Lattice dynamics and methyl rotational excitations of 2-butyne

Abstract

On the basis of the recently determined low temperature crystal structure the lattice dynamics of 2-butyne and a single particle methyl rotational potential are calculated using pair potential parameters given by Williams in 1974 within the model of semirigid molecules. In the regime of lattice modes the existence of four methyl librational bands with significant dispersion can explain the measured density of states. The single particle librational energy obtained for the rotational potential from the Schrödinger equation of the methyl rotor coincides well with the mean librational band energy. The calculated single particle tunneling frequency is only 17% lower than observed in experiment. Similarly well the activation energy is reproduced. Although the pair potential parameters developed for aromatic compounds need some scaling they are useful for materials with triple bond carbons.

Rotational dynamics of methyl groups in m-xylene

“…The single particle model ͑SPM 6 ͒, which describes the dynamics of many molecular crystals containing, e.g., CH 3 groups, assumes an isolated group whose reorientations are hindered via its interaction with its environment. Often these a͒ Author to whom correspondence should be addressed.…”

“…The most important point of this approach lies in its transferability which, of course, simplifies the theoretical description molecular crystals see, e.g., Ref. 3 for the dynamics of CH 3 groups in 2-butyne or Ref. 4 for the dynamics in acetamide.…”

Quasielastic neutron scattering experiments including activation energies and mathematical modeling of methyl halide dynamics

“…Since dispersion is not contained in the SPM the calculated transition can only define a mean librational transition. Such effect is clearly observed in dimehtylacetylene ͑DMA͒ 20 where strong dispersion of librational modes resulted in two distinct and well separated peaks in the DOS. Secondly, the unusual temperature evolution of the tunneling transition in methyl iodide could be explained by such coupling of methyl rotation to phonons.…”

The lattice and rotational dynamics of the methyl halides described by pair potentials based on universal force fields

“…9,10,20 Phonon energies and eigenvectors are obtained by solving the set of coupled differential equations (␣,␤ϭx,y,z)…”

“…6 -8, and allow the description of pure hydrocarbon systems like p-xylene ͑Ref. 9͒ or 2-butyne, 10 it is more difficult to get reliable pair potential parameters from the literature for less common atoms. One successful description of a compound containing H, C, and N, 2,6-dimethylpyrazine, is given in Ref.…”