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  Orientational disorder in 1,2,3-trichloro-4,5,6-trimethylbenzene. A single crystal deuterium NMR study of the site populations and dynamics

Bräuninger, T., Poupko, R., Luz, Z., Reichert, D., Zimmermann, H., Schmitt, H., et al. (2001). Orientational disorder in 1,2,3-trichloro-4,5,6-trimethylbenzene. A single crystal deuterium NMR study of the site populations and dynamics. Physical Chemistry Chemical Physics, 3(10), 1891-1903. doi:10.1039/B008999K.

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Alternative Title : Orientational disorder in 1,2,3-trichloro-4,5,6-trimethylbenzene. A single crystal deuterium NMR study of the site populations and dynamics

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PhysChemChemPhys_3_2001_1891.pdf (Any fulltext), 417KB
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Bräuninger, Thomas, Author
Poupko, Raphy, Author
Luz, Zeev, Author
Reichert, Detlef, Author
Zimmermann, Herbert1, 2, Author              
Schmitt, Heike3, Author              
Haeberlen, Ulrich4, Author              
Affiliations:
1Department of Molecular Physics, Max Planck Institute for Medical Research, Max Planck Society, ou_1497705              
2Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Max Planck Society, ou_1497700              
3Department of Molecular Neurobiology, Max Planck Institute for Medical Research, Max Planck Society, ou_1497704              
4Research Group Prof. Dr. Haeberlen, Max Planck Institute for Medical Research, Max Planck Society, ou_1497739              

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 Abstract: Deuterium NMR measurements on powder and single crystal samples of 1,2,3-trichloro-4,5,6-trimethylbenzene (TCTMB) specifically deuterated at the central methyl group (TCTMB-d3) are reported. The compound exhibits three solid phases: (I) A high temperature phase, just below the melting point (400 to 499 K), of unknown structure. (II) An intermediate monoclinic phase (268 to 400 K). (III) A low temperature triclinic phase (<268 K). Earlier X-ray investigations of the latter two phases indicate that they are, respectively, well ordered (Phase III at 173 K), and orientationally disordered, with the TCTMB molecules statistically aligned along the six possible orientations of the benzene core (Phase II at 298 K). In contrast to these observations the deuterium NMR results show a continuous growth, with temperature, of orientational disorder already in Phase III. The transition to Phase II involves a discontinuous increase in the disorder, but a considerable degree of order is retained in this phase even at high temperatures. This ordering reflects the non-equal population distribution of the molecular orientation in the crystal lattice sites. A quantitative analysis of the deuterium NMR spectra provided detailed information on this distribution and its temperature dependence in the solid phases of TCTMB. The deuterium NMR spectra also exhibit characteristic dynamic effects, As a function of increasing temperature, four dynamic regions can be distinguished: (i) Below 15 K, the spectrum exhibits features typical of coherent quantum-mechanical tunneling of the CD3groups, (ii) Above 15 K, up to about 170 K, there is fast classical (incoherent) reorientation of the methyl groups, but otherwise a rigid molecular lattice. (iii) Between 1 80 and 250 K, the spectra exhibit dynamic lineshape changes reflecting the onset of six-fold jumps of the TCTMB molecules, (iv) Above 250 K the spectra correspond to a dynamically averaged, but temperature dependent, quadrupolar splitting, reflecting the changes in the orientational distribution of the TCTMB molecules. This region includes the phase transitions, III to II and II to I. T1 relaxation measurements over the entire temperature range yielded kinetic parameters for the molecular six-fold jumps (k(260 K) = 7 × 106s-1, ΔE = 38.5 kJ mol-1), as well as for the incoherent methyl group reorientation (k(25 K) = 1 × 109 s-1, ΔE = 2.8 kJ mol-1)

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Language(s): eng - English
 Dates: 2000-11-082000-02-272001-04-112001
 Publication Status: Published in print
 Pages: 13
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 Table of Contents: -
 Rev. Type: Peer
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Title: Physical Chemistry Chemical Physics
  Abbreviation : Phys. Chem. Chem. Phys.
Source Genre: Journal
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Publ. Info: Cambridge, England : Royal Society of Chemistry
Pages: - Volume / Issue: 3 (10) Sequence Number: - Start / End Page: 1891 - 1903 Identifier: ISSN: 1463-9076
CoNE: https://pure.mpg.de/cone/journals/resource/954925272413_1