AB-INITIO AND DENSITY-FUNCTIONAL STUDIES ON INTERNAL-ROTATION AND CORRESPONDING TRANSITION-STATES IN CONJUGATED MOLECULES

A comparative study between a high-level ab initio molecular orbital m ethod and density functional theory (DFT) employing local density appr oximation with nonlocal gradient corrections to the exchange-correlati on potential (NLGC) included has been performed on the rotational barr iers around the single bond for 11 molecules. Ah of these molecules ar e constitutive parts of large biomolecules and have single bonds with varying degrees of partial double bond character. All conformers were optimized by the both methods, using comparable basis sets (double zet a plus polarization in valence orbital) including correlation effect, followed by single-point energy calculations using even larger basis s ets. An excellent agreement between the two methods was obtained in lo cating the transition states. Although reasonable agreement between th e two methods was obtained for the barriers in non-amide molecules, fa irly large discrepancies were found for amide bonds.