Abstract
Protonated carboxylic acids, (RCOOH)H+, are the initial intermediates in acid-catalyzed (Fischer) esterification reactions. However, the identity of the isomeric form has been debated. Surprisingly, no optical spectra have been reported for any isomer of the protonated carboxylic acid monomer, despite it being a fundamental organic cation. Here, we address these issues by using a new approach to prepare cold He-tagged cations of protonated acetic acid (AA), which entails electron ionization of helium nanodroplets containing metastable dimers of AA. The protonated species is subsequently probed using infrared photodissociation spectroscopy, and following a comparison with calculations, we identify the two isomers whose roles in Fischer esterification are debated. These are the carbonyl-protonated E, Z isomer and the metastable hydroxyl-protonated isomer. Our technique provides a novel approach that can be applied to other elusive ionic species.
Highlights
The mechanism of the acid-catalyzed (Fischer) esterification of carboxylic acids was first explored in detail in the 1930s
This work suggests that carbonyl protonation produces an intermediate which is too stable to react with an alcohol to yield an ester
We introduce an experimental technique for measuring IR spectra of He-tagged cations, which utilizes electron ionization of neutral clusters embedded in helium nanodroplets
Summary
The mechanism of the acid-catalyzed (Fischer) esterification of carboxylic acids was first explored in detail in the 1930s.1,2 This early mechanistic work suggested that initial protonation occurs at the hydroxyl oxygen atom. We introduce an experimental technique for measuring IR spectra of He-tagged cations, which utilizes electron ionization of neutral clusters embedded in helium nanodroplets.
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