Quantitative rate determination by dynamic nuclear polarization enhanced NMR of a Diels-Alder reaction

Emerging techniques for hyperpolarization of nuclear spins, foremost dynamic nuclear polarization (DNP), lend unprecedented sensitivity to nuclear magnetic resonance spectroscopy. Sufficient signal can be obtained from a single scan, and reactions even far from equilibrium can be studied in real-time. When following the progress of a reaction by nuclear magnetic resonance, however, spin relaxation occurs concomitantly with the reaction to alter resonance line intensities. Here, we present a model for accounting for spin-relaxation in such reactions studied by hyperpolarized NMR. The model takes into account auto- and cross-relaxation in dipole-dipole coupled spin systems and is therefore applicable to NMR of hyperpolarized protons, the most abundant NMR-active nuclei. Applied to the Diels-Alder reaction of 1,4-dipheneylbutadiene (DPBD) with 4-phenyl-1,2,4-triazole-3,5-dione (PTD), reaction rates could be obtained accurately and reproducibly. Additional parameters available from the same experiment include relaxation rates of the reaction product, which may yield further information about the molecular properties of the product. The method presented is also compatible with an experiment where a single spin in the reactant is labeled in its spin-state by a selective radio frequency pulse for subsequent tracking through the reaction, allowing the unambiguous identification of its position in the product molecule. In this case, the chemical shift specificity of high-resolution NMR can allow for the simultaneous determination of reaction rates and mechanistic information in one experiment.