Carbon group contributions

While there is some dispute about how universal the universal methylene increment really is (cf Reference 1), it is nonetheless generally conceded that a methylene group affixed to two carbons usually contributes ca — 21 kJ mol-1 to the gas phase enthalpy of formation. 

Aspartate now donates its amino group in two steps ((8) and (9)) formation of an amide bond, followed by elimination of the carbon skeleton of aspartate (as fu-marate). Recall that aspartate plays an analogous role in two steps of the urea cycle (see Fig. 18-10). The final carbon is contributed by N1 "-formyltetrahydrofolate (step ), and a second ring closure takes place to yield the second fused ring of the purine nucleus (step (Q)). 

Figure 1.6.1 Various group contribution types for propene nitrile. The SMILES rotation is used to indicate the groups. The Csp hybridize carbon atom is in bold.

The left-hand term in eq. 9.3.6 corresponds to the n-alkyl contribution and the right-hand term to the functional group contribution. Nc is the number of carbon atoms in the molecule and rc is a constant. The contribution parameters a and be refer to the alkyl group and the parameters bf and bfc to the functional group. For compounds with homogeneous multifunctional groups (e.g., alkanediols or polychlorinated alkanes), the model takes the following form ... 

HIAs is only O.lkcalmol-1. This reflects almost complete cancellation of contributions from the extra CH2 group in the butyl structure between the cation and hydrocarbon. It indicates that HIAs provide a good approximation to differences in stability between a carbocation center and the corresponding group contribution from a hydrocarbon, independently of structural variations at carbon atoms not attached to the carbocation center. Moreover, a comparison between two secondary carbocations leads to almost complete cancellation of the contributions from the parent hydrocarbons and from alkyl groups of the carbocations too far removed from the charge center to influence stability. One is very close therefore to a comparison of stabilities comparable to that between isomeric cations. It should be noted that such intrinsic stabilities are not expressed in heats of formation of carbocations because they include uncanceled contributions from more remote portions of the structure. 

The H NMR spectra of the related [La(THED)]3+ as a function of temperature reveal a dynamic process at room temperature similar to that observed for [Ln(DOTA)] complexes [143]. At ambient temperature, the 13C NMR spectra (methanol-d, ) consists of two sharp resonances assigned to the pendant arms and one broad resonance attributed to the ethylene ring carbons, which sharpens as the fast exchange limit is approached (ca. 50°C). Likewise, at -20°C the broad resonance resolves into two peaks. The increased flexibility observed for [La(THED)]3+ as compared to DOTA complexes suggests that the pendant groups contribute to the structural rigidity of the macrocyclic ring. 

As an example of the group contribution concept, consider the Benson methylene groups shown below, where C represents a tetrahedral carbon, and Cb represents an aromatic carbon, Cd represents a doubly bonded carbon. Each of these values was determined from experimental thermodynamic data. Note that the contribution of the C-(H)2(X)(Y) group to the enthalpy of formation is different in each bonded environment. 

A sample calculation for a generic polyester carbonate (Figure 16.7) [29] with corresponding group contributions (Table 16.2) illustrates how the deterministic and statistical models were used to screen for flammability. 

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