UCLA

A first complete μSR study of the T dependences of the (reduced) muon, Aμ′(T), and proton, Ap(T), β-hyperfine coupling constants (hfcc) of the muoniated t-butyl radical is reported in the faujasitic zeolites NaY and USY, and the results are compared with similar data and with early EPR results in condensed bulk phases. The results are also compared with single-molecule UMP2 and DFT/B3LYP calculations in the bulk and in an NaY zeolite fragment of Si and O atoms with both OH- and H-capping. Muon hfcc are reported for the first time for the Mu-isobutyl radical in the bulk phase and are also compared with theory and with EPR data. The present results for the muon and proton hfcc of Mu-t-butyl in the bulk complement earlier work published elsewhere at higher temperatures but are extended here down to 5 K to facilitate comparisons with in vacuo theory at 0 K. Good fits to the data for both Aμ′(T) and Ap(T) for Mu-t-butyl are found from the calculated hfcc in both the bulk and in NaY, assuming a Boltzmann-weighted energy dependence given by a simple twofold torsional potential, providing an estimate of the barrier to internal rotation. In contrast to the bulk data, there is no clear discontinuity seen in Aμ′(T) for Mu-t-butyl in NaY or USY at the melting point of isobutene, demonstrating the dominance of single-molecule guest-host interactions in the faujasite supercage. In contrast to the Aμ′(T) dependence in the bulk, there is no discontinuity seen for either of the proton hfcc, Ap,CH3(T) or Ap,CH2Mu(T), at the melting point, which also exhibit similar behavior in NaY, suggesting that its observation in the bulk for only the muon hfcc arises from a specific effect of the intermolecular interactions on the vibrational averaging of the muon hfcc. The measured muon hfcc for Mu-t-butyl in NaY fall below those in the bulk at low temperatures, indicating some transfer of electron spin density to the Na cation, which is confirmed by specific additional level-crossing resonances not observed in USY. The Na nuclear hfcc, ANa(T), follow a similar trend with temperature as Aμ′(T), with an estimate of the Na spin density at 0 K that also agrees well with theory. © 2013 American Chemical Society.