our needs, and for the time being, Eq. (6.8) solves the problem. Moreover, the reasoning is that if this approximation holds in the close neighborhood of nitrogen, it should be all the more acceptable for carbons in positions y, S, and so on. 

The first thing to do is to learn how to write the bond energy formula [Eq. (10.37)] for carbon-nitrogen bonds... 

Straightforward applications of the theory are presented in the atom-by-atom approach, as exemplified in Table 15.4, using charges deduced from NMR shifts, Eq. (6.8) for the carbon atoms, and Eqs. (6.12)-(6.14) for the nitrogen atoms. (CN bond dissociation energies and comparisons with the corresponding intrinsic bond energies are described in Chapter 12 for both alkylamines and selected nitroalkanes.)... 

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