Direct NMR spinning sideband analysis for a dipolar coupled system: structure elucidation of a proton—trimethylphosphine complex in LaH-Y zeolites

Abstract

A new approach is proposed to determine the dipolar coupling constant, and hence the internuclear distance through direct NMR sample spinning sideband intensity analysis. This highly sensitive method reveals a small change in either the structure or the dynamics of a protonated trimethylphosphine complex, [(CH 3) 3P−H] +, in a series of La-exchanged zeolite-Y of different La loading. The dipolar coupling constants exhibits an inverse correlation with the activity of the zeolite for n-hexane cracking, which is a measure of the Bro̵nsted acidity. Assuming a rigid lattice model, the P—H bond distances in the various LaH-Y zeolites were found to range from 1.454 to 1.419 Å. The distances are in good agreement with typical P—H bond distances found in various phosphines. Molecular dynamics of the [(CH 3) 3P−H] + complex for samples with different acidity are proposed to account for the variation in the dipolar coupling constant. This direct and sensitive approach is expected to be readily applicable to general structural and dynamic studies of polycrystalline or amorphous samples involving two-body dipolar interactions.