The Surprising Dynamics of the McLafferty Rearrangement

The McLafferty rearrangement in radical cations is a well-known reaction that occurs in mass spectrometry. In this rearrangement, a γ-hydrogen transfers to a double-bonded atom through a six-membered transition state. Subsequently, the β-carbon bond breaks, resulting in π-electron rearrangement and the formation of a neutral olefin as well as a positive charged enol. However, the timescale of this reaction was not known, and literature supported a stepwise or concerted mechanism. This work used disruptive probing following strong-field tunnel ionization to find the timescale of the McLafferty rearrangement for 2-pentanone, 4-methyl-2-pentanone, and 4,4 dimethyl-2-pentanone. The transient ion yield of the McLafferty product, m/z 58, fits to a biexponential function, with a fast (~100 fs) and slow (~10 ps) component, thus this study supports the stepwise nature of the rearrangement. Furthermore, the fast timescale is attributed to the internal rotation of the molecule and proton transfer. The slow timescale, the π-electron rearrangement, is surprising. If this step only required electron motion, it would be orders of magnitude faster. We find that this step requires extensive exploration of internal degrees of freedom to attain the molecular geometry necessary for electron transfer. These findings were corroborated with ab initio molecular dynamic simulations.