کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
9575177 | 1504342 | 2005 | 10 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Lattice modes of hexamethylbenzene studied by inelastic neutron scattering
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
شیمی
شیمی تئوریک و عملی
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چکیده انگلیسی
The combination of inelastic neutron scattering and detailed ab initio calculations has been used to arrive at accurate assignments of the low energy lattice mode region of hexamethylbenzene (HMB) across the low temperature first order phase transition at 117.5Â K. This was also extended well into the mid-infrared spectral region and a good agreement was found between observed and calculated frequencies, which were also confirmed with isotopically substituted d-HMB. At low temperature, the lattice region is dominated by the methyl group torsions around 15 and 20Â meV, which soften dramatically on passing into the higher temperature phase. The lowest energy methyl torsion corresponds to a coherent gear wheel motion, observed here for the first time and predicted in previous numerical studies of HMB. The three acoustic phonons lie to lower energy, centered around 6-7Â meV, whilst the three optic phonons are very close in energy to the lowest methyl torsions. Other assignments are found to be in accord with literature values and so an unambiguous assignment of all spectral modes has been obtained for the first time. We conclude that due to the behaviour of the lattice modes either side of the phase transition, its nature is predominantly that of a thermally activated dynamic order-disorder transition.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Chemical Physics - Volume 317, Issues 2â3, 31 October 2005, Pages 143-152
Journal: Chemical Physics - Volume 317, Issues 2â3, 31 October 2005, Pages 143-152
نویسندگان
J.A. Stride, J.M. Adams, M.R. Johnson,