An analysis of the conformers of 1,5-hexadiene

Abstract

High level theoretical methods have been used to predict the energetic ordering of the conformations of 1,5-hexadiene. Presented are optimized geometries and relative energies for ten energetically distinct conformations of the molecule and two enantiomeric conformations. Previous work had predicted that the gauche (or skew) conformation about the central carbon—carbon bond is the lowest in energy. However, we have found that the conformational energies of 1,5-hexadiene, unlike n-butane, are remarkably similar. We postulate this to be the reason for poor performance (for example, in comparisons with variable temperature NMR studies) of some molecular mechanics simulations on molecules of this type in the past. With the DZP CCSD(T) method, the gauche(120)/anti/gauche(120) conformer is predicted to be the global minimum, followed 0.06kcalmol⁻¹ higher in energy by the gauche(120)/anti/gauche(−120) structure, followed another 0.03 kcal mol⁻¹ higher in energy by the gauche(120)/gauche(60)/gauche(−120) arrangement. The three earlier reported molecular mechanics conformational energy surfaces all rise much more steeply from the gauche(120)/anti/gauche(120) and gauche(120)/gauche(60)/gauche(−120) structures.