Proton-detected solution-state NMR at 14.1 T based on scalar-driven ¹³C Overhauser dynamic nuclear polarization

Overhauser dynamic nuclear polarization (ODNP) NMR of solutions at high fields is usually mediated by scalar couplings that polarize the nuclei of heavier, electron-rich atoms. This leaves ¹H-detected NMR outside the realm of such studies. This study presents experiments that deliver ¹H-detected NMR experiments on relatively large liquid volumes (60 ∼ 100 μL) and at high fields (14.1 T), while relying on ODNP enhancements. To this end ¹³C NMR polarizations were first enhanced by relying on a mechanism that utilizes e^--¹³C scalar coupling interactions; the nuclear spin alignment thus achieved was then passed on to neighboring ¹H for observation, by a reverse INEPT scheme relying on one-bond J_CH-couplings. Such ¹³C →¹H polarization transfer ported the ¹³C ODNP gains into the ¹H, permitting detection at higher frequencies and with higher potential sensitivities. For a model solution of labeled ¹³CHCl₃ comixed with a nitroxide-based TEMPO derivative as polarizing agent, an ODNP enhancement factor of ca. 5x could thus be imparted to the ¹H signal. When applied to bigger organic molecules like 2-¹³C-phenylacetylene and ¹³C₈-indole, ODNP enhancements in the 1.2-3x range were obtained. Thus, although handicapped by the lower γ of the ¹³C, enhancements could be imparted on the ¹H thermal acquisitions in all cases. We also find that conventional ¹H-¹³C nuclear Overhauser enhancements (NOEs) are largely absent in these solutions due to the presence of co-dissolved radicals, adding negligible gains and playing negligible roles on the scalar e^-→¹³C ODNP transfer. Potential rationalizations of these effects as well as extensions of these experiments, are briefly discussed.