On the structure of the C2H4O2 neutrals (acetic acid versus 1,1-dihydroxyethene) generated from ionized n-hexanoic acid and n-butyl acetate in the gas phase. Implications for the mechanism of the McLafferty rearrangement

Results are reported of CIDI and NRMS experiments which show that the C2H4O2 neutral co-generated with C4H+8 from the metastable n-hexanoic acid ion and the n-butyl acetate molecular ion is acetic acid and not its stable enol, CH2C(OH)2. This is in marked contrast to the structure of the C2H4O+2 ion, co-generated with C4H8 from the metastable n-hexanoic acid ion, which was earlier shown to be exclusively the enol form of acetic acid. The implication of this and earlier findings for the mechanism of McLafferty-type processes is discussed and it is suggested that this ubiquitous reaction is more complex than hitherto expected. The reaction may well proceed via long-lived ion/dipole or hydrogen-bridged intermediates.