Gas phase proton affinities of carbonyl compounds by ion cyclotron resonance spectroscopy

Abstract

Pressure studies and the double resonance technique of ion cyclotron resonance spectroscopy have been used to establish the following orders of proton affinities for carbonyl compounds in the gas phase: HCOOH < HCOOCH₃ < CH₃COOH < HCOOC₂H₅ < CH₃COOCH₃≈ CH₃COCH₃ < CH₃COOC₂H₅; CH₃COCH₃ < CH₃COC₂H₅ < C₂H₅COC₂H₅. The order of proton affinities in these compounds is shown to follow the decrease in ionisation potential of homologous compounds and the inductive effect of alkyl substituents. An ester and its corresponding thioester have been found to have approximately equal proton affinity.