Competing mechanisms for methyl cation formation upon collision-induced dissociation of protonated acetaldehyde

Isotopic labeling and energy-resolved collision-induced dissociation (CID) experiments show that methyl cation loss from protonated acetaldehyde occur s by at least two different mechanisms, one involving direct cleavage and t he second involving rearrangement. A statistical scrambling mechanism is ru led out from the CID appearance curves for the CH3+ and CH2D+ products from CH3C(OD)H+, whereas a possible kinetic isotope effect is ruled out by usin g the alternately labeled CD3C(OH)H+ acetaldehyde ion. The results show tha t dissociation of protonated aldehydes is even more complex than is suggest ed by standard CID spectra. (C) 2001 Elsevier Science B.V.