Tim Clark Story

Clark et al. also noted in their work that force-field calculations would be a quick and easy way to obtain quantities such as the heats of formation of these compounds, if the force fields proved to be reliable. They accordingly used two of the available molecular mechanics force fields of that day to calculate these quantities and compared the results with experiment. They concluded that the force fields contained fairly large systematic errors, and while they gave useful information in terms of trends, they needed to be improved if they were going to be competitive in accuracy with experiments. Indeed, force fields were fairly primitive prior to 1975. These particular compounds are relatively free from stretching, bending, and torsional strain. They do, however, show significant van der Waals interactions. These are mainly between the carbons in the interiors of the molecules, which turn out to be mostly small repulsions. The sizes of these repulsions were not yet known very accurately in 1975. 

The present author was particularly interested in this problem, and when the experimental heats of formation for compounds 1-9 were furnished to him by Dr. Clark before publication, he carried out calculations on the series. While it was found that the experimental heats of formation for the other compounds were well fit by the MM2 calculations, compound 6 was found to be clearly out of line by about 2kcal/mol. 

TABLE 11.3. Heats of Formation of Adamantane Derivatives (kcal/mol) 

Source From Reference 14a. Copyright 1975 by the American Chemical Society. Reprinted by permison of the American Chemical Society. 

TABLE 11.4. Increments for Adding Methyl Groups to Adamantane, Diamantane, and Norbornane (kcal/mol) 

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