On the formation of C2H5O2+ ions having the structure of hydroxy-protonated acetic acid

Probing Elusive Cations: Infrared Spectroscopy of Protonated Acetic Acid

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.

The water elimination from the ethyl acetate radical cation — Answers from theory to a longstanding mechanistic problem

Previous experimental studies of the water elimination from ionized ethyl acetate (EA-1) and its enol, CH2=C(OH)OC2H5·+ (EA-2), provide no mechanistic proposals, but note that the reaction is complex because all hydrogen atoms and both oxygen atoms participate. Loss of H2O is the predominant process observed for the metastable ions, yielding ionized methyl vinyl ketone, CH3C(=O)CH=CH2·+ (MVK), as the product ion. Metastable keto-alcohol ions CH3C(=O)CH2CH2OH·+ (HB-1) also abundantly lose H2O, yielding MVK at the thermochemical threshold. Using the CBS-QB3 model chemistry and complementary RRKM calculations, we have examined plausible mechanisms for the water elimination from metastable ions EA-1/2 and HB-1. Our calculations support the proposal that (i) ions HB-1 dissociate into MVK at the thermochemical threshold via a 1,4-H shift and a consecutive [1,2]-hydroxycarbene shift, and (ii) ions EA-1/2 undergo a three-step isomerization reaction into HB-1, which serves as the precursor for the ensuing water elimination. It is further shown that synergy between theory and experiment leads to a complex, yet transparent, mechanistic picture of the isomerization and dissociation behaviour of ionized ethyl acetate, which accounts for the labelling results.Key words: tandem mass spectrometry, CBS-QB3 model chemistry, reaction mechanism, hydroxycarbene shift.