Probing the Reactivity Limits of Microdroplets via Thermometer Ions: A Case of Unimolecular Dissociation

Recent studies have demonstrated that chemical reactions that are considered unlikely to happen in bulk water can occur spontaneously in microdroplets. This phenomenon raises fundamental questions: to what extent can this reactivity be enhanced, and how much energy can microdroplets impart to the reactants? To address this, we used high-resolution mass spectrometry to investigate the direct C-N bond cleavage of benzylamines in aqueous microdroplets. Our results reveal that ions formed in microdroplets possess exceptionally high internal energy, which facilitates facile C-N bond cleavage. Using the survival yield method, a mean internal energy of 93 kJ/mol for these reactive ions is determined. Further analysis highlighted key interfacial effects responsible for this reactivity. The superacidic interface promotes precursor protonation, making the heterolytic cleavage of the C-N bond favorable. Moreover, based on DFT calculations, it is proposed that the interfacial electric field induces C-N bond cleavage in protonated benzylamines. These findings deepen our understanding of the capability of microdroplets to make "impossible" reactions possible from an energy scale.