Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

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.

Isotope Effects and the Temperature Dependences of the Hyperfine Coupling Constants of Muoniated sec-Butyl Radicals in Condensed Phases

“…Though hyperfine anisotropy and molecular reorientation of muoniated free radicals can give rise to asymmetric line shapes, 11,30,31,33 in the present study of neat butenes these are invariably symmetric lines and hence are amenable to Lorentzian fits. It is the positions of the ALC resonances, B r (Δ 1 ) and B r (Δ 0 ), that are important here and that are well determined from the fits, even for broad line resonances, giving the isotropic Hfcc of interest: for the muon, A μ , from the Δ 1 resonance, and for the proton, A p , for each group of magnetic equivalent nuclei, from the Δ 0 resonances, as defined by eqs 4 and 5,…”

“…Like the FT data, the Δ 1 ALC lines are particularly broad in the solid phase (J2000 G), partly due perhaps to the nonplanar nature of the dominant gauche conformer of 1-butene (paper I), giving rise to a more polycrystalline environment and hence to a larger expected hyperfine anisotropy. 30,31,33 This could also explain the broad FT lines seen in Figure 1 (top).…”

“…Line shapes of this nature have mainly been investigated for the Mucyclohexadienyl radical in zeolites 31,33 and in soft matter 10 but also for the muoniated ethyl radical in zeolites. 11 However, fitting these data for the Δ 1 resonance with the assumption of an axial tensor at 10 and 60 K gave noticeably worse fits in the region of the minimum compared to the Lorentizan fits shown.…”

Muon Spin Spectroscopy of Transient Radicals in Zeolites

“…The muoniated cyclohexadienyl radical has been observed in benzene-loaded NaY and the ALC-µSR spectrum is very dissimilar to the spectrum obtained for pure frozen benzene or benzene-loaded ZSM-5 [22]. There are four broad resonances in the | M| = 1 and methylene proton | M| = 0 region of the spectrum, as opposed to two in benzene and ZSM-5.…”

Presenting Muon Thermalization with Feynman QED

“…The majority of muons that do not capture electrons will react with the medium, forming diamagnetic species [15]. This process occurs within a picosecond through the following two reactions [3]…”