Dc. Doetschman et al., PHYSICAL CHARACTERIZATION OF THE STATE OF MOTION OF THE PHENALENYL SPIN-PROBE IN CATION-EXCHANGED FAUJASITE ZEOLITE SUPERCAGES WITH PULSED EPR, Chemical physics, 185(3), 1994, pp. 343-356
The molecular motion of the phenalenyl (PNL) spin probe in the superca
ges of cation-exchanged X and Y zeolites (faujasites) has been physica
lly characterized by pulsed and continuous wave (CW) electron paramagn
etic resonance (EPR). Both X and Y zeolites, whose cation sites were e
xchanged with the alkali metal ions, Li+, Na+, K+, Rb+ and Cs+ were ex
amined. There is a good correspondence between the temperature depende
nces 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 rotat
ion (Das et al., Chem. Phys. 143 (1990) 253). The rate of in-plane rot
ation is an activated process, with E(parallel-to)/R = 1289 +/- 35 K
and 1462 +/- 47 K in NaX and KX zeolites, respectively. The rotation a
ppears 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(perpe
ndicular-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 bo
nding decreases with increasing cation atomic number as indicated by E
(perpendicular-to) and the peripheral repulsion (crowding) of PNL inc
reases with cation size as indicated by E(perpendicular-to). There ar
e qualitative indications that the binding of PNL to the cations in th
e Y zeolite is stronger than in the X zeolite.