Testing signal enhancement mechanisms in the dissolution NMR of acetone

In cryogenic dissolution NMR experiments, a substance of interest is allowed to rest in a strong magnetic field at cryogenic temperature, before dissolving the substance in a warm solvent, transferring it to a high-resolution NMR spectrometer, and observing the solution-state NMR spectrum. In some cases, negative enhancements of the ¹³C NMR signals are observed, which have been attributed to quantum-rotor-induced polarization. We show that in the case of acetone (propan-2-one) the negative signal enhancements of the methyl ¹³C sites may be understood by invoking conventional cross-relaxation within the methyl groups. The ¹H nuclei acquire a relative large net polarization through thermal equilibration in a magnetic field at low temperature, facilitated by the methyl rotation which acts as a relaxation sink; after dissolution, the ¹H magnetization slowly returns to thermal equilibrium at high temperature, in part by cross-relaxation processes, which induce a transient negative polarization of nearby ¹³C nuclei. We provide evidence for this mechanism experimentally and theoretically by saturating the ¹H magnetization using a radiofrequency field pulse sequence before dissolution and comparing the ¹³C magnetization evolution after dissolution with the results obtained from a conventional ¹H-¹³C cross relaxation model of the CH₃ moieties in acetone.