Radical cations of diesters: generation by irradiation and detection by electron spin resonance

Abstract

Exposure of dilute solutions of a range of dimethyl diesters in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave the corresponding radical cations. For the oxalate, malonate, and succinate esters the primary cations were detected. This is in contrast with results for various monoesters, which generally gave alkylradical-derived cations formed by hydrogen-atom migration to the carbonyl oxygen. The primary diester cations are characterised by fairly large positive g-shifts, small proton hyperfine splittings and, in some cases, intense violet colours. Rearranged products were detected from the malonate and succinate esters on annealing above 77 K.

The extra stability of the parent cations is attributed to delocalisation between the two carbonyl groups, which confers extra stability on the primary cations and inhibits hydrogen-atom transfer. For the malonate and succinate cations, weak bonding between two adjacent carbonyl oxygen atoms is proposed.

Dimethyl maleate and fumarate cations have also been studied. The former was also violet and had a triplet spectrum similar to that for the succinate. However, the latter, which cannot readily form an oxygen–oxygen bond, was colourless, the e.s.r. spectrum comprising an anisotropic triplet characteristic of an H₂ĊO–R radical formed by hydrogen-atom transfer. Thus, in this case, it seems that the primary cation has a localised SOMO, facilitating rearrangement.