Molecular mechanics. The MM3 force field for hydrocarbons. 3. The van der Waals' potentials and crystal data for aliphatic and aromatic hydrocarbons

The van der Waals' potentials used for interactions between carbon and hydrogen in both aliphatic and aromatic systems have been improved from those available in MM2, and the new values are used in MM3. The atoms are slightly larger and somewhat softer than they were with MM2. These values were optimized by fitting to the crystal parameters (six cell constants) and the heats of sublimation for the normal alkanes from C6 to C,,, plus C,2, and also diamond, graphite, benzene, biphenyl, and hexamethylbenzene, in addition to fitting structural and energy data on congested molecules as reported earlier. The parameters developed give good crystal structures and heats of sublimation for these molecules. Biphenyl is calculated to be twisted about 40' in the gas phase, but lattice forces cause it to flatten into a planar conformation in the crystal.