Reference

Dynamics in molecular crystals: an application of proton NMR to selectively protonated biphenyl

Dynamics in molecular crystals: an application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently?. Meinel, C.; Zimmermann, H.; Schmitt, H.; Haeberlen, U. (AG Molekuelkristalle, Max-Planck-Institut fuer Medizinische Forschung, Heidelberg, Germany). Applied Magnetic Resonance, 24(1), 25-40 (English) 2003 Springer-Verlag Wien. CODEN: APMREI. ISSN: 0937-9347. DOCUMENT TYPE: Journal CA Section: 22 (Physical Organic Chemistry) Section cross-reference(s): 68 The synthesis of a specific isotopomer, C6D4H(ortho)-H(ortho)D4C6 of biphenyl is reported. The intramol.In this article, certain chemicals are used. Some of their cas registry numbers are 81103-79-9 and 7782-92-5 dipolar coupling of the protons leads to a well-resolved single-crystal 1H NMR spectrum and allows one to study the dynamics of the Ph rings in a unique way. At room temp. 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 and T = 250 K, the prominent dynamical mode consists of oscillatory twists f(t) of the two rings. The data allow us to infer the mean square, Wf2V, of these twists. Wf2V Is found to grow linearly with T for 50 < T < 200 K. From the slope of Wf2V vs. T the frequency (the wave no. ~v) is derived. The result is ~v = 20 cm-1. For T < 38 K, the spectra give direct evidence of the IC phase transition and its nature (stripe-like rather than quilt-like). The temp. dependence of the magnitude of the order parameter of the IC phase is obtained. .