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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 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 2-13C-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.
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Comment on “Chirality-Induced Electron Spin Polarization and Enantiospecific Response in Solid-State Cross-Polarization Nuclear Magnetic Resonance”
Recently, Santos et al. published an article titled “Chirality-Induced Electron Spin Polarization and Enantiospecific Response in Solid-State Cross-Polarization Nuclear Magnetic Resonance” in ACS Nano. In this article it was claimed that crystalline amino acid enantiomers can give rise to 1H-15N and 1H-13C cross-polarization magic angle spinning (CPMAS) solid-state NMR spectra with different relative signal intensities. The authors attributed such differences to transient changes in T1 relaxation times resulting from an interaction between the electron spins and the radiofrequency contact pulses used in the CPMAS experiment, and discussed this proposed phenomenon in terms of the chirality-induced spin selectivity (CISS) effect. We disagree with the authors conclusion that the CISS effect plays a role in the different signal intensities observed in the CPMAS solid-state NMR spectra of crystalline enantiomers. Quantitative 13C CPMAS experiments on aspartic acid enantiomers demonstrate that CPMAS signal variations can likely be attributed to sample dependent differences in T1 relaxation times rather than any chirality effects.
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- Award ID(s):
- 1709972
- PAR ID:
- 10094913
- Date Published:
- Journal Name:
- ACS nano
- ISSN:
- 1936-0851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsnano.9b00221
