Comprehensive quantum chemical and mass spectrometric analysis of the McLafferty rearrangement of methyl valerate.

The McLafferty rearrangement (McLR) of the methyl valerate molecular ion has been comprehensively studied from the standpoints of the timescale for the keto-enol transformation and the change of the configuration of intermediates and transition state (TS), using mass spectrometry with electron ionization, strong-field tunnel ionization and collision-induced dissociation methods, and the global reaction route mapping (GRRM) program with quantum chemical calculations (QCCs). The timescales estimated from mass spectrometric results suggested that the McLR starts at 100 fs after ionization and is completed at least within 100 ns in the ion source. Whereas the timescales are consistent with a stepwise mechanism of fast (100 fs) and slow (10 ps) steps presented by Stamm , the QCCs put forth the possibility that an unanticipated, rapid, concerted process may be involved in completing the McLR reaction. It is worth noting that there appears to be a concerted process as a potential direct route to form the McLR fragments, while the slower step seems to involve a dynamic rearrangement in the configuration of the propene moiety in the TS of the molecular ion.