Conformational Analysis of Planar Enol Rotamers of 2,4-Pentanedione: an Ab Initio Study

Abstract

All conformations among different planar enol conformers (rotamers) of 2,4-pentanedione were studied by means of the Hartree-Fock method using the STO-3G** basis set. The calculations were carried out with the Gaussian-98 program. For each conformation, stationary points with the highest energy on the energy curve were found graphically. Several conformations have low energy barriers and correspond to rotations around single bonds. They describe the spatial motion of only one (in most cases, hydrogen) atom or a small molecular fragment. All low energy barriers are in the interval 13-59 kJ·mol-1. As would be expected, the lowest energy barrier is exhibited by the conformation that leads to the formation of an enol rotamer having an intramolecular H-bond (so-called ω-shaped form). On the other hand, conformations in which rotation around a bond leads to a break of the intramolecular hydrogen bond have the highest energy barriers. Conformations in which rotation occurs around the double bond have high energy barriers. The influence of the solvents CHCl3 and CH3CN on the intramolecular H-bond has also been studied by means of IPCM at the HF/6-31G** level.