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Carbon Dioxide in 1-Butyl-3-methylimidazoliumAcetate.I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations.

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Title: Carbon Dioxide in 1-Butyl-3-methylimidazoliumAcetate.I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations.
Authors: Cabaço, M. Isabel1, Besnard, M.1, Danten, Y.1, Coutinho, J. A.P.1
Source: Journal of Physical Chemistry A. Feb2012, Vol. 116 Issue 6, p1605-1620. 16p.
Subjects: Carbon dioxide, Solubility, Aromatic compounds, Raman spectroscopy, Carboxylation, Chemical reactions, Density functionals
Abstract: The unusual solubility of carbon dioxide in 1-butyl-3-methylimidazoliumacetate (Bmim Ac) has been studied by Raman spectroscopy and DFT calculations.It is shown that the solubility results from the existence of twodistinct solvation regimes. In the first one (CO2molefraction ≤ 0.35), the usual Fermi dyad is not observed, a factnever reported before for binary mixtures with organic liquids orionic liquids (IL). Strong experimental evidence complemented by effectiveDFT modeling shows that this regime is dominated by a chemical reactionleading to the carboxylation of the imidazolium ring accompanied byacetic acid formation. The reactive scheme proposed involves two concertedmechanisms, which are a proton exchange process between the imidazoliumcation and the acetate anion and the carboxylation process itselfinitiated from the formation of “transient” CO2–1-butyl-3-methylimidazole 2-ylidene carbene species. In thatsense, CO2triggers the carboxylation reaction. Moreover,this dynamic picture circumvents consideration of a long-lived carbeneformation in dense phase. The second regime is characterized by thedetection of the CO2Fermi dyad showing that the carboxylationreaction has been strongly moderated. This finding has been interpretedas due to the interaction of the acetic acid molecules with the COOgroup of acetate anions involved in monodentate forms with the cation.The observation of the Fermi doublet allows us to infer that CO2essentially preserves its linear geometry and that the natureand strength of the interactions with its environment should be comparableto those existing in organic liquids and other IL as well. These resultshave been supported by DFT calculations showing that the CO2molecule interacts with energetically equivalent coexisting structuresand that its geometry departs only slightly from the linearity. Finally,we find that the CO2solvation in Bmim Ac and 1-butyl-3-methylimidazoliumtrifluoroacetate (Bmim TFA) cannot be straightforwardly compared neitherin the first regime due to the existence of a chemical reaction norin the second regime because CO2interacts with a varietyof environments not only consisting of ions pairs like in Bmim TFAbut also with carboxylate and acetic acid molecule. [ABSTRACT FROM AUTHOR]
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Abstract:The unusual solubility of carbon dioxide in 1-butyl-3-methylimidazoliumacetate (Bmim Ac) has been studied by Raman spectroscopy and DFT calculations.It is shown that the solubility results from the existence of twodistinct solvation regimes. In the first one (CO2molefraction ≤ 0.35), the usual Fermi dyad is not observed, a factnever reported before for binary mixtures with organic liquids orionic liquids (IL). Strong experimental evidence complemented by effectiveDFT modeling shows that this regime is dominated by a chemical reactionleading to the carboxylation of the imidazolium ring accompanied byacetic acid formation. The reactive scheme proposed involves two concertedmechanisms, which are a proton exchange process between the imidazoliumcation and the acetate anion and the carboxylation process itselfinitiated from the formation of “transient” CO2–1-butyl-3-methylimidazole 2-ylidene carbene species. In thatsense, CO2triggers the carboxylation reaction. Moreover,this dynamic picture circumvents consideration of a long-lived carbeneformation in dense phase. The second regime is characterized by thedetection of the CO2Fermi dyad showing that the carboxylationreaction has been strongly moderated. This finding has been interpretedas due to the interaction of the acetic acid molecules with the COOgroup of acetate anions involved in monodentate forms with the cation.The observation of the Fermi doublet allows us to infer that CO2essentially preserves its linear geometry and that the natureand strength of the interactions with its environment should be comparableto those existing in organic liquids and other IL as well. These resultshave been supported by DFT calculations showing that the CO2molecule interacts with energetically equivalent coexisting structuresand that its geometry departs only slightly from the linearity. Finally,we find that the CO2solvation in Bmim Ac and 1-butyl-3-methylimidazoliumtrifluoroacetate (Bmim TFA) cannot be straightforwardly compared neitherin the first regime due to the existence of a chemical reaction norin the second regime because CO2interacts with a varietyof environments not only consisting of ions pairs like in Bmim TFAbut also with carboxylate and acetic acid molecule. [ABSTRACT FROM AUTHOR]
ISSN:10895639
DOI:10.1021/jp211211n