Dc. Doetschman et Gd. Thomas, MOLECULAR MOTIONS OF NITROXYL RADICAL SPIN PROBES IN X-ZEOLITES - DEPENDENCE ON ZEOLITE CATION AND SPIN-PROBE CHEMICAL FUNCTIONAL-GROUP, Chemical physics, 228(1-3), 1998, pp. 103-114
The nitroxyl radical spin probes, 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO), 4-oxo-TEMPO (TEMPONE) and 4-amino-TEMPO (TEMPAMINE) were exa
mined by continuous wave (CW) electron paramagnetic resonance (EPR) in
a series of cation-exchanged X-type faujasite zeolite supercages. The
spectra in Li-X, Na-X and K-X between 10 and 300 K were fitted by var
ying the rotational correlation constants, the homogeneous linewidth,
and the principal nitrogen nuclear hyperfine coupling matrix element (
A(zz)) for the axis near the C-N-C plane normal. The TEMPONE activatio
n energies for rotation, 12.1-3.1 kJ mol(-1), decrease monotonically w
ith cation size, suggesting an attractive interaction with the cation
that depends on cation Lewis activity. TEMPO and TEMPAMINE have activa
tion energies for rotation, 6.7-12.8 and 2.8-10.0 kJ mol(-1), respecti
vely, that increase with cation size, suggesting non-bonded repulsion
of the molecules by the cations. We propose that the attraction of the
cation to TEMPONE is via its carbonyl pi electrons, an interaction fo
und for the larger phenalenyl pi systems in previous work (D.C. Doetsc
hman, D.W. Dwyer, J.D. Fox, C.K. Frederick, S. Scull, G.D. Thomas, S.G
. Utterback, J. Wei, Chem. Phys. 185 (1994) 343). In contrast, the ami
no group of TEMPAMINE is sp(3) hybridized and has essentially no pi ch
aracter. It is well known from X-ray diffraction studies, in which the
NO group is found to project out of the C-N-C plane, and from the mag
nitude of the A(zz) nuclear hyperfine coupling element that the NO gro
up has relatively little rr character. The activation energies of the
TEMPO rotation are consistently larger than those of TEMPAMINE. We pro
pose that the volume of TEMPO is effectively larger than that of TEMPA
MINE because TEMPO is undergoing rapid piperidine ring inversion which
does not occur in TEMPAMINE. Preliminary pulsed EPR results for TEMPO
are presented that appear to indicate a second type of molecular moti
on associated with the piperidine ring inversion. (C) 1998 Elsevier Sc
ience.