Overhauser Dynamic Nuclear Polarization Enables Single Scan Benchtop 13C NMR Spectroscopy in Continuous-Flow

Abstract

Benchtop ¹³C NMR spectroscopy is highly attractive for reaction and process monitoring. However, insufficient premagnetization and low signal intensities largely prevent its application to flowing liquids. We show that hyperpolarization by Overhauser dynamic nuclear polarization (ODNP) can be used to overcome these problems, as ODNP operates on short time scales and results in strong ¹³C signal enhancements. Benchtop ¹³C NMR spectra with ODNP enhancement acquired in continuous-flow are reported here for the first time. We have investigated two ODNP approaches: direct ODNP, which transfers the polarization of unpaired electrons to ¹³C nuclei via direct hyperfine coupling, and indirect ODNP, in which the electron polarization is first transferred to ¹H nuclei before a polarization transfer pulse sequence finally transfers the polarization to the ¹³C nuclei. Experiments were carried out for three pure solvents and a mixture for different flow rates. The results show significant ¹³C signal enhancements for both approaches. However, their performance varies for different substances, depending on the strength and type of the hyperfine interaction as well as on the relaxation properties, but by selecting a suitable approach, good single-scan ¹³C NMR spectra can be obtained with benchtop NMR, even at high flow rates.