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 method and density functional theory (DFT) employing local density approximation with nonlocal gradient corrections to the exchange-correlation potential (NLGC) included has been performed on the rotational barriers around the single bond for 11 molecules. Ah of these molecules are 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 zeta plus polarization in valence orbital) including correlation effect, followed by single-point energy calculations using even larger basis sets. An excellent agreement between the two methods was obtained in locating the transition states. Although reasonable agreement between the two methods was obtained for the barriers in non-amide molecules, fairly large discrepancies were found for amide bonds.