The Geometries of the s-Cis Conformers of Some Acyclic 1,3-Dienes: Planar or Twisted?

Article abstract of DOI:10.1021/ja00310a016

We have established a method to determine the geometry of the minor forms of acyclic 1,3-dienes.A high vacuum cryogenic trapping technique was used to obtain spectral data of these metastable conformers from mixtures equilibrated at 1100 K.This has enabled us to characterize the minor form of 2,3-dimethyl-1,3-butadiene (DMB) by IR and UV spectroscopy.Barriers to rotation of the metastable forms of isoprene and DMB to their stable s-trans forms have also been obtained.The UV spectra of acyclic 1,3-dienes are found to be determinant of structure and, on this basis, weassign a planar s-cis molecular framework to 1,3-butadiene and isoprene, and a gauche s-cis geometry to DMB.