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

Deuterium nuclear magnetic resonance measurements on single crystals of azu lene, specifically deuterated in the 1 and 3 positions, are reported. The q uadrupole coupling tensor of these deuterons was determined by rotation exp eriments, yielding Q(zz)=182.2 kHz and eta=0.056, with Q(xx), the intermedi ate component (magnitude wise), oriented perpendicular to the molecular pla ne. The deuterium signals are inhomogeneously broadened and their widths ar e strongly anisotropic. This is quantitatively interpreted in terms of alig nment disorder, induced by polar (up-down) disorder already known to be pre sent in azulene from earlier x-ray measurements. It is shown that the align ment disorder is due to a planar distribution in the orientation of the mol ecules about the short axis, with a root mean square deviation of +/- 1 deg rees. The linewidths are strongly temperature dependent and reduce from abo ut 8.9 kHz (maximum width) at room temperature to 1.6 kHz at 67 degrees C. This effect is interpreted in terms of molecular "up-down" flips, which ave rage out both the polar and the alignment disorder. The rate of this proces s is found to be 10(4) s(-1) at 40 degrees 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. Th e results show the presence of additional dynamic processes in the ultraslo w motion regime. These include molecular pi flips about their long axes, as well as jumps between different sites in the lattice. The rate of both pro cesses is about 0.084 s(-1) at 57 degrees C. The intersite jumps are predom inantly of the flip type, which interchange crystallographic symmetry relat ed deuterons. From the rate of this process, a self-diffusion constant of 0 .35x10(-22) m(2) s(-1), at 57 degrees C, is estimated. (C) 2000 American In stitute of Physics. [S0021-9606(00)00424-4].