PHYSICAL CHARACTERIZATION OF THE STATE OF MOTION OF THE PHENALENYL SPIN-PROBE IN CATION-EXCHANGED FAUJASITE ZEOLITE SUPERCAGES WITH PULSED EPR

The molecular motion of the phenalenyl (PNL) spin probe in the supercages of cation-exchanged X and Y zeolites (faujasites) has been physically characterized by pulsed and continuous wave (CW) electron paramagnetic resonance (EPR). Both X and Y zeolites, whose cation sites were exchanged with the alkali metal ions, Li+, Na+, K+, Rb+ and Cs+ were examined. There is a good correspondence between the temperature dependences of the PNL electron spin phase memory time and the CW EPR spectra. Both display evidence of a thermal activation from a stationary, non-rotating molecular state to a low-temperature state of in-plane rotation (Das et al., Chem. Phys. 143 (1990) 253). The rate of in-plane rotation is an activated process, with E(parallel-to)*/R = 1289 +/- 35 K and 1462 +/- 47 K in NaX and KX zeolites, respectively. The rotation appears to be about an axis along which the half-filled, non-bonding pi orbital interacts with the exchanged cation in the supercage. Both CW and pulsed EPR also show a higher temperature activation from the in-plane rotating state to an effectively isotropic state of rotation of PNL in which the PNL-cation bond is thought to be broken, with E(perpendicular-to)*/R = 2050 +/- 11 0 K, 1956 +/- 46 K, 1335 +/- 97 K in LiX, NaX and KX zeolites, respectively. The strength of the PNL-cation bonding decreases with increasing cation atomic number as indicated by E(perpendicular-to)* and the peripheral repulsion (crowding) of PNL increases with cation size as indicated by E(perpendicular-to)*. There are qualitative indications that the binding of PNL to the cations in the Y zeolite is stronger than in the X zeolite.