EPR Study of the Hindered Motion of NO2 and ClO2 Adsorbed in Synthetic Zeolites

The temperature-dependent EPR spectra of NO2 and ClO2 adsorbed in sodium (NaX) and calcium (CaX) cation type-X zeolite were investigated. Motional narrowing occurs above 77°K in each case. The time-dependent spectra of NO2 in CaX zeolite were simulated by linear combinations of stationary-state solutions of the spin Hamiltonian. The model used to produce these linear combinations required that there be no preferred axis of rotation for NO2 in this system and that rotation greater than ∼ 35° from the equilibrium position be strongly hindered. Although no comparably detailed study has been made for the other systems, it appears that NO2 in NaX also has a high barrier to rotation but that ClO2 in both NaX and CaX zeolite has a greater degree of rotational freedom.