Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ-irradiated 2-methyltetrahydrofuran glass

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Abstract
Three specifically deuterated 2‐methyltetrahydrofurans (MTHF) have been synthesized to delineate the proton (deuteron) arrangement and hence the molecular arrangement around a trapped excess electron in γ‐irradiated glassy MTHF at 77 K. The electron–deuteron anisotropic hyperfine interaction was detected by its modulation of the two‐pulse electron spin echo envelope. Comparison of experimental and simulated modulation patterns shows that the excess electron is located about 3.7 Å below the center of the plane of the ring carbons in MTHF on the side opposite the methyl group and that three or four MTHF molecules constitute the first solvation shell. A second moment analysis of the EPR linewidths of the excess electron in the variously deuterated MTHF’s independently supports the same structural conclusions. The shortest electron–proton distance is then ∼3.1 Å, which compares moderately well with previous, less accurate ENDOR results. This structure seems somewhat more expanded than that indicated by theoretical calculations based on the semicontinuum model, but does not seem incompatible with that theoretical model. A small positive isotropic coupling to the ring protons (∼1 MHz) is also indicated by the spin echo results.