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Characterization of a trans-trans Carbonic Acid-Fluoride Complex by Infrared Action Spectroscopy in Helium Nanodroplets

MPS-Authors
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Thomas,  Daniel
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Mucha,  Eike
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Lettow,  Maike
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Meijer,  Gerard
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Rossi,  Mariana
Theory, Fritz Haber Institute, Max Planck Society;

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Helden,  Gert von
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Citation

Thomas, D., Mucha, E., Lettow, M., Meijer, G., Rossi, M., & Helden, G. v. (2019). Characterization of a trans-trans Carbonic Acid-Fluoride Complex by Infrared Action Spectroscopy in Helium Nanodroplets. Journal of the American Chemical Society, 141(14), 5815-5823. doi:10.1021/jacs.8b13542.


Cite as: http://hdl.handle.net/21.11116/0000-0003-421F-1
Abstract
The high Lewis basicity and small ionic radius of fluoride promote the formation of strong ionic hydrogen bonds in the complexation of fluoride with protic molecules. Herein, we report that carbonic acid, a thermodynamically disfavored species challenging to investigate experimentally, forms a complex with fluoride in the gas phase. Intriguingly, this complex appears highly stable and is observed in abundance upon nanoelectrospray ionization of an aqueous sodium fluoride solution in the presence of gas-phase carbon dioxide. We characterize the structure and properties of the carbonic acid-fluoride complex, F-(H2CO3), and its deuterated isotopologue, F-(D2CO3), by helium nanodroplet infrared action spectroscopy in the photon-energy range of 390–2800 cm-1. The complex adopts a C2v-symmetry structure with the carbonic acid in a planar trans-trans conformation and both OH groups forming ionic hydrogen bonds with the fluoride. Substantial vibrational anharmonic effects are observed in the infrared spectra, most notably a strong blue-shift of the symmetric hydrogen stretching fundamental relative to predictions from the harmonic approximation or vibrational second-order perturbation theory. Ab initio thermostatted ring-polymer molecular dynamics simulations indicate that this blue-shift originates from strong coupling between the hydrogen stretching and bending vibrations, resulting in an effective weakening of the OH…F- ionic hydrogen bonds.