NMR studies of rotational motion at low viscosity

We have measured the rotational correlation time of dilute solutions of several small molecules dissolved in supercritical fluids using ¹⁴N NMR spectroscopy. In supercritical fluids at the densities employed, the solubility is sufficiently high to obtain rapid acquisition of the spectra, while the viscosity is extremely small. Comparison of the rotational correlation times with the literature values obtained from light-scattering measurements at higher viscosity demonstrates that the Stokes-Einstein-Debye plot deviates from linearity as the inertial limit is approached. The measured correlation times compare favorably with the free-rotor correlation time.