NMR Studies of the Rotational Diffusion of Symmetric Top Molecules in Liquids

Abstract

Temperature‐dependent nuclear magnetic resonance relaxation times of quadrupolar nuclei in several symmetric top molecules (as neat liquids) are reported. These include ⁵¹V and ³⁵Cl in VOCl₃, ¹¹B and ³⁵Cl in BCl₃, and ¹⁴N and ³⁵Cl in CCl₃CN; sufficient in each case to calculate the molecule's complete rotational diffusion tensor as a function of temperature. These results, along with previously reported results for other symmetric top molecules, are discussed. It is found that for small molecules in the liquid phase rotational motions which reorient an electric dipole invariably proceed by small‐step Brownian diffusion and that the rates of such reorientations can be calculated with good accuracy by hydrodynamic (microviscosity) theory. The success of the hydrodynamic calculation, the agreement with all available reorientational rates determined by dielectric relaxation, and the semiquantitative “$\chi$ test” are used to confirm that these motions are indeed in the diffusion limit. On the other hand, strong inertial effects are usually found in the parallel motions which do not reorient the dipole and in both motions of the nonpolar BCl₃. In these cases, the rates of reorientation are faster than predicted hydrodynamically, especially when the relevant moment of inertia is small.