Studying porous materials with krypton-83 NMR spectroscopy

Abstract

This report is the first review of (83)Kr nuclear magnetic resonance as a new and promising technique for exploring the surfaces of solid materials. In contrast to the spin I = 1/2 nucleus of (129)Xe, (83)Kr has a nuclear spin of I = 9/2 and therefore possesses a nuclear electric quadrupole moment. Interactions of the quadrupole moment with the electronic environment are modulated by surface adsorption processes and therefore affect the (83)Kr relaxation rate and spectral lineshape. These effects are much more sensitive probes for surfaces than the (129)Xe chemical shielding and provide unique insights into macroporous materials in which the (129)Xe chemical shift is typically of little diagnostic value. The first part of this report reviews the effect of quadrupolar interactions on the (83)Kr linewidth in zeolites and also the (83)Kr chemical shift behavior that is distinct from that of its (129)Xe cousin in some of these materials. The second part reviews hyperpolarized (hp) (83)Kr NMR spectroscopy of macroporous materials in which the longitudinal relaxation is typically too slow to allow sufficient averaging of thermally polarized (83)Kr NMR signals. The quadrupolar-driven T(1) relaxation times of hp (83)Kr in these materials are sensitive to surface chemistry, surface-to-volume ratios, coadsorption of other species on surfaces, and surface temperature. Thus, (83)Kr T(1) relaxation can provide information about surfaces and chemical processes in macroscopic pores and can generate surface-sensitive contrast in hp (83)Kr MRI.

Physical methods and techniques : NMR spectroscopy

“…125 A review of 83 Kr as a probe for exploring surfaces in porous solids highlights some of the advantages of the quadrupolar nature of 83 Kr over spin 1/2 129 Xe. 126 The existence of a helium molecule trapped inside an open-cage fullerene can be inferred from by the chemical shift of a proton directly attached to the outside. 127 A review of solid-state NMR investigations of endohedral hydrogen-fullerene complexes presents 1 H, 13 C, relaxation and dipole-dipole recoupling experiments.…”

QUADRELAX: A Program for Computing Relaxation Data for Quadrupolar Nuclei, Including Chemical Exchange Effects

“…At the end, the effects of chemical exchange on the NMR line shape for spin 5 2 and 7 2 nuclei in nonextreme narrowing condition was reported in 1982 by Westlund (13). Although spin 9 2 nuclei have been studied well (14)(15)(16), there were no available theoretical data about their relaxation. Published papers on this topic only consider the effects of chemical exchange for I5 3 2 ; 5 2 , and 7 2 nuclei; furthermore, these data are too limited as they are not applicable to all molecular movement regimes, they also cannot be used for any value of some exchange parameters such as the lifetime of a nucleus at its specific location.…”

“…Thus, it is obvious that macromolecules does not satisfy the extreme narrowing condition, and the extension of the relaxation theories and data to other molecular movement regimes beyond the extreme narrowing condition was a need. Although spin 9 2 nuclei have been studied well (14)(15)(16), there were no available theoretical data about their relaxation. Although Bull had considered chemical exchange effect in his 1972 paper, his work was restricted to spin 3 2 nuclei (9).…”

83Kr nuclear magnetic moment in terms of that of 3He

“…The first attempts at 83 Kr spectroscopy were made on pure gaseous krypton and krypton dissolved in several liquids . Up to now, several applications in NMR and MRI were found – the inclusion of krypton inside fullerene molecules, as a probe of surfaces in different solid substances, as well as for inhalation in the magnetic imaging of lungs . Use of polarized krypton to substantially increase the sensitivity of such analysis has become the generally adopted method.…”