dc.description |
Exposure of dilute solutions of chloromethane in [<sup>2</sup>H3]cyanomethane to <sup>60</sup>Co γ-rays at 77 K gave .rad;CH3â â â Cl<sup>â </sup> adducts by electron capture. These were characterised by their e.s.r. spectra, which showed a slightly reduced proton hyperfine coupling (â 22 G) and a clear quartet splitting from <sup>35</sup>Cl and <sup>37</sup>Cl [A â ¥â 4 G and A â ¥â (â )2 G]. On annealing, normal methyl radicals were formed irreversibly.Also, t-butyl chloride in tetramethylsilane or adamantane matrices gave two types of rotating t-butyl radicals, one having A(<sup>1</sup>H)= 22.7 G and the other having A(<sup>1</sup>H)= 21.1 G. The former is certainly normal t-butyl radicals and the latter, converted irreversibly to the former on annealing to ca. 180 K, is assigned to chloride-ion adducts with negligibly small isotropic coupling to chlorine. These features were broad at 77 K but did not exhibit well defined anisotropic splitting from chlorine nuclei.We conclude that, as with alkyl bromides and iodides, halide-ion adducts are formed on electron capture and that these are best viewed as â collision complexesâ or charge-transfer complexes, held together by the rigid matrices. They are not properly described as radical anions, and in our view their ready formation and low-temperature stability precludes the possibility that true radical anions are formed in condensed phases.The results show that the extent of charge transfer (ca. 4%) is less than that deduced for R.rad;Br<sup>â </sup> adducts (ca. 10%) or R.rad;I<sup>â </sup> adducts (ca. 17%). The trend follows the ionization potentials of the halide ions, as expected.These results are also compared with those for the isoelectronic radicals H3N[graphic omitted]Cl, H3N[graphic omitted]Br and H3N[graphic omitted]I, which are clearly Ï <sup>*</sup> radicals rather than being halide-ion complexes of H3N<sup>.rad;+</sup> radicals. |