Introducing krypton NMR spectroscopy as a probe of void space in solids

A wealth of information about porous materials and their void spaces has been obtained from the chemical shift data in Xe-129 NMR spectroscopy during the past decades. In this contribution, the only NMR active, stable krypton isotope Kr-83 (spin / = (9)/(2)) is explored as a novel probe for porous materials. It is demonstrated that Kr-83 NMR spectroscopy of nanoporous or microporous materials is feasible and straightforward despite the low gyromagnetic ratio and low abundance of the Kr-83 isotope. The Kr-83 line width in most of the studied cases is quadrupolar dominated and field-strength independent. A significant exception was found in calcium-exchanged zeolites where the field dependence of the line width indicates a distribution of isotropic chemical shifts that may be caused by long-range disorder in the zeolite structure. The Kr-83 chemical shifts observed in the investigated materials display a somewhat different behavior than that of their 129Xe counterparts and should provide a great resource for the verification or refinement of current Xe-129 chemical shift theory. In contrast to xenon, krypton with its smaller atomic radius has been demonstrated to easily penetrate the porous framework of NaA. Chemical shifts and line widths of 83Kr are moderately dependent on small fluctuations in the krypton loading but differ strongly between some of the studied samples.