Carbon-13 hyperfine splittings of normal and abnormal methyl radicals trapped on the porous Vycor glass surface

To clarify the electronic structure and motional state of the abnormal methyl radical, we studied the ¹³C‐hf splittings of normal and abnormal methyl radicals photoproduced and stabilized on the surface of porous Vycor glass, as well as that of the methyl radical‐iodide ion adduct prepared by γ‐irradiation of ¹³CH₃I in a CD₃CN matrix at 77 K. The ESR spectra of all the methyl radicals quoted above exhibited axially symmetric features with respect to the ¹³C‐hf (and ¹²⁷I‐hf) tensors at low temperatures. A significant decrease in the isotropic ¹³C‐hf constant of the abnormal methyl radical compared with that of the normal one is interpreted in terms of the spin‐delocalization onto the oxygen atom of the surface siloxane bridge, keeping its structure planar. The fact that the ¹³C‐hf tensors of all the methyl radicals show a trend of $A_{\parallel }{\mathrm{\hspace{0.17em}(}}^{13}\mathrm{C}{\mathrm{)\; >\; A}}_{\perp }{\mathrm{\hspace{0.17em}(}}^{13}\mathrm{C})$ implies that the normal and abnormal methyl radicals interact with the surface via the 2pπ unpaired‐spin orbital and rotate around the C₃ axis. The temperature dependence of the isotropic ¹³C‐hf splitting of the normal methyl radical was also reexamined in the temperature range 77–300 K, and, accordingly, the result was found to agree well with the theoretical calculation by Schrader based on the extra coupling due to the out‐of‐plane vibration.