μSR Studies of Hyperfine Couplings and Molecular Interactions of the Mu-Cyclohexadienyl Radical in Y-Zeolites and in Solid Bulk Benzene

Abstract

The interaction of muoniated cyclohexadienyl radicals with zeolite environments in NaY, HY, and USY has been studied using mainly avoided level crossing muon spin rotation (ALC-μSR) spectroscopy, which utilizes spin-polarized positive muons as local probes. A strong interaction of C 6 H 6 Mu with sodium cations in NaY is indicated and leads to significant distortion of the C 6 H 6 Mu structure from planarity, accompanied by large shifts in hyperfine coupling constant (hfcc) values compared to those in bulk benzene. Much weaker interactions of this radical with Brønsted acid sites in USY/HY are deduced from the measured muon and proton hfcc values, indicative of a largely planar radical, that, in contrast to those in NaY, also exhibit a strong dependence on benzene loading. The small shifts in hfcc values seen in USY/HY compared to those in the bulk relate to the nature of OH binding sites and possibly also to effects arising from varying local dielectric constants at different benzene packing densities. The muon ALC resonances in these frameworks exhibit a marked increase in widths at temperatures near 300 K and at low benzene loadings, evidence for molecular dynamics, indicating desorption and reorientation of C 6 H 6 Mu, in marked contrast to the nearly static widths seen in NaY up to 470 K.

Exploring the Temperature Dependent Solid-State ALC Spectrum of the C6H6Mu•Radical with Ab-Initio Simulation Techniques

“…The experimental data was acquired at the muon beamline in ISIS, RAL. The acquired data closely reproduces and expands upon results that can be found in literature [2]. …”

Level Crossing Resonance Studies of the Muoniated Cyclohexadienyl Radical (MuĊ6H6) Interacting with Uncapped Gold Nanoparticles in Porous Silica Hosts

“…There is also a very weak/narrow ΔM = 2(Δ 2 ) double-quantum resonance expected, which corresponds to a muon flip−flip between states |α e α μ α p ⟩ and |α e β μ β p ⟩, also arising only in the second order but from the anisotropic muon−electron hyperfine interaction only (terms E μ and F μ for muon and proton in eq 3). These contribute much less than C μ and D μ , as seen earlier in solid benzene 7,50,51,58 and in Figure 2 here as well. Since this is such a weak line, it is only the Δ 0 and Δ 1 resonances that are of interest in the present study.…”

“…In magnetic fields well above the hyperfine coupling field for the muoniated radical of interest (∼185 G for MuC ̇6H 6 7,45,46 ), in the Zeeman region, the product eigenstates of muon, electron, and protons exhibit avoided level crossings, giving rise to ALC resonances at specific applied longitudinal magnetic fields. 7,22,42,[50][51][52]56,58 This is shown in Figure 1 for a single proton, 22 which in fact is the case of interest here for the "ipso" position where Mu adds to the benzene ring, giving a methylene −CHMu group. The top part of this figure shows schematic results for the energies of this three spin ( 1 / 2 ) system, assuming an axial hyperfine interaction (2), labeled by their usual spin projections α and β for electron, muon, and nuclear k = proton spins.…”

“…Noteworthy are the amplitudes of the ALC signals seen here, as measured by their peak intensities, which qualitatively scale linearly with Bz loading (Table 1, Figure 9 below), and which are much larger than those seen in the AuNP/silica samples. In a previous study of MuC ̇6H 6 in the siliceous zeolites, HY and USY, 7 it was assumed that the muoniated radical was weakly interacting with hydroxyl (−OH) groups in the zeolite supercages. That is also expected to be mainly the case in the SBA-15 silica here, consistent with the elemental analysis discussed in paper P1.…”

Muon spin spectroscopy of ferrocene: characterization of muoniated ferrocenyl radicals

“…The use of a high-order polynomial is a common procedure for dealing with the background in ALC-mSR measurements. 31,32 The low field portion of the resonance deviates significantly from the Lorentzian lineshape above 50 K. This was observed in the spectra of both Fe(C 5 H 5 ) 2 and Fe(C 5 D 5 ) 2 , which were taken approximately 6 months apart, and was reproducible so we can conclude it was not due to the background changing but due to the samples themselves. Our conclusion is that this is due to the repolarization.…”