Optimal control-based nuclear spin cross-polarization in the presence of complicating anisotropic interactions

Abstract

Cross-polarization is an indispensable part of solid state nuclear magnetic resonance spectroscopy to enhance sensitivity and extract structural information. However, the presence of certain anisotropic interactions, including chemical shift anisotropy and quadrupolar coupling, makes the inter-nuclear spin correlation experiments challenging. This impedes characterization of numerous materials and pharmaceutical compounds containing isotopes, such as ¹⁹F with large chemical shift anisotropy and ^6/7Li, ²³Na, ²⁷Al, etc., with quadrupolar coupling. To address this problem, we introduce a new optimal control simulation-generated pulse sequence for Optimal Polarization Transfer In the presence of Anisotropic Nuclear Spin interactions (OPTIANS). Numerical simulations show high efficiency and robustness against experimental imperfections under a broad range of anisotropic interaction strengths for ¹⁹F–⁷Li, ¹⁹F–²³Na, ¹⁹F–²⁷Al, and ¹⁹F–¹³C polarization transfers. The polarization transfer curves show transient oscillations, which make the pulse sequence a quantitative method for dipolar coupling measurements. Experiments on a multi-metal fluoride system validate the predictions of the simulations by showing efficient PT in three spin pairs at varying experimental conditions. Remarkably, this method shows 50% better ¹⁹F–⁷Li PT efficiency at 14.1 T compared to the ramped cross-polarization experiment. The underlying polarization transfer mechanism is analyzed using the Fourier transform of the polarization transfer curves revealing that this optimal control method utilizes the chemical shift anisotropy and quadrupolar coupling to facilitate robust and efficient cross-polarization.