A general, most basic rule for ion dissociation: Protonated molecules.

Area of Science:

• Mass Spectrometry
• Computational Chemistry
• Physical Chemistry

Background:

• Protonated molecules are common in mass spectrometry.
• The dissociation of excited protonated molecules is often assumed to involve the most stable protomer.

Purpose of the Study:

• To challenge the "most stable protomer" rule in ion chemistry.
• To introduce and validate the "most labile protomer" rule.
• To provide a new framework for understanding dissociation pathways of protonated molecules.

Main Methods:

• Electrospray Ionization-Tandem Mass Spectrometry (ESI(+)-MS/MS) on selected molecules.
• Density Functional Theory (DFT) calculations, specifically PM7.
• Potential energy surface diagrams construction.
• Reinterpretation of dissociation processes using the "most labile protomer" rule.

Main Results:

• Demonstrated that the "most labile protomer" dictates dissociation, not the most stable.
• Illustrated the mobile proton model's role in proton transfer and protomer formation.
• Showcased lower dissociation thresholds for less stable, more labile protomers.
• Successfully reinterpreted existing dissociation mechanisms.

Conclusions:

• The "most labile protomer" rule provides a more accurate model for protonated molecule dissociation.
• The mobile proton model is crucial for understanding ion fragmentation.
• This work necessitates a revision of how ion dissociation is predicted and rationalized in mass spectrometry.