A cross-polarization based rotating-frame separated-local-field NMR experiment under ultrafast MAS conditions

Abstract

Rotating-frame separated-local-field solid-state NMR experiments measure highly resolved heteronuclear dipolar couplings which, in turn, provide valuable interatomic distances for structural and dynamic studies of molecules in the solid-state. Though many different rotating-frame SLF sequences have been put forth, recent advances in ultrafast MAS technology have considerably simplified pulse sequence requirements due to the suppression of proton–proton dipolar interactions. In this study we revisit a simple two-dimensional 1H–13C dipolar coupling/chemical shift correlation experiment using 13C detected cross-polarization with a variable contact time (CPVC) and systematically study the conditions for its optimal performance at 60kHz MAS. In addition, we demonstrate the feasibility of a proton-detected version of the CPVC experiment. The theoretical analysis of the CPVC pulse sequence under different Hartmann–Hahn matching conditions confirms that it performs optimally under the ZQ (w1H−w1C=±wr) condition for polarization transfer. The limits of the cross polarization process are explored and precisely defined as a function of offset and Hartmann–Hahn mismatch via spin dynamics simulation and experiments on a powder sample of uniformly 13C-labeled L-isoleucine. Our results show that the performance of the CPVC sequence and subsequent determination of 1H–13C dipolar couplings are insensitive to 1H/13C frequency offset frequency when high RF fields are used on both RF channels. Conversely, the CPVC sequence is quite sensitive to the Hartmann–Hahn mismatch, particularly for systems with weak heteronuclear dipolar couplings. We demonstrate the use of the CPVC based SLF experiment as a tool to identify different carbon groups, and hope to motivate the exploration of more sophisticated 1H detected avenues for ultrafast MAS.