(1)H NMR spectra of the methyl group in an oriented crystal sample of methylmalonic acid with all three non-methyl protons replaced by deuterons are interpreted in terms of the damped quantum rotation (DQR) theory of NMR line shapes. The DQR approach offers a perfect theoretical reproduction of the observed spectra while the conventional Alexander-Binsch line-shape model shows evident defects in the present case. The temperature trends of the quantities characterizing the coherent and incoherent dynamics of the methyl group in the DQR approach (the effective tunnelling frequency and two coherence-damping rates) derived from the spectra are fairly reproduced using a model reported previously. The present findings provide further evidence of limitations to the validity of the common belief that molecular rate processes in condensed phases are necessarily classical.

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Deuterium nuclear magnetic resonance measurements on single crystals of azulene, specifically deuterated in the 1 and 3 positions, are reported. The quadrupole coupling tensor of these deuterons was determined by rotation experiments, yielding Qzz=182.2 kHz and η=0.056, with Qxx, the intermediate component (magnitude wise), oriented perpendicular to the molecular plane. The deuterium signals are inhomogeneously broadened and their widths are strongly anisotropic. This is quantitatively interpreted in terms of alignment disorder, induced by polar (up–down) disorder already known to be present in azulene from earlier x-ray measurements. It is shown that the alignment disorder is due to a planar distribution in the orientation of the molecules about the short axis, with a root mean square deviation of ±1°. The linewidths are strongly temperature dependent and reduce from about 8.9 kHz (maximum width) at room temperature to 1.6 kHz at 67 °C. This effect is interpreted in terms of molecular “up–down” flips, which average out both the polar and the alignment disorder. The rate of this process is found to be 104 s−1 at 40 °C, with an activation energy of 65 kJ/mol. Magnetization transfer experiments were performed by selectively inverting the magnetization of one of the deuterium doublets, followed by monitoring the subsequent approach to equilibrium of the whole spectrum. The results show the presence of additional dynamic processes in the ultraslow motion regime. These include molecular π flips about their long axes, as well as jumps between different sites in the lattice. The rate of both processes is about 0.084 s−1 at 57 °C. The intersite jumps are predominantly of the flip type, which interchange crystallographic symmetry related deuterons. From the rate of this process, a self-diffusion constant of 0.35×10−22 m2 s−1, at 57 °C, is estimated.

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