Frequency-selective Heteronuclear Correlation (FS-HETCOR) Experiments in Solid-State NMR

Heteronuclear correlation (HETCOR) is critical to obtain structural information in solid-state nuclear magnetic resonance (NMR). We propose novel frequency-selective Heteronuclear correlation (FS-HETCOR) experiments to selectively enhance the inter-atomic correlations of interest. FS-HETCOR relies on heteronuclear selective phase-optimized recoupling (SPRx), which is frequency-selective in heteronuclear recouping without using selective pulses. Compared to regular HETCOR, FS-HETCOR selectively enhances the desired heteronuclear correlations by a factor of up to 5 and suppresses the unwanted ones to 10% as demonstrated in 1H-19F and 1H-13C experiments under fast magic-angle spinning (MAS). Moreover, FS-HETCOR can theoretically be applied at arbitrary MAS rates by utilizing various SPRx schemes. We believe that the method will enhance the ability of solid-state NMR to probe heteronuclear structural information.

Using nutation-frequency-selective pulses to reduce radio-frequency field inhomogeneity in solid-state NMR

Radio-frequency (rf) field inhomogeneity is a common problem in NMR which leads to non-ideal rotations of spins in parts of the sample. Often, a physical volume restriction of the sample is used to reduce the effects of rf-field inhomogeneity, especially in solid-state NMR where spacers are inserted to reduce the sample volume to the centre of the coil. We show that band-selective pulses in the spin-lock frame can be used to apply B1-field selective inversions to spins that experience selected parts of the rf-field distribution. Any frequency band-selective pulse can be used for this purpose, but we chose the family of I-BURP pulses (Geen and Freeman, 1991) for the measurements demonstrated here. As an example, we show that the implementation of such pulses improves homonuclear frequency-switched Lee–Goldburg decoupling in solid-state NMR.

Using nutation-frequency-selective pulses to reduce radio-frequency field inhomogeneity in solid-state NMR

Radio-frequency (rf) field inhomogeneity is a common problem in NMR which leads to non-ideal rotations of spins in parts of the sample. Often, a physical volume restriction of the sample is used to reduce the effects of rf-field inhomogeneity especially in solid-state NMR where spacers are inserted to reduce the sample volume to the centre of the coil. We show that band-selective pulses in the spin-lock frame can be used to apply B1-field selective inversions to spins that experience selected parts of the rf-field distribution. Any frequency band-selective pulse can be used for this purpose but we chose the family of I-BURP pulses (H. Geen, R. Freeman, Band-Selective Radiofrequency Pulses, J. Magn. Reson. 93 (1991) 93–141) for the measurements demonstrated here. As an example, we show that the implementation of such pulses improves homonuclear frequency-switched Lee-Goldburg decoupling in solid-state NMR.