Probing O–H Bonding through Proton Detected¹H–¹⁷O Double Resonance Solid-State NMR Spectroscopy

Abstract

The ubiquity of oxygen in organic, inorganic, and biological systems has stimulated the application and development of ¹⁷O solid-state NMR spectroscopy as a probe of molecular structure and dynamics. Unfortunately, ¹⁷O solid-state NMR experiments are often hindered by a combination of broad NMR signals and low sensitivity. Here, it is demonstrated that fast MAS and proton detection with the D-RINEPT pulse sequence can be generally applied to enhance the sensitivity and resolution of ¹⁷O solid-state NMR experiments. Complete 2D ¹⁷O → ¹H D-RINEPT correlation NMR spectra were typically obtained in less than 10 h from less than 10 mg of material, with low to moderate ¹⁷O enrichment (less than 20%). Two-dimensional ¹H-¹⁷O correlation solid-state NMR spectra allow overlapping oxygen sites to be resolved on the basis of proton chemical shifts or by varying the mixing time used for ¹H-¹⁷O magnetization transfer. In addition, J-resolved or separated local field (SLF) blocks can be incorporated into the D-RINEPT pulse sequence to allow the direct measurement of one-bond ¹H-¹⁷O scalar coupling constants (¹ J_OH) or ¹H-¹⁷O dipolar couplings ( D_OH), respectively, the latter of which can be used to infer ¹H-¹⁷O bond lengths. ¹ J_OH and D_OH calculated from plane-wave density functional theory (DFT) show very good agreement with experimental values. Therefore, the 2D ¹H-¹⁷O correlation experiments, ¹H-¹⁷O scalar and dipolar couplings, and plane-wave DFT calculations provide a method to precisely determine proton positions relative to oxygen atoms. This capability opens new opportunities to probe interactions between oxygen and hydrogen in a variety of chemical systems.