EMPIRICAL FORCE-FIELD STUDY OF GEOMETRIES AND CONFORMATIONAL TRANSITIONS OF SOME ORGANIC-MOLECULES

Minimum energy geometries and conformational energy differences of two classes of organic molecules-saturated hydrocarbons and molecules with methyl groups adjacent to double bonds-are investigated in detail by using a molecular mechanics force field of the type commonly employed for macromolecules. The potential energy function is parameterized to reproduce relevant properties of the simpler molecules and is transferred to more complicated compounds. Although the form of the potential function is simpler than those most frequently employed in small molecule calculations, the properties examined are determined with comparable accuracy; of particular interest is that single torsional terms can be used to achieve good agreement with experiment for relative barrier values in molecules like butane. The relationship between torsional, van der Waals, and electrostatic effects within molecules is investigated. The calculations on the molecules containing methyl groups adjacent to double bonds, including carbon-carbon and carbon-oxygen double bonds, test whether the force field can accurately represent widely varying rotational barriers in a class of similar molecules. Detailed calculations are reported for the butane rotational potential, cyclohexane ring inversion, and the geometry of the overcrowded molecule tri-tert-butylmethane.