Rotational isomerism and structure of penta-1,4-diene: Raman spectrum and ab initio calculations

Abstract

Several points on the potential energy surface of penta-1,4-diene as a function of the two torsional angles around the C—C single bonds have been computed using a SCF MO LCAO scheme with a basis of contracted gaussian functions. Three stable conformations are predicted for the molecule at very similar total energies and a fourth metastable form is found at a higher energy. The stable isomers belong to the point groups C_s, C₂ and C₁; the metastable conformation has C₁ symmetry. Following a very simple mechanism suggested by the analysis of the theoretical results, the isomerization paths are also investigated, and estimates of the appropriate barrier heights obtained.

The Raman spectra of the liquid and of the solid phase have been recorded and show the existence of three conformers. The results prove that the three isomers have C_s, C₂ and C₁ symmetry, and have nearly equal enthalpies.