Induced Infrared Absorption of Molecular Oxygen Sorbed in Exchanged A Zeolites. 2. Frequency Shift Calculation

Abstract

O2 sorbed in NaA, NaCaA, and CaA exhibits induced infrared bands whose frequencies are differently shifted with respect to the gas phase frequency. In the preceding article, we analyzed the intensity of these bands. The shift depends on the molecule−zeolite interaction, by the variation of the latter with respect to the normal coordinate of the molecule. We have calculated this interaction, to determine the adsorption sites of O2 in these zeolites. It was shown that the main adsorption site in NaA is in front of the NaIII cation. The oxygen molecule can take two orientations, parallel and perpendicular, with respect to the field of the Na+ cation. In NaCaA the adsorption site is in front of a Na+ or Ca2+ cation in a site SI. It was shown that the frequency shift depends strongly on the location of the cation in the 6-ring. In CaA, the strongest site seems to be located near a Ca2+ cation in a SII site in a 8-ring window. The frequency shifts, calculated for all these adsorption sites, depend strongly on the variation of the quadrupole moment and of the parallel and perpendicular polarizability component of the admolecule with respect to the normal coordinate, which are only known up to first order. By adjusting the computed shift to the experimental one, we have been able to calculate a crude value of the second derivative of these molecular quantities with respect to the normal coordinate.