Librational motions in dimethylacetylene studied by inelastic neutron scattering

Rotational Isomerism Involving Acetylene Carbon

“…A sample of crystalline 2-butyne was investigated by 1 H NMR at low temperature, and the analysis of T 1 values gave a barrier of 3.5 kJ/mol . Neutron scattering measurement at 4.5 K gave a comparable barrier of 4 kJ/mol. , On the basis of the X-ray crystal structure of DPE, the rotational barrier (4.9 kJ/mol) was calculated with a model of semirigid molecules . As far as we know, there is no report of the internal rotation in 2,4-hexadiyne in the fluid state.…”

Lattice dynamics and methyl rotational excitations of 2-butyne

“…There is good agreement between the calculated ͑shaded͒ and measured ͑open dots͒ DOS although there are differences in the intensities of the peaks especially at ϳ6 meV and ϳ13 meV. In Tilli's work 12 it was the lack of knowledge of the low temperature crystal structure which allowed only a phenomenological access to the problem. The authors introduced a rotor-rotor coupling between neighboring methyl groups and therewith dispersion to explain two peaks of the DOS.…”

“…This part of the spectrum was measured already earlier at the direct time-of-flight spectrometer IN4 of the ILL, France. 12 Small differences in peak positions are likely due to the different ways the used instruments integrate over the dispersion curves.…”

“…10,11 It was difficult to relate this tunnel splitting to the librational modes measured in a forthcoming experiment. 12 In this paper the missing knowledge of the crystal structure did not allow a mode analysis and the description had finally to be based on an ad hoc model allowing for dispersion of the librational modes.…”