Unveiling the impact of second-order 1H-17O Quadrupolar-Dipolar interaction on solid-state NMR spectroscopy

Abstract

Recent advances in the ¹⁷O-enrichment techiques and high-resolution nuclear magnetic resonance (NMR) methods have opened new opportunities to utilize ¹⁷O NMR to disentangle the zeolitic structure−property relationship that has not been well resolved through traditional ¹H, ²⁷Al, and ²⁹Si NMR spectroscopy. Compared with one-dimensional ¹⁷O magic angle spinning (MAS) NMR experiments, ¹H-¹⁷O correlation spectrocopy has become a crucial method for revealing the structures and dynamics of reactive hydroxyl species in zeolites with higher resolution and precision. However, the introduction of ¹⁷O can induce changes in ¹H MAS NMR signals due to the second-order ¹H-¹⁷O quadrupolar-dipolar (2nd-QD) cross interaction, which has recently been revealed by us on H-ZSM-5 (H-MFI) zeolites with 10-membered-ring (MR) channels. Herein, we performed various ¹H-¹⁷O correlation experiments (¹H{¹⁷O}-J-heteronuclear multiple quantum coherence (HMQC), ¹H{¹⁷O}-D-HMQC, and ¹H→¹⁷O-D-RINEPT) on two other types of ¹⁷O-enriched zeolites, i.e., H-Mordenite (H-MOR) with 8-/12-MR channels and H-ZSM-35 (H-FER) with 8-/10-MR channels. Notably, unusual ¹H-¹⁷O correlation signals with tilted patterns and magnetic-field-dependent shifts were observed on both samples and all tested correlation experiments at high fields up to 18.8 T. These observations were further comprehensively explained by theoretical analysis of the ¹H-¹⁷O quadrupolar-dipolar interaction, thus demonstrating that the ¹H-¹⁷O 2nd-QD interaction generally affects the ¹H and ¹H-¹⁷O correlation MAS NMR spectra of the dehydrated ¹⁷O-enriched zeolites, irrespective of the framework types. Beyond zeolites, the non-ignorable 2nd-QD interaction on NMR spectroscopy can complicate NMR identification of ¹⁷O-labeled hydroxyls in many other inorganic materials and biomolecules. The analysis methods proposed in this study are expected to effectively address these challenges and provide clearer insights into such systems.