Radical-cations of aldehydes and ketones generated by ionizing radiation: an electron spin resonance study

Abstract

Exposure of dilute solutions of various aldehydes and ketones in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave, in most cases, the corresponding cations, identified by e.s.r. spectroscopy. The acetaldehyde cation is characterized by a very large proton hyperfine coupling, the SOMO being the in-plane, formally non-bonding, p-orbital on oxygen. At 77 K there was a weak interaction with one chlorine atom of a solvent molecule, but this was lost reversibly on annealing to ca. 120 K. That the quartet splitting was due to chlorine rather than to the methyl protons as had been previously supposed was established by studying the (CD₃CDO)⁺ cation. The acetone cation spectrum comprised a single asymmetric feature with no evidence of solvent interaction or hyperfine coupling to the γ-protons. In contrast, aldehyde and ketone cations with δ-protons showed hyperfine coupling in the 10–30 G range to conformationally selected protons. For methyl groups, one proton only is selected, the coupling being lost reversibly on annealing. These findings are semi-quantitatively supported by INDO calculations.