Electron spin resonance spectra of carboxylic acid radical anions at 77°K

Abstract

Sodium atoms have been deposited on a series of monocarboxylic acids, R—COOH, at 77° K in the rotating cryostat and the deposits examined by electron spin resonance (e.s.r.) spectroscopy. The spectra show that the primary paramagnetic species in each case is the corresponding radical O^– anion, [graphic omitted]. The spectra are similar to those observed for the primary radicals formed by X-, β- or γ- irradiation of mono-or di-carboxylic acids at 77° K and thus confirm that these radicals are the radical anions, and not cations, of the corresponding acids. The carbon-13 hyperfine coupling constants have been determined for the radical anion, [graphic omitted], of fully deuterated acetic acid. The results show that the free valence centre has a pyramidal configuration, in contrast to the planar structure found in many alkyl radicals.

The radical anions are able to abstract hydrogen from adjacent neutral molecules at 77° K to give secondary radicals. The extent of reaction depends on the strength of the CH bonds in the acid. Negligible reaction occurs with acetic acid which possesses only primary hydrogens, whereas the radical anion reacts completely with the tertiary hydrogen in isobutyric acid to give the radical (CH₃)₂CCOOH.