B. Chiavarino ; O. Dopfer ; ME. Crestoni ; D. Corinti ; P. Maitre ; S. Fornarini
Chemphyschem 21 749-761 (2020). DOI

Travail réalisé sur le site de l’Université Paris-Saclay.

Abstract

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multi photon dissociation (IRMPD) spectra of protonated ortho, meta, and para-hydroxybenzaldehydes (OH-BZH+), delivered into the gas phase by electrospray ionization of hydro-alcoholic solutions, are reported in the 3200-3700 cm-1 spectral range. This range is characteristic of O-H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6-311++G(2df2p) level suggests that for both p-OH-BZH+ and m-OH-BZH+ only cis conformers are present in the ion population analyzed. For o-OH-BZH+, IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformer. The energy barrier for cis-trans isomerization calculated for each OH-BZH+ isomer is a measure of the degree of pi-electron delocalization. As further inquiry, IRMPD spectra of both p-OH-BZH+ m-OH-BZH+ and protonated phenol, (this last used as reference) were recorded also in the fingerprint range. Both the observed C-O and O-H stretching vibrations appears to be a measure of pi-electron delocalization in the ions.