Abstract

Low-γ nuclei signal enhancement in solid-state NMR spectroscopy is typically achieved via cross-polarization (CP) using abundant 1H polarization in organic solids. Nevertheless, direct low-γ nuclei signal detection via a single CP process is quite challenging with minute quantities of samples due to the extremely limited signal-to-noise ratio (SNR) of the acquired spectra. Herein, we demonstrated the robust performance of a multiple-contact CP experiment with multiple acquisition periods (MCP) in each transient scan, leading to several-fold SNR enhancement over a conventional single-CP experiment at fast MAS conditions with slightly increased experimental time. Spin thermodynamic analysis was further performed to achieve maximum SNR by adding the obtained Nmax CP spectra from each transient, where Nmax ∼ T1ρ/τcw. Here, T1ρ is the proton spin–lattice relaxation time in the rotating frame, and τcw is the total time of CP and a heteronuclear decoupling period. The theoretical analysis is in good agreement with experimental results, and more than 4.5-fold SNR enhancement can be achieved for the pharmaceutical danazol/vanillin cocrystals. Besides, MCP was also used for proton T1 and T1ρ measurement with high-resolution 13C detection, where both proton T1 and T1ρ can serve as the spectral-editing basis to identify different immiscible components in complex molecular systems.