An ESR investigation of ester .pi.-cation radicals in a Freon matrix at low temperatures: evidence for unusual barriers to methyl group rotation and intramolecular bonding

Abstract

..gamma..-Irradiation of dilute solutions of a number of esters in CFCl/sub 3/ at 77 K is shown by ESR spectroscopy to produce the ..pi..-cation radicals of these molecules. The ..pi..-cations formed are those of the methyl and ethyl formates, acetates, and propionates. Larger esters containing propyl and butyl side chains are suggested to be unstable toward deprotonation at 77 K. Synthesis of deuterium-labeled compounds was employed to assign proton hyperfine couplings to specific sites in the ..pi..-cation radicals. The ..pi..-cations of the ethyl esters show unusually large couplings to the terminal methyl groups. In addition, large barriers to methyl group rotation are found for both the methyl and ethyl ester cation radicals. The large barriers suggest intermolecular bonding between the alkyl group and the carbonyl oxygen in the ester functional group. Such a hypothesis is supported by the finding of a torsional motion in the terminal methyl group of ethyl formate ..pi..-cation which has a ca. 1.7 kcal/mol activation energy barrier. It is suggested that a new intramolecular ..pi..*-bond is responsible for the formation of cyclic rings in the methyl- and ethyl-substituted ester cation radicals.