Optimal control-based solid-state NMR cross-polarization between anisotropic nuclear spins

Cross-polarization (CP) is an indispensable part of solid-state nuclear magnetic resonance (NMR) spectroscopy to enhance sensitivity and extract quantitative structural information. However, the most common versions of the CP experiments are susceptible to the presence of anisotropic interactions, including chemical shift anisotropy (CSA) and quadrupolar coupling (QC). Numerous materials and pharmaceutical compounds commonly contain isotopes, such as 19F with large CSA and 6/7Li, 23Na, 27Al, etc., with QC. Inter-nuclear spin correlation experiments can provide crucial local structural information using an efficient cross-polarization method. We introduce a new Optimal Control (OC) simulation generated pulse sequence for Optimal Polarization Transfer In Anisotropic Nuclear Spins (OPTIANS) aiming at 19F-7Li correlations essential for characterizing materials for batteries, heterogeneous catalysis, and many other applications. Numerical simulations showing an improved efficiency for 19F-7Li, 19F-23Na, 19F-27Al and 19F-13C polarization transfer compared to the standard CP are shown. Moreover, robustness against variations in the experimental parameters and the strength of the internal anisotropic interactions is demonstrated using simulations for 19F-7Li case. 19F-7Li polarization transfer experiments on a multi-metal fluoride (MMF) system validate the predictions of the simulations. A comparison with the standard CP experiment shows a gain of 50% in polarization transfer efficiency in the presence of large anisotropic spin interactions at 14.1 T.