Dynamic disorder in 2,3-dimethylnaphthalene: a single crystal deuteron nuclear magnetic resonance study

Abstract

Deuteron NMR experiments on single crystals of 2,3-dimethylnaphthalene (DMN) deuterated in the aromatic sites are reported. The deuteron resonances are inhomogeneously broadened and their widths are strongly anisotropic. This is shown to be due to orientational disorder of the DMN molecules. Two types of disorder are distinguished: polar disorder, also seen by X-ray and neutron diffraction, in which the direction of the long molecular axis alternates between ‘up’ and ‘down’, and alignment disorder, in which the direction of this axis varies statistically about an average direction with a mean deviation of about 1.8°. It is argued that the two types of disorder are linked. Their coexistence suggests that the polar disorder in DMN is statistical rather than correlated. One- and two- dimensional magnetization transfer experiments show effects due to spin diffusion, molecular reorientations (π-flips) and molecular self-diffusion. The self-diffusion process involves two types of molecular jumps called flips and slips. These jumps cause both types of disorder to become dynamic. The mean time between such jumps is about 3 s at T = 338 K.