Strainless parameters

At the top of File Segment 5-1 is a heat of fomiation information block. Two sums are listed One is a sum of nomial bond enthalpies for ethylene, and the other is a sum selected from a parameter set of stiainless bonds. Both sets of bond enthalpies have been empirically chosen. A group of molecules selected as nomial generates one parameter set, and a group supposed to be strainless is selected to generate a second set of str ainless bond enthalpies designated SBE in Eile Segment 5-1. The subject of parameterization has been treated in detail in Chapter 4. See Computer Projects 3-6 and 3-7 for the specific problem of bond enthalpies. 

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. 

It follows from the determination of the main forces ft = dF(x)/dx. in accordance with the Eq. (29) that at the equilibrium values x =Xj these forces are equal to zero. That is why let s determine the main forces as those, which should be applied to the strainless conformational volume, for which the conformational radius is equilibrium, with respect to in order to transform it into the deformed state of the conformational ellipsoid with the semi-axes, equilibrium with respect to cr,.. This determination means, that in Eq. (29) parameter a. should be replaced on (t , and values X. should be replaced on X,.. Then we will obtain ... 

The strainless standard for alkanes (MM4 and earlier) is defined by five parameters. These are the C-C and C-H bonds and three branching parameters. Me, Iso, and Neo, where the methylene group is a default quantity (with an energy value of zero). Methane is also given a strain energy of zero. 

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