Parameters, heat of formation

These heat of formation parameters may be considered as shifting the zero point of Fpp to a common origin. Since corrections from larger moieties are small, it follows that energy differences between systems having the same groups (for example methyl-cyclohexane and ethyl-cyclopentane) can be calculated directly from differences in steric energy. 

If the heat of fonnation parameters are derived on the basis of fitting to a large variety of compounds, a specific set of parameters is obtained. A slightly different set of parameters may be obtained if only certain strainless molecules are included in the parameterization. Typically molecules like straight chain alkanes and cyclohexane are defined as strainless. Using these strainless heat of formation parameters, a strain energy may be calculated as illustrated in Figure 2.14. 

Deriving such heat of formation parameters requires a large body of experimental Ai/f values. For many classes of compound there are not sufficient data available. Only a few force fields, notably MM2 and MM3, attempt to parameterize also heats of formation. Most force fields are only concerned with reproducing geometries and possibly conformational relative energies, for which the steric energy is sufficient. 

If the heat of formation parameters are denved on the basis of fitting to a large vanety of cnmpniinHs a specific set of parameters is obtained. A slivhtlv different set of... 

To convert the steric energy to heat of formation, terms can be added depending on the number and types of bond present in the molecule. This again rests on the assumption of transferability, e.g. all C-H bonds have a dissociation energy close to lOOkcal/mol. A heat of formation parameter can be assigned to each bond type, and the numerical... 

TABLE 11.2. MM4 Alkane Heat of Formation Parameters (kcal/mol)... 

The alkane heat of formation parameters have values more or less as we expect, but there are some peculiarities. Of course, they were obtained by a least-squares fitting of data points to experimental information, and this is the parameter set that gives the best fit. However, one wonders why the R< parameter is needed (it was not needed in early force fields when the statistical tteatment was omitted see later). And one might wonder why the TOR value is as large as it is, for example. 

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