The role of polarization of Xe by di- and monovalent cations in Xe NMR studies in zeolite A

We consider the role of polarization in the adsorption of Xe in zeolites of type A by direct comparative analysis of the adsorption isotherms, distributions of occupancies, and ¹²⁹Xe NMR chemical shifts of Xe_n in cages containing Ca_xNa_{12 ?2x} ions per alpha cage (x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, and in the progressions of Xe_n chemical shifts (for n = 0–8 in cages with x = 0, 1 Ca²⁺ ions and for n = 0–5 in cages with x = 2, 3 Ca²⁺ ions) upon increasing the level of Ca²⁺ ion for Na⁺ ion substitution could only be accounted for by including polarization of the Xe atom by the zeolite framework and its ions. This system, which permits observation of individual Xe_n peaks and of directly comparable adsorption isotherms in several cage types, provides a good model system for the interpretation of the more general case in which only the overall average ¹²⁹Xe NMR chemical shift is observed in open network zeolites, arising from free exchange of Xe among cavities of variable occupancy and variable cation distribution.