SIGMA-STAR-RADICALS R3N-HAL - ELECTRON-PARAMAGNETIC RESONANCE STUDIES OF THE RADIOLYSIS OF ALKYLAMMONIUM HALIDES

Exposure of various ammonium and alkylammonium salts to Co-60 gamma-rays at 77 K gives a range of paramagnetic products including several R3N. hal radicals (R = H, alkyl; hal = CI, Br, I) characterised by their EPR spectra. These showed large hyperfine couplings to N-14 and to halogen nuclei, indicating that they are genuine neutral radicals whose SOMO comprises, primarily, sigma-orbitals from N and hal. Hence these species can be classified as sigma-* radicals. The stability of these R3N. hal radicals is contrasted with the instability of the isoelectronic R3C. 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 N-14 hyperfine coupling constants show that the R3N- moieties in these sigma-* radicals are pyramidal, in contrast with the parent amminium cations, R3N.+ which are planar. The extent of deviation from planarity increased on going from -CI to -I, 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.