APPLICATION OF FORCE-FIELD CALCULATIONS TO ORGANIC-CHEMISTRY .8. INTERNAL-ROTATION IN SIMPLE TO CONGESTED HYDROCARBONS INCLUDING 2,3-DIMETHYLBUTANE, 1,1,2,2-TETRA-TERT-BUTYLETHANE, 2,2,4,4,5,5,7,7-OCTAMETHYLOCTANE, AND CHOLESTANE

Allinger's new force field MM2 was tested for the calculation of barrier heights of internal rotation about the C-C bond in simple to congested acyclic hydrocarbons. This force field performs satisfactorily for simple hydrocarbons but systematically underestimates barrier heights for highly congested molecules. MM2 calculations were performed to investigate beyond experimental limits novel features of internal rotation in several molecules of current interest. Conformers having nonalternating Newman projections proposed recently by Mislow are confirmed to appear in the torsional itinerary of 2, 3-dimethylbutane. 1, 1, 2, 2-Tetra-tert-butylethane (1) is predicted to possess a distorted gauche ground-state conformation of (FB)3, or essentially FBE2, rotamer type. Between this global energy minimum and gauche-anti barrier is a wide torsional range in which the novel F2BFB2 rotamer type dominates. Internal compression effects were postulated in order to explain the unusually high rotational barrier of the central C-C bond of 2, 2, 4, 4, 5, 5, 7, 7-octamethyloctane (2) and these were analyzed in some detail and given strong support. In addition, skeletal twisting and unique valence angle variations accompanying the internal rotation of 2 are also attributed to the compression effects. Steric energies of rotamers of cholestane (3) regarding the rotation about the C17-C20 bond have been calculated to clarify the controversy over their relative stabilities. They are separated by low barriers (at most 12 kcal/mol). Rotational barriers about the bond between C17 and various alkyl substituents are calculated for models of 3, and the reason for the reported failure of freezing C17-tert-butyl bond rotation is rationalized. © 1979, American Chemical Society. All rights reserved.