The dynamics of the plastically crystalline phase of cyanoadamantane revisited by NMR line shape analysis and field-cycling relaxometry

Abstract

The dynamics of cyanoadamantane (CN-ADA) in its plastically crystalline phase encompasses three processes: overall tumbling of the rigid molecule, rotation around the molecular symmetry axis, and vacancy diffusion. This makes CN-ADA a prototypical case to be studied by field-cycling as well as by conventional NMR relaxometry. Data are collected from 430 K down to about 4 K and frequencies in the range of 10 kHz–56 MHz are covered. The overall tumbling is interpreted as a cooperative jump process preceding along the orthogonal axis of the cubic lattice and exhibiting a temperature independent non-Lorentzian spectral density. Consequently, a master curve is constructed, which yields model-independent correlation times, which agree well with those reported in the literature. It can be interpolated by a Cole-Davidson function with a width parameter βCD = 0.83. The uniaxial rotation persisting in the glassy crystal (T < Tg = 170 K) is governed by a broad distribution of activation energies, g(E). In this case, the standard master curve construction applied for the overall tumbling, for example, fails, as the actually probed distribution of correlation times G(ln τ) strongly changes with temperature. We suggest a scaling method that generally applies for the case that a relaxation process is determined by a distribution of thermally activated processes. Frequency as well as temperature dependence of the relaxation rate can be used to reconstruct g(E). In addition, g(E) is extracted from the proton line-shape, which was measured down to 4 K. Vacancy diffusion governs the relaxation dispersion at highest temperatures; yet, a quantitative analysis is not possible due to instrumental limitations.

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation

“…Furthermore, they point out that MBBA exhibits a plastic crystalline phase . NMR relaxometry studies of molecular dynamics of plastic crystal phases due to small globular molecules have been reported elsewhere. − Molecular dynamics in the soft crystal, smectic E phase of NCS-substituted biphenyl namely 4-butyl-4′-isothiocyano-1,1′-biphenyl (4TCB) were probed by the analysis of variable-temperature 1 H NMR spin–lattice relaxation time ( T 1 ) data . Earlier thermodynamic and spectroscopic studies have focused on the successive disordering transitions of the alkyl groups in discotic mesogen benzene-hexa- n -hexanoate and concluded the high-temperature solid phase to be a highly disordered crystalline phase concerning conformations of the paraffinic chains .…”