Determination of molecular self-diffusion coefficients using pulsed-field-gradient NMR: An experiment for undergraduate physical chemistry laboratory

NMR spectroscopy has become one of the primary tools that chemists utilize to characterize a range of chemical species in the solution phase, from small organic molecules to medium-sized proteins. A discussion of NMR spectroscopy is an essential component of physical and biophysical chemistry lecture courses, and a number of instructional laboratory exercises have been described. The latter includes experiments to understand restricted rotations, measure relaxation times, and run two-dimensional NMR experiments. This note describes how NMR spectroscopy can be used to measure the translational diffusion coefficients using pulsed-field-gradients (PFG). Though the principle of the diffusion coefficient measurements is based on one of the earliest pulse-sequences proposed, the advent of standard availability of PFG in commercial NMR spectrometers has made the implementation of this experiment straightforward. In addition to learning the basic operation of an NMR spectrometer, the specific goals of the experiment may include understanding the effect of temperature, solvent viscosity, and concentration on molecular motions as well as the analysis of a mixture. Complete details on how to implement the experiment and perform data analysis are provided in the Supporting Information.