Exploring Field-Induced Fragmentation of Protonated Alcohols: Mechanistic Insights and Stabilizing Ion–Solvent Clusters

Field-induced ion activation in medium to high pressure regions of a mass spectrometer or ion mobility spectrometer can lead to changes in the ion structure, namely unfolding, tautomerization, or fragmentation. To either prevent mislabeling of spectra or utilize these effects efficiently, the underlying ion dynamics need to be understood. Hydroxyl-containing compounds in particular show significant fragmentation (loss of H₂O), yet the energetics and mechanisms are not well studied. This is particularly true for primary hydroxyl groups, as the presumably formed primary carbocations are highly instable. In this study, we investigate the dynamics of the field-induced fragmentation of protonated primary and secondary alcohols using a combined theoretical and experimental approach. Specifically, we combine density functional theory and reaction kinetics modeling with fragmentation measurements using a HiKE-IMS-MS and tandem IMS device. We find that the fragmentation mechanism of both primary and secondary protonated alcohols proceeds via a protonated cyclopropane (PCP⁺) moiety. Especially for primary alcohols, this moiety enables an intramolecular S_N2 reaction where the neutral H₂O at the terminal carbon is substituted by an H-shift, directly yielding a secondary carbocation. Our results suggest quite high fragmentation rates, even at moderate ion activations, rendering protonated alcohols very unstable. However, we also find that neutral background water can form ion–solvent clusters with the protonated alcohols that effectively prevent the fragmentation. This could also help stabilize other labile ions in the future.