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

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.