Hyperfine Interactions and Molecular Motion of the Mu−Ethyl Radical in Faujasites:  NaY, HY, and USY

Abstract

The adsorption and dynamical behavior of the Mu−ethyl radical (MuC2H4) in NaY, HY, and USY faujasites was investigated by the muon spin resonance (μSR) technique, at loadings of one to five ethene molecules per supercage and over a temperature range of ca. 5−500 K (for NaY). The temperature dependences of both the muon and proton hyperfine coupling constants (Hfc's) are reported and compared with similar studies of MuC2H4 in different environments. Both transverse field (TF) μSR and avoided level crossing resonance (ALC) μSR spectra were recorded, with information on molecular motion mainly provided by the ALC line shapes. The muon Hfc's show only a small sensitivity to different frameworks and loadings but exhibit significant (∼10%) shifts at low temperatures, in comparison with bulk values, due to binding of the ethyl radical to cations at S II sites in NaY and to framework hydroxyls in the case of HY(USY). The Δ1 resonances are symmetric and quite broad at the lower temperatures studied, but dramatically further broaden near room temperature, seen also in the TF relaxation rates, suggesting that the Mu−ethyl radical either desorbs from or hops between its binding sites at the higher temperatures. An Arrhenius estimate of the activation energy for desorption gives ∼ 20 kJ/mol, consistent with the dipolar interaction energy between the Mu−ethyl radical and an NaY cluster. The observation of such highly broadened Δ1 ALC lines at the higher temperatures contrasts with the largely static line widths reported previously for the Mu−cyclohexadienyl radical (MuC6H6) in NaY. Sharper Δ0 ALC lines for both the α and β protons of MuC2H4 appear near the same temperatures at which the Δ1 lines overly broaden, and which persist to the highest temperatures (350 K). For NaY, the α proton resonances also broaden further at these temperatures. For both NaY and particularly HY, the temperature dependence of the α proton Hfc's indicates considerable distortion of the Mu−ethyl radical geometry, due to its binding to zeolite sites. Recently published calculations of binding energies and Hfc's for ethyl radicals in NaY and HY suggest a much stronger binding of the MuC2H4 radical than seems warranted by the data and pose as well a conundrum in comparison with earlier results for MuC6H6 in NaY. On the other hand, the temperature dependence of the isotropic muon Hfc's found from the T-atom model for NaY employed in these calculations is in excellent agreement with experiment.

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,…”

“…For the Mu radicals of interest, in TFs J 1.5 kG, there are three principal frequencies, corresponding to muons in diamagnetic environments (ν D ) and two radical frequencies, ν 12 and ν 34 for a given radical, that correspond to the allowed transitions of the spin Hamiltonian. 8,29 These are most easily seen in Fourier transform (FT-μSR) spectra at the characteristic precession frequencies, 7,8,11,13 …”

“…In these cases, the muon Hfcc can be found by the ALC-μSR technique in a longitudinal field, which also provides the important measurement of the nuclear (here proton only) Hfcc, A p . 7,8,10,11 An ALC signal appears as a "dip" in the time-integrated decay asymmetry at a particular value of the LF, corresponding to a resonant transfer of muon spin polarization from the backward to the forward direction as the magnetic field is varied. 8,11,13,14,20,31,32 In the present study, typically several sweeps of the LF range were carried out, in alternating directions, incremented in steps of about 100 G, depending on conditions.…”

“…It is noteworthy that, in contrast to A p,CH 3 (T), there is no discontinuity in A p,CHMu (T), for either the cis-or trans-2-butyl radicals at their respective melting points, a situation that also prevails for the proton Hfcc of -CH 2 Mu in the sec-butyl radical from 1-butene ( Figure 3), and for the same group in the muoniated ethyl radical. 11,14 4. ASPECTS OF THEORY 4.1.…”

“…6 The μ þ is produced 100% spin polarized at a nuclear accelerator (TRIUMF in the present study) and this polarization can be effectively transfered to a free radical by Mu addition reactions, and then sensitively monitored by the μSR (muon spin relaxation or resonance) technique. [6][7][8][9][10][11][12][13][14] The most extensive studies of Hfcc in alkyl radicals to date has been for the ethyl radical and its isotopomers, both by EPR 1,5,12,15 and μSR 12,14 (see also citations in ref 11). Of relevance here are the torsional barriers to internal rotation.…”