σ*-Radicals R3N — hal: electron paramagnetic resonance studies of the radiolysis of alkylammonium halides

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Abstract
Exposure of various ammonium and alkylammonium salts to 60Co γ-rays at 77 K gives a range of paramagnetic products including several R3N [graphic omitted] hal radicals (R = H, alkyl; hal = Cl, Br, l) characterised by their EPR spectra. These showed large hyperfine couplings to 14N and to halogen nuclei, indicating that they are genuine neutral radicals whose SOMO comprises, primarily, σ-orbitals from N and hal. Hence these species can be classified as σ* radicals. The stability of these R3N [graphic omitted] hal radicals is contrasted with the instability of the isoelectronic R3C [graphic omitted] hal– radical-anions, which normally have no significant lifetime, but dissociate directly to give R3C˙ radicals sometimes weakly associated with the ejected halide ions. Proton and 14N hyperfine coupling constants show that the R3N— moieties in these σ* radicals are pyramidal, in contrast with the parent amminium cations, R3N˙+ which are planar. The extent of deviation from planarity increased on going from —Cl to —l, as does the extent of delocalisation onto halogen. For the iodide complexes, the spin density on iodine decreases from ca. 70% to ca. 40% on going from the NH3 derivative to the Et3N derivative. A range of other radicals, also detected in the irradiated salts, are briefly described.

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