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

摘要

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 C6H6Mu with sodium cations in NaY is indicated and leads to significant distortion of the C6H6Mu 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 Bronsted 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 C6H6Mu, in marked contrast to the nearly static widths seen in NaY up to 470 K.