Unstable Intermediates in X-Irradiated Clathrate Hydrates: ESR and ENDOR of Tetramethylammonium Hydroxide Pentahydrate (TMNOH)

Abstract

X-irradiation of tetramethylammonium hydroxide pentahydrate (TMNOH) at 77 K produces trapped electrons, and CH3• and (CH3)3N+CH2• radicals. The trapped electrons were detected by bleaching with visible light and by subtraction of spectra from bleached and unbleached samples. The decay of the methyl radicals starts at ≈100 K and that of the (CH3)3N+CH2• radicals at ≈150 K. The ESR spectrum measured at 130 K has the best resolution and is a superposition of contributions from the methyl radicals (13%) and from the (CH3)3N+CH2• radical (87%). The reversibility of the line shapes with temperature variations suggested that the increased resolution observed at 130 K is due to dynamical effects involving the (CH3)3N+CH2• radicals. ESR spectra from these radicals can be simulated by assuming that the hyperfine tensor components for the two α protons are averaged by two types of motions: rotation of the CH2 group about the C2v symmetry axis and precession or wobbling of this axis. The parameters used in the simulation are giso = 2.0022, two equivalent α protons with axial hyperfine components A∥av = 25.0 G and A⊥av = 23.0 G, one 14N nucleus with AN = 3.9 G, and one remote proton with AH,remote = 4.2 G. The remote proton is identified with a lattice proton. If the precession model is adopted, the precession angle calculated from the principal values of the hyperfine tensor for the α protons used in the simulations is 47°. For the wobbling model, the wobbling angle deduced is 72°. These results suggest that the hydrogen-bonded cage around the guest allows large-scale dynamical effects even at 130 K.