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Probing elusive cations: infrared spectroscopy of protonated acetic acid

Davies, Julia A.; Besley, Nicholas A.; Yang, Shengfu; Ellis, Andrew M.


Julia A. Davies

Nicholas A. Besley

Shengfu Yang

Andrew M. Ellis


Protonated carboxylic acids, (RCOOH)H+, are the initial intermediates in acid-catalyzed (Fischer) esterification reactions. However the identity of the isomeric form is under debate. 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 are debated in Fischer esterification. 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.


Davies, J. A., Besley, N. A., Yang, S., & Ellis, A. M. (2019). Probing elusive cations: infrared spectroscopy of protonated acetic acid. Journal of Physical Chemistry Letters, 10(9), 2108-2112.

Journal Article Type Article
Acceptance Date Apr 11, 2019
Online Publication Date Apr 11, 2019
Publication Date Apr 11, 2019
Deposit Date Apr 18, 2019
Publicly Available Date Apr 12, 2020
Journal Journal of Physical Chemistry Letters
Print ISSN 1948-7185
Electronic ISSN 1948-7185
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
Volume 10
Issue 9
Pages 2108-2112
Keywords General Materials Science
Public URL
Publisher URL
Additional Information This document is the unedited Author’s version of a submitted Work that was subsequently accepted for publication in Journal of Physical Chemistry Letters, copyright © American Chemical Society after peer review. To access the final edited and published work see


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