Transferred Overhauser DNP: A Fast, Efficient Approach for Room Temperature ¹³C ODNP at Moderately Low Fields and Natural Abundance

Abstract

Overhauser dynamic nuclear polarization (ODNP) is investigated at a moderately low field (1.2 T) for natural abundance C NMR of small molecules in solution state at room temperature. It is shown that ODNP transferred fromH to C by NMR coherence transfer is in general significantly more efficient than direct ODNP ofC. Compared to direct C ODNP, we demonstrate over 4-fold higherC sensitivity (signal-to-noise ratio, SNR), achieved in one-eighth of the measurement time by transferred ODNP (t-ODNP). Compared to the C signal arising from Boltzmann equilibrium in a fixed measurement time, this is equivalent to about 1500-fold enhancement ofC signal by t-ODNP, as against a direct C ODNP signal enhancement of about 45-fold, both at a moderate ESR saturation factor of about 0.25. This owes in part to the short polarization times characteristic ofH. Typically, t-ODNP reflects the essentially uniform ODNP enhancements of all protons in a molecule. Although the purpose of this work is to establish the superiority of t-ODNP vis-à-vis direct C ODNP, a comparison is also made of the SNR in t-ODNP experiments with standard high resolution NMR as well. Finally, the potential of t-ODNP experiments for 2D heteronuclear correlation spectroscopy of small molecules is demonstrated in 2DH-C HETCOR experiments at natural abundance, with decoupling in both dimensions.

Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1H DNP for Insight into Oxygen Reduction Reactions

“…In previous studies, it has been established that a number of NMR nuclides (e.g., 1 H and 13 C) exhibit polarization transfer with unpaired electron systems, , and a variety of enhanced NMR signals have been observed in materials such as activated carbons, chars, and tars. ,,− There are various mechanisms of continuous wave DNP, and among them, the Overhauser effect (OE) and solid effects (SE) are widely described in studies of liquids as well as solids. , In liquids, the OE is the only mechanism for the direct polarization of liquids. , The molecular rotational and translational motion and chemical exchange lead to time-dependent dipolar and scalar interactions between the electrons and the nuclei and then relaxation processes. In solids, the OE effects are dominated by mobile electrons, such as those within the conduction band of materials.…”

“…23,33,35−37 There are various mechanisms of continuous wave DNP, and among them, the Overhauser effect (OE) and solid effects (SE) are widely described in studies of liquids as well as solids. 38,39 In liquids, the OE is the only mechanism for the direct polarization of liquids. 28,38 The molecular rotational and translational motion and chemical exchange lead to time-dependent dipolar and scalar interactions between the electrons and the nuclei and then relaxation processes.…”

Overhauser dynamic nuclear polarization (ODNP)-enhanced two-dimensional proton NMR spectroscopy at low magnetic fields

“…However, the method almost completely vanished with the push of magnetic resonance methods to ever higher magnetic fields but was resurrected in the early 2000s because of its potential to determine local hydration dynamics on surfaces (Armstrong and Han, 2007). Today, it is an active field of research and the theory, instrumentation and application of ODNP spectroscopy is constantly developing (Armstrong and Han, 2009;Doll et al, 2012;Franck, 2020;Franck et al, 2013;Franck and Han, 2019;Han et al, 2008;Keller et al, 2020). While the observed enhancements can be large at low fields, a major drawback of low-field NMR spectroscopy, with or without the addition of ODNP, is the decreased resolution.…”

1H Hyperpolarization of Solutions by Overhauser Dynamic Nuclear Polarization with 13C–1H Polarization Transfer

“… 45 − 51 Notably, Dey et al have previously performed 1 H→ 13 C transfers to enhance the 13 C sensitivity using Overhauser DNP at low fields where 1 H hyperpolarization is efficient. 52 INEPT is a well-established method for enhancing the NMR sensitivity of low-gyromagnetic ratio nuclei and/or obtaining through-bond correlation spectra via J couplings. 1 For a fully 13 C enriched sample at thermal equilibrium, the 1 H magnetization produced by a 13 C→ 1 H INEPT experiment will be 25% of that of a directly excited 1 H NMR spectrum (in an ideal NMR spectrometer).…”

“…Here, we suggest that a simple way to obtain 1 H enhancements is to transfer the hyperpolarization from 13 C to 1 H using, for example, a reverse INEPT sequence (Figure B) ,, or cross-polarization in a 13 C-enriched substrate. Combining these polarization transfer tools with other approaches to hyperpolarization, including dissolution DNP and PHIP, has been demonstrated. − Notably, Dey et al have previously performed 1 H→ 13 C transfers to enhance the 13 C sensitivity using Overhauser DNP at low fields where 1 H hyperpolarization is efficient . INEPT is a well-established method for enhancing the NMR sensitivity of low-gyromagnetic ratio nuclei and/or obtaining through-bond correlation spectra via J couplings .…”