NBO approach to evaluate origin of rotational barrier of diformylhydrazine

Abstract

To evaluate the origin of rotational barrier and the structures of N, N'-diformylhydrazine as a model compound of azapeptide, the natural bond orbital (NBO) analysis was performed at the HF/6-31G* level. The rotational barrier energies as a function of structural isomers were dissected into structural, total exchange repulsion and delocalization energy changes. The preferred dihedral angles of CO-N-N-CO were known to be orthogonal minimizing the repulsion between nitrogen lone pairs, which means that nitrogen lone pair repulsion is dominant in determining the preferred angles of CO-N-N-CO. However, the dihedral angles are deviated from 90degrees within the range of +/- 20degrees, and the NBO analysis reveals that these preferred dihedral angles of CO-N-N-CO as a function of rotamers are involved with many other individual terms of exchange and delocalization energy changes as well as nitrogen lone pair energy term of structural energy change. The pyramidality of nitrogen in transition states of N, N-diformylhydrazine was simply explained by the repulsion of nitrogen lone pairs. However, NBO analysis shows that this pyramidalization of nitrogen atoms leads to decrease of structural energy change and increase of the total exchange repulsion and delocalization energies. The intramolecular hydrogen bonding energy could be important term in forming planar as found in crystal structure, but is not dominant energy term to lower the barrier in comparison to the amide resonance and total exchange repulsion. This NBO analysis to elucidate the origins of the perturbation of dihedral angles and pyramidality of nitrogen atoms as well as the role of intramolecular hydrogen bonding in hydrazine derivative will shed some lights on the understanding of the dynamics and structures of azapeptide analogues. (C) 2003 Elsevier B.V. All rights reserved.