Heats of Formation from Quantum Mechanics Alkanes

HEATS OF FORMATION FROM QUANTUM MECHANICS ALKANES 

Basically, the application of this Wiberg-Schleyer method proceeds in a manner quite similar to the method used in molecular mechanics, previously described for the MM4 calculation of the heats of formation . One needs to define a set of parameters that will be required in order to represent the molecules with appropriate generality and accuracy, and then one needs to calculate the energies of particular molecules using quantum mechanics, in place of molecular mechanics. The same eight parameters were used here as with MM4. If the quantum calculation could be done with sufficiently high accuracy, the parameters (other than the statistical ones and the numbers and kinds of bonds) would presumably tend to the value zero, and the heats of formation would be obtained with only minimal effort as outlined earlier. 

The difficulty here, of course, is that we cannot usually at present carry out the calculations with sufficient accuracy so that these parameters will go to zero. Thus, we have to determine exactly what parameters are required, and to evaluate their numerical values for a specific level of quantum mechanical calculation (basis set/correlation). For sure one will want to use bond energies and stfuctural features, presumably primary, secondary, and so forth, and the TOR parameter will be specifically required, as will Boltzmann distributions. We will here consider in detail the alkanes, and we can use 

In more recent work we have extended those calculations to the same 58 compounds used as the standard for MM4 heats of formation (Table 11.1), so that we can make comparisons of the results obtained by these and other methods. Since the original quantum mechanical work was published, computer capabilities have improved to where it has become possible to carry out MP2 and MP4 calculations on this same group of compounds. The optimized parameter set, together with the overall accuracy of the results, is shown for each of several methods in Table 11.6. There is 

TABLE 11.6. Alkane Heat of Formation F rameters (kcal/mol) for Various Calculations  

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