No abstract

The adsorption and dynamical behavior of the muonated cyclohexadienyl radical (C 6 H 6 Mu) in NaY zeolite, formed by muonium (Mu) addition on adsorbed benzene, was investigated by the muon spin resonance (µSR) technique, primarily at loadings of 2-3 C 6 H 6 molecules per supercage of NaY. The dynamics of this radical are expected to be the same as its isotopic analogue, C 6 H 7 , for which there are no similar data available. Both TF-µSR and ALC-µSR spectra were recorded, with the most detailed information provided by the positions and line widths of the avoided level crossing resonances. In concert with 2 H NMR, neutron diffraction and molecular dynamics studies of the parent benzene molecule, as well as current theoretical calculations, the dominant adsorption site for the C 6 H 6 Mu radical is believed to be the S II Na cation, within a supercage, which gives rise to three observed ALC lines, corresponding to two different orientations for the muon (proton) of the CHMu methylene group: pointing toward (endo) and away (exo) from the Na cation. The cation interaction gives rise to unprecedentedly large (≈20%) shifts in hyperfine coupling constants, indicative of a strong bond formed with the π electrons of the C 6 H 6 Mu radical. An additional but weaker resonance line is also seen, which is interpreted as being due to adsorption at the window sites between supercages. The ALC lines associated with the C 6 H 6 Mu radical bound to both the Na cation and window sites are all broad, ≈1 kG, change little with temperature and exhibit mainly static line shapes over the whole temperature range studied, from 3 to 322 K. This indicates a much stronger host-guest interaction for C 6 H 6 Mu, particularly with the Na cation, than is known for benzene, to the extent that this site acts as an effective trap for the free radical, over the critical µSR time scale of 50 ns.

Abstraet. Muon spin rotation 0tSR) and avoided level erossing resonanee (ALCR) have been used to determine the hyperf'me eoupling eonstants (hfes) of the muonium-substituted eyclohexadienyl radieals C6H6Mu, C6D6Mu and C6F6Mu in the gas phase, at pressures ~ 1 and 15 atm and temperatures in the range 40-80~ Equivalent studies of polyatomie free radieals in gases, by electron spin resonanee (ESR) speetroscopy, ate generally not possible in this pressure range. The present gas phase results support the findings of earlier studies of eyelohexadienyl radicals in the eondensed phase, by borla ~tSR and ESR. Minor but not insignifieant (~1%) effeets on the hfes ate observed, whieh can be qualitatively understood for sueh nonpolar media in terms of their differing pola¡ This is the first time that eomparisons of this nature have been possible between different phases at the same temperatures. These ~tSR/ALCR gas-phase results provide a valuable benehmark for eomputational studies on radieals, free from possible effeets of solvent or matrix environments.