Radiolytically generated radical-cations of simple esters: an electron spin resonance study

Abstract

Exposure of dilute solutions of simple esters in fluorotrichloromethane to ⁶⁰Co γ-rays at 77 K gave the corresponding cations in several instances (methyl formate and acetate, ethyl formate and acetate). In all cases, the SOMO of the stable cation was π, with major spin density on the ester oxygen atom and low negative spin density on carbon. This result makes an intersting contrast with that from photoelectron spectroscopy. For the ethyl esters, a large hyperfine coupling (ca. 16 G) was observed for one of the γ-protons, in addition to the two β-protons (ca. 22 G). The first formed product obtained from methyl formate is thought to have the less stable σ-structure, comparable with those for ketones, stabilised by weak σ-bonding via the carbonyl oxygen to one chlorine atom in the solvent. This results in a large chlorine hyperfine splitting. This complex cation decomposed irreversibly on annealing to give the π-cation, with no chlorine hyperfine coupling. This result is discussed in terms of the mechanistic pathway leading to cation formation. The isopropyl and t-butyl esters gave unimolecular breakdown products, identified as the cation (H₂C[graphic ommitted]CMe₂)⁺ for t-butyl and as a mixture of (H₂C[graphic ommitted]CHMe)⁺ and Me₂ĊO(OH)CH₃⁺ for the isopropyl derivative. Acetates with more remote hydrogen atoms in the alkyl groups gave alkyl radicals thought to be formed by internal attack of the carbonyl oxygen, resulting in hydrogen-atom transfer.