Unstable intermediates. Part 187. The structure of β-bromoalkyl radicals: an electron spin resonance study of the radiolysis of iso- and t-butyl bromides

Iso- and t-butyl bromides have been exposed to 60Co γ-rays in a range of matrices at 77 K and the radical products studied by e.s.r. spectroscopy. Two quite different species have previously been detected in such systems. One (species S), having a very large hyperfine coupling to 79Br and 81Br, was identified as Me2Ċ–CH2Br in which the bromine is out of the radical plane to maximise hyperconjugative overlap. The other (species W), also identified originally as Me2Ċ–CH2Br, had a very small coupling to 79Br and 81Br (ca. 6 G) and it was concluded that the bromine atom is twisted ca. 75° away from the maximum overlap position and that the radical is strongly pyramidal at trigonal carbon. We find that after radiolysis at 77 K, Me2CHCH2Br in adamantane gives a novel species identified as the Me2CHCH2· radical interacting weakly with the ejected bromide ion (Me2CHCH2·, Br– adduct). On warming to ca. 140–170 K this species was lost irreversibly and species W was detected. The spectrum for W comprises a 10 × 4 group of hyperfine features which we assign to the Me3C·, Br– adduct. On cooling to 77 K the spectrum for W becomes much more complicated because the protons become inequivalent. At ca. 4 K the 79Br and 81Br A values become anisotropic, the data being compatible with those previously assigned to such adduct radicals. Species S is also formed from Me2CHCH2Br in adamantane, showing that S and W cannot be the same species, modified by different environments, as has been suggested. Me3CBr gave species W directly at 77 K, together with species S and a minor production of Me3C· radicals. Thus W is a primary product in this case. Using tetramethyl-p-phenylenediamine to generate photoelectrons we have established that W is formed by electron addition, in accord with the formulation Me3C·,Br–. By treating Me3CBr with Me3CO· radicals formed photochemically, we have shown that species S is formed by hydrogen atom abstraction. We conclude that species S is Me2ĊCH2Br, and that the structure of this radical is comparable with that of Me2ĊCH2Cl, the halogen atoms being in the maximum σ–π overlap position.