Kinetic energy release during CO loss by rearrangement of α-benzoylcarbenium ions

Primary α-benzoylcarbenium ions (a) and tertiary α-benzoyldimethylcarbenium ions (b) are obtained by chemical ionization of ω-hydroxyacetophenone and its dimethyl derivative, respectively. Both α-acylcarbenium ions decompose by a rearrangement reaction and subsequent loss of a CO molecule. The kinetic energy released during this process by metastable ions a and b has been determined under different experimental conditions. The kinetic energy released during the CO elimination from the tertiary α-benzoyldimethylcarbenium ions b is independent of the experimental conditions and gives rise to dish-topped peaks with T₅₀=440±20 meV in the MIKE spectra of b. In contrast to this the kinetic energy releases and the peak-shapes in the MIKE spectra of the primary α-benzoylcarbenium ion a varies with the experimental conditions. The mechanism of this rearrangement reaction is discussed, and it is shown by a MNDO calculation of the heats of formation of the relevant ions that the different characteristics of the kinetic energy release during the fragmentation of primary and tertiary carbenium ions can be attributed to different types of reaction energy profiles.