A double quantum 129Xe NMR experiment for probing xenon in multiply-occupied cavities of solid-state inclusion compounds

Abstract

A method is presented for detecting multiple xenon atoms in cavities of solid-state inclusion compounds using ¹²⁹Xe double quantum NMR spectroscopy. Double quantum filtered ¹²⁹Xe NMR spectra, performed on the xenon clathrate of Dianin’s compound were obtained under high-resolution Magic-Angle Spinning (MAS) conditions, by recoupling the weak ¹²⁹Xe–¹²⁹Xe dipole–dipole couplings that exist between xenon atoms in close spatial proximity. Because the ¹²⁹Xe–¹²⁹Xe dipole–dipole couplings are generally weak due to dynamics of the atoms and to large internuclear separations, and since the ¹²⁹Xe Chemical Shift Anisotropy (CSA) tends to be relatively large, a very robust dipolar recoupling sequence was necessary, with the symmetry-based SR26¹¹₄ dipolar recoupling sequence proving appropriate. We have also attempted to measure the ¹²⁹Xe–¹²⁹Xe dipole–dipole coupling constant between xenon atoms in the cavities of the xenon–Dianin’s compound clathrate and have found that the dynamics of the xenon atoms (as investigated with molecular dynamics simulations) as well as ¹²⁹Xe multiple spin effects complicate the analysis. The double quantum NMR method is useful for peak assignment in ¹²⁹Xe NMR spectra because peaks arising from different types of absorption/inclusion sites or from different levels of occupancy of single sites can be distinguished. The method can also help resolve ambiguities in diffraction experiments concerning the order/disorder in a material.