The dynamic disorder of azulene: A single crystal deuterium nuclear magnetic resonance study

Abstract

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.