Forcing the ‘lazy’ protons to work

Abstract

The combination of cross-polarization (CP) with flip-back (FB) pulse has enabled in NMR the enhancement of ¹³C sensitivity and the decrease of the recycling delay at both moderate and fast magic-angle spinning (MAS) frequencies. However, only continuous-wave (CW) decoupling is presently compatible with FB-pulse (FB-CW), and depending on the CW radio-frequency (rf) field, either an insignificant sensitivity gain or an acquisition time-dependent gain and a low ¹³C resolution are obtained. In this study, we propose a new FB-pulse method in which radio frequency-driven recoupling (RFDR) is used as the ¹H–¹³C decoupling scheme to overcome these drawbacks. The performances of FB-RFDR in terms of decoupling efficiency and sensitivity gain are tested on both natural abundance (NA) and uniformly ¹³C–¹⁵N labeled L-histidine·HCl·H₂O (Hist) samples at a MAS frequency of ν_R = 70 kHz. The results show the superiority of RFDR over the CW decoupling with respect to these criteria. Importantly, they reveal that the sensitivity gain offered by FB-RFDR is nearly independent of the decoupling/acquisition duration. The application of FB-RFDR on NA-Hist and sucrose yields a sensitivity gain between 60 and 100% compared to conventional FB-CW and CPMAS-SPINAL experiments. Moreover, we compare the ¹³C sensitivities of NA-Hist obtained by our 1D FB-RFDR method and 2D ¹H–{¹³C} double-CP acquisition. Both methods provide similar ¹³C sensitivity and are complementary. Indeed, the 2D method has the advantage of also providing the ¹H–¹³C spatial proximities, but its sensitivity for quaternary carbons is limited; whereas our 1D FB-RFDR method is more independent of the type of carbon, and can provide a ¹³C 1D spectrum in a shorter experimental time. We also test the feasibility of FB-RFDR at a moderate frequency of ν_R = 20 kHz, but the experimental results demonstrate a poor resolution as well as a negligible sensitivity gain.