Proton nuclear magnetic resonance (NMR) in solid state has gained significant attention in recent years due to the remarkable resolution and sensitivity enhancement afforded by ultrafast magic-angle spinning (MAS). In spite of the substantial suppression of 1H-1H dipolar couplings, the proton spectral resolution is still poor compared to that of 13C or 15N NMR, rendering it challenging for the structural and conformational analysis of complex chemicals or biological solids. 

Herein, by utilizing the benefits of double-quantum (DQ) and triple-quantum (TQ) coherences, we propose a 3D single-channel pulse sequence that correlates proton triple-quantum/double-quantum/single-quantum (TQ/DQ/SQ) chemical shifts as shown in Figure 1.

Figure 1. 3D 1H TQ/DQ/SQ chemical shift correlation NMR experiment.

In addition to the two-spin proximity information, this 3D TQ/DQ/SQ pulse sequence enables more reliable extraction of three-spin proximity information compared to the regular 2D TQ/SQ correlation experiment, which could aid in revealing the proton network in solids. Furthermore, the TQ/DQ slice taken at a specific SQ chemical shift only reveals the local correlations to the corresponding SQ chemical shift, and thus it enables accurate assignments of the proton peaks along the TQ and DQ dimensions and simplifies the interpretation of proton spectra especially for dense proton networks. 

Figure 2. 2D NMR spectra of alanine. (a) Regular 2D DQ/SQ spectrum; (b) Regular 2D TQ/SQ spectrum; (c) 2D DQ/SQ spectrum projected from the 3D TQ/DQ/SQ spectrum; (d) 2D TQ/SQ spectrum projected from the 3D TQ/DQ/SQ spectrum;

Figure 3 (a) Schematic illustration of identifying the three spins involved in TQ coherences. (b) TQ/DQ slices taken at the isotropic SQ chemical shift of -CH group of alanine. For simplifying the representation, we used label A, B, and C to denote the protons of -NH3+, -CH and -CH3 groups, respectively. The solid line indicates the diagonal line with a slope of 1, while the dot line is in parallel with the solid line at a distance of the isotropic chemical shift of spin B.

Rongchun Zhang*, Nghia Tuan Duong, and Yusuke Nishiyama*. Resolution Enhancement and Proton Proximity Probed by 3D TQ/DQ/SQ Proton NMR Spectroscopy Under Ultrafast Magic-Angle-Spinning beyond 70 kHz. J. Magn. Reson. 2019, https://doi.org/10.1016/j.jmr.2019.05.006

Link: https://www.sciencedirect.com/science/article/pii/S1090780719300965?via%3Dihub