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

Abstract

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.