Methyl group rotation in 2,4-hexadiyne

Abstract

Rotational tunnelling spectra for the methyl group in 2,4-hexadiyne are observed to collapse into broadened quasielastic scattering as the temperature is raised from 20 to 53 K. The temperature dependence of the frequency and of the width of the tunnelling lines is reported as well as the temperature dependence of the resultant quasielastic scattering up to and through the phase transition at 117·9 K. It is shown that the activation energy for the broadening of the tunnelling transitions at very low temperatures and the activation energy for the width of the quasielastic scattering are the same and that the pre-exponential factors are also the same within experimental limits. This suggests that classical hops over the barrier are the principal mechanism for broadening of the tunnelling transitions even at the lowest temperatures and that these hops are co-existent with quantum mechanical tunnelling in this temperature range.