Reorientation and Angular Momentum Correlation Times in Fluorobenzene-d5 in the Liquid State

The NMR spin-lattice relaxation times of deuterium and fluorine have been measured in fluorobenzene-d5 over the temperature range from 30 to 309°C and pressures from 1 to 2000 bar. Quadrupolar interactions represent the sole relaxation mechanism for deuterium. The spin-rotation interactions provide the dominant relaxation mechanism for fluorine but small corrections for intra- and interdipolar, and anisotropy of chemical shift contributions were carried out. Since the 2D quadrupole coupling constant is known and the 19F spin-rotation interaction tensor was determined by molecular beam measurements, the experimental 2D and 19F T1 data allowed us to calculate the angular position correlation time, τθ, and the angular momentum correlation time, τJ, respectively. The relationship of τθ and τJ over the range of temperatures and pressures provides useful information about the mechanism of molecular reorientation in liquid fluorobenzene. The experimental data indicate that at lower temperatures (≤ 100°C) the rotational diffusion theory describes well the reorientation and that the Hubbard's relation between τθ and τJ holds. At temperatures near to critical temperature the experimental results can be interpreted in terms of the extended J-diffusion model.