Boyd’s force field

Exhaustive catalytic hydrogenation of triptycene affords an equilibrium mixture of perhydrotriptycene isomers. As expected, Boyd s force field (37) calculations, with a modified torsional constant, reproduced the observed composition fairly well (Table 6). All important conformations were taken into account for each isomer. The most stable conformations agree with the results of the X-ray analysis (131) and have the characteristic that the cyclohexane rings are invariably either boat or deformed chair. The most stable conformation of all is 20 (ttt). The predominant conformation of ccc, in which all cyclohexane rings are boat, has an enthalpy only 2.56 kcal/mol above that of 20. The difference is virtually all due to angle and torsional terms. 

Three MM programs are reported to be able to handle metal chelates (Table 10). Those originating from Boyd s force field do not consider nonbonded interactions beyond 1.2 times the sum of the van der Waals radii (218-221). Rasmussen s version of CFF is much more sophisticated (33,222). However, none of them explicitly considers induced bond dipoles in chelate ions that contain a number of highly polar bonds. This is probably why the previous calculations of metal chelates only compare closely related isomeric structures having the same metal ion. 

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