MOLECULAR MOTIONS OF NITROXYL RADICAL SPIN PROBES IN X-ZEOLITES - DEPENDENCE ON ZEOLITE CATION AND SPIN-PROBE CHEMICAL FUNCTIONAL-GROUP

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.