Skeletal deformation vibrations and rotational isomerism of methylbutenes and methylpentenes as model compounds of 1,4‐polyisoprene

Infrared spectra of methylbutenes and methylpentenes in the skeletal deformation region have been measured. The force constants for the skeletal deformation vibrations are determined from the frequencies of isobutylene, isobutylene‐d6, and methylbutenes. The frequencies of methylpentenes are calculated for the various conformations, and the stable rotational isomers determined. The energy differences between the isomers are determined from the temperature dependence of the spectra. These results are correlated with the conformations of cis‐and trans‐1,4‐polyisoprene chains, and possible conformations are suggested.