Highly sensitive measurements of methylene dynamics with a frequency-selective double-quantum sideband method

Probing the fast dynamics of surface sites using NMR spectroscopy is highly challenging owing to the sites' high dilution and the difficulties often associated with isotopic enrichment. Intra-CH₂¹H-¹H dipolar couplings are ideal probes of motions given that they only involve ¹H's and the tensor has a well-defined size and orientation. We introduce a frequency-selective variant of the double-quantum sideband method to measure like-spin ¹H-¹H dipolar coupling constants. The experiment dramatically reduces the instrument time required to measure dynamically-averaged intra-CH₂ dipolar couplings. We demonstrate the performance of the sequence using silica-supported silanes as model highly-mobile surface species.