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.