Soundararajan, Murari, Thierry Dubroca, Johan van Tol, Stephen Hill, Lucio Frydman, and Sungsool Wi. “Proton-Detected Solution-State NMR at 14.1 T Based on Scalar-Driven 13C Overhauser Dynamic Nuclear Polarization.” Journal of Magnetic Resonance 343 (October 2022): 107304.
https://doi.org/10.1016/j.jmr.2022.107304.
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 lL) 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 JCH-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 2-13Cphenylacetylene and 13C8-indole, ODNP enhancements in the 1.2-3x range were obtained. Thus, although handicapped by the lower c 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.