Abstract
We present a computational analysis of the complex proton-transfer processes in two protic ionic liquids based on phosphorylated amino acid anions. The structure and the short time dynamics have been analyzed via ab initio and semi-empirical molecular dynamics. Given the presence of mobile protons on the side chain, such ionic liquids may represent a viable prototype of highly conductive ionic mediums. The results of our simulations are not entirely satisfactory in this respect. Our results indicate that conduction in these liquids may be limited due to a quick quenching of the proton-transfer processes. In particular, we have found that, while proton migration does occur on very short timescales, the amino groups act as proton scavengers preventing an efficient proton migration. Despite their limits as conductive mediums, we show that these ionic liquids possess an unconventional microscopic structure, where the anionic component is made by amino acid anions that the aforementioned proton transfer has transformed into zwitterionic isomers. This unusual chemical structure is relevant because of the recent use of amino acid-based ionic liquids, such as CO2 absorbent.
Highlights
One of the key features of protic ionic liquids (PILs) is the presence of mobile protons that can act as light and fast charge carriers
Further insights into the complexity of certain PILs has been obtained by us;[19−21] we have shown that proton transfer in fully ionized liquids can be mediated by the clustering of like-charge ions
We have decided to expand the range of compounds we have examined so far and explore, by means of molecular dynamics (MD) techniques, the proton-transfer dynamics in PILs, where the amino acid (AA) anions contain a phosphate group on the side chain
Summary
One of the key features of protic ionic liquids (PILs) is the presence of mobile protons that can act as light and fast charge carriers. We showed that it is possible to overcome this latter problem by identifying a subset of PILs, which possesses mobile protons, but in which the occurrence of a neutral moiety is only a transient event, which should not induce dramatic effects in the bulk structure Such ionic liquids are made by cholinium cations (hereafter [Ch]+) coupled to amino acid (AA) anions (general formula HR−CH(NH2)− COOH) that have a protic function on the side chain (−HR) with a different acidity with respect to the primary carboxylic function. The quenching of proton migration due to the amino group is less effective because the phosphate group has more than one proton that can act as a charge carrier in the fluid
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