Unimolecular dissociation of energy-selected ethyl formate ions

Abstract

The unimolecular dissociation of ethyl formate ions has been investigated using threshold photoelectron photoion coincidence mass spectrometry. The fragment ions appearing at low internal energies are C 2H 4CO + and C 2H 4 ·+, whose appearance potentials are higher than the ionization energy of the ethyl formate molecule by 0.18 eV and 0.28 eV respectively. Observed decay rates for the formation of C 2H 4CO + were found to be different to those of C 2H 4 ·+, although the direct fragmentation mechanism indicates that they should be identical. Furthermore, the rate constants predicted by RRKM/QET calculations are 10 4–10 5 times higher than the observed rate constants for both ions. The different decay rates probably result from different isomerization processes. One of the reaction channels is a stepwise McLafferty rearrangement involving a distonic intermediate, with charge retention on the olefin.