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 CH 3 • and (CH 3 ) 3 N + CH 2 • 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 (CH 3 ) 3 N + CH 2 • 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 (CH 3 ) 3 N + CH 2 • 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 (CH 3 ) 3 N + CH 2 • radicals. ESR spectra from these radicals can be simulated by assuming that the hyperfine tensor components for the two R protons are averaged by two types of motions: rotation of the CH 2 group about the C 2V symmetry axis and precession or wobbling of this axis. The parameters used in the simulation are g iso ) 2.0022, two equivalent R protons with axial hyperfine components A | av ) 25.0 G and A ⊥ av ) 23.0 G, one 14 N nucleus with A N ) 3.9 G, and one remote proton with A H,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 R 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.

Structure and dynamics of radicals in zeolite matrices: ESR and theoretical studies

“…It is therefore probable that the rotation about the N-C'H 2 bond is not suffŸ to average out the anisotropy, but a precession or wobbling motion must also occur as assumed here and previously [14,39]. The calculated value of the ~4N hf coupling constant is in excellent agreement with the experimental value, while the couplings for the CH 3 protons depended on the angle of rotation about the N-CH 3 bonds as observed experimentally.…”

“…= 2.24 toT, A H = 2.47 toT ( Table 2) estimated from simulations is much less than that observed for the ethyl radical A l = 2.0 toT, Arj = 3.0 toT undergoing rapid rotation about the C-C bond [38]. Similar data have been reported for the [(CH3)3NCH2] "+ radical in a clathrate system and ah additional averaging of the anisotropy was proposed to occur by a precessional or wobbling motion of the N-CH 2 axis [39]. A large-scale motion is predieted in either case for the N-CH 2 fragment, e.g., an angle of 43 ~ between the N-C bond and the precession axis [14].…”

Organic Free Radicals in Clathrate Hydrates Investigated by Muon Spin Spectroscopy

“…17,18 There is also related research on γ-irradiation of so-called ionic clathrate hydrate systems, composed of tetramethylammonium hydroxide pentahydrate and nonionic guest molecules. 19,20 A common problem with conventional irradiation methods is that multiple transient species are formed. It is sometimes possible to study individual components by means of annealing studies, but in the absence of fast detection methods ambiguities remain about which are primary radicals, which secondary, etc.…”

Characterization of free radicals in clathrate hydrates of pyrrole, thiophene, and isoxazole by muon spin spectroscopy

“…[18][19][20] Similarly, small organic free radicals have been created by radiolysis at low temperature and their behaviour monitored by ESR as the temperature was raised. [21][22][23][24][25] There is clear evidence from some of these experiments of H-atom transfer from an alkyl radical in one cavity to a guest molecule in a neighbouring cage. [26][27][28] Our own experiments make use of muonium as an H-atom analogue.…”