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 ¹²⁹Xe NMR spectroscopy during the past decades. In this contribution, the only NMR active, stable krypton isotope ⁸³Kr (spin I = 9/2) is explored as a novel probe for porous materials. It is demonstrated that ⁸³Kr NMR spectroscopy of nanoporous or microporous materials is feasible and straightforward despite the low gyromagnetic ratio and low abundance of the ⁸³Kr isotope. The ⁸³Kr 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 ⁸³Kr 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 ¹²⁹Xe 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.