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

## TL;DR

This study explores how protonated alcohols fragment under electric fields in mass spectrometers and reveals a stabilizing effect from water clusters.

## Contribution

The study identifies a protonated cyclopropane intermediate and shows how water clusters stabilize protonated alcohols during field-induced fragmentation.

## Key findings

- Fragmentation of protonated alcohols occurs via a protonated cyclopropane intermediate.
- Primary alcohols undergo intramolecular SN2 reactions facilitated by the intermediate.
- Water clusters can stabilize protonated alcohols and prevent fragmentation.

## Abstract

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 SN2 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.