The ¹³C hyperfine splittings in ESR spectra of prop-2-ynyl and 1-(trimethylsiloxy)prop-2-ynyl radicals

The ¹³C satellite lines were observed in the 9.4 GHz ESR spectra of prop-2-ynyl and 1-trimethylsiloxyprop-2-ynyl radicals generated from precursors containing ¹³C in natural abundance. The ¹³C hfs indicate that prop-2-ynyl radicals adopt the π-delocalized structure, not the σ-allenyl form. The hfs of 1-trimethylsiloxyprop-2-ynyl radicals indicate that there is appreciable delocalization of spin onto the trimethylsiloxy group and that the radical is essentially planar at C(1). A linear correlation of a(C) with a(H) is shown to hold for a number of neutral hydrocarbon π-delocalized radicals. The experimental hfs are compared with spin densities calculated by a variety of semiempirical and ab initio SCF MO methods.