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Dynamics in molecular crystals: An application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently?

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Meinel,  Christoph
Research Group Prof. Dr. Haeberlen, Max Planck Institute for Medical Research, Max Planck Society;

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Zimmermann,  Herbert
Department of Molecular Physics, Max Planck Institute for Medical Research, Max Planck Society;
Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Max Planck Society;

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Schmitt,  Heike
Department of Molecular Neurobiology, Max Planck Institute for Medical Research, Max Planck Society;

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Haeberlen,  Ulrich
Research Group Prof. Dr. Haeberlen, Max Planck Institute for Medical Research, Max Planck Society;

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Citation

Meinel, C., Zimmermann, H., Schmitt, H., & Haeberlen, U. (2003). Dynamics in molecular crystals: An application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently? Applied Magnetic Resonance, 24(1), 25-40. doi:10.1007/BF03166676.


Cite as: https://hdl.handle.net/11858/00-001M-0000-002A-0B6A-8
Abstract
The synthesis of a specific isotopomer, C6D4H(ortho)-H(ortho)D4C6 of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other. Between the incommensurate (IC) phase transition of biphenyl at 38 K andT=250 K, the prominent namical mode consists of oscillatory twists ϕ(t) of the two rings. The data allow us to infer the mean square, (φ2), of these twists. (φ2) is found to grow linearly withT for 50<T<200 K. From the slope of (φ2) vs.T the frequency (the wave numberv~) is derived. The result isv~=20 cm−1. ForT<38 K, the spectra give direct evidence of the IC phase transition and its nature (stripelike rather than quiltlike). The temperature dependence of the magnitude of the order parameter of the IC phase is obtained.