Use or Misuse of NMR to Test Molecular Mobility during Chemical Reaction

We evaluate critically the use of pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) to measure molecular mobility during chemical reactions. With raw NMR spectra available in a public depository, we confirm boosted mobility during the click chemical reaction (Science 2020, 369, 537) regardless of the order of magnetic field gradient (linearly-increasing, linearly-decreasing, random sequence). We also confirm boosted mobility for the Diels-Alder chemical reaction. The conceptual advantage of the former chemical system 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 discussion in this paper show the reliability of experiments when one avoids convection and allows decay of nuclear spin magnetization between successive pulses and recovery of its intensity between gradients. Especially important is to satisfy quasi-steady-state during the time window to acquire each datum and compare to the time scale of actual chemical reaction kinetics. We discuss possible sources of mistaken conclusions that are desirable to avoid.

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