Kinetic and Electron Impact Studies of the Hco⁺/COH⁺ Isomers

Abstract

The ionization efficiency curve for the CHO+ ion, m/z 29, formed from the near-monochromatic electron impact ionization of CH₃OH has been found to exhibit two distinct appearance energies (thresholds) which can be assigned to the formyl, HCO⁺, and the higher energy isoformyl, COH⁺, isomers. The energy separation between the appearance energies of 138 kJ mol^-1 provides an estimate of the difference between the enthalpies of formation of the formyl-isoformyl isomers. Acetaldehyde undergoes dissociative ionization to yield only the lower energy, formyl, isomer. This has been confirmed from a study of the energetics of the proton transfer reaction of CHO⁺ on N₂O over the centre-of-mass collision energy range 0.3-4 eV measured by using a drift-tube mass spectrometer.