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We evaluate critically the use of pulsed gradient spin-echo nuclear magneticresonance (PGSE NMR) to measure molecular mobility during chemical reactions.With raw NMR spectra available in a public depository, we confirm boostedmobility during the click chemical reaction (Science 2020, 369, 537) regardlessof the order of magnetic field gradient (linearly-increasing,linearly-decreasing, random sequence). We also confirm boosted mobility for theDiels-Alder chemical reaction. The conceptual advantage of the former chemicalsystem is that constant reaction rate implies constant catalyst concentration,whereas that of the latter is the absence of a paramagnetic catalyst,precluding paramagnetism as objection to the measurements. Data and discussionin this paper show the reliability of experiments when one avoids convection,allows decay of nuclear spin magnetization between successive pulses andrecovery of its intensity between gradients, and satisfies quasi-steady stateduring the time window to acquire each datum. Especially important is to makecomparisons on the time scale of actual chemical reaction kinetics. We discusspossible sources of mistaken conclusions that are desirable to avoid.

Use of NMR to Test Molecular Mobility during Chemical Reaction