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.

Dynamics of guest molecules in PHTP inclusion compounds as probed by solid-state NMR and fluorescence spectroscopy

“…The derived correlation times and the activation energy for the ring flip motion fall within the typical range which is discussed for solids with phenyl rings that undergo rotational motion around a C-C single bond (polymers, 22,64,65 proteins, molecular crystals, etc.). [66][67][68][69][70][71][72][73] For instance, 13 C NMR studies were performed on poly(p-phenylene vinylene) (PPV) films, which have shown that the phenyl rings in both the amorphous and crystalline regions undergo 1801-ring flips with activation energies ranging from 20 to 60 kJ mol À1 . 59,73 It is interesting to note that in inclusion compounds also other intramolecular processes, like methyl group rotation, ring inversion or conformational changes, have been examined which again were found to remain almost unaffected by the surrounding host lattice.…”

About the Interactions Controlling Nafion’s Viscoelastic Properties and Morphology

“…Measurements of the orientation dependence of 2 H NMR spectra have been successfully applied for the study of crystalline substances, − but are equally suitable for the measurement of residual anisotropies of the pseudonematic effect in polymers and ionomers. − ,− In such experiments, the sample is mounted in a goniometer, allowing for measurements of 2 H NMR spectra at different relative orientations to the magnetic field B 0 . These orientations are varied through rotating the sample by defined angles ϕ.…”

Seeking Small Molecules for Singlet Fission: A Heteroatom Substitution Strategy

“…We further suggest a herringbone structure for 18, based on its parent azulene. 99,100 To the best of our knowledge, none of the molecules we propose have been synthesized; however, molecules with related structures have been reported. Introducing two BN units into a benzene ring has been achieved (see ref 53 and the references therein).…”