Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids

Kundu, Kaushik and Chandra, Goutam K and Umapathy, Siva and Kiefer, Johannes (2019) Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids. In: PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 21 (37). pp. 20791-20804.

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Abstract

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate (Chl]Pro]) and the protic N-methyl-2-pyrrolidonium propionate (NMP]Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in Chl]Pro]-H2O systems compared to NMP]Pro]-H2O mixtures. This can be explained by the tightly packed ion-pair associations in NMP]Pro] comprising the protic -N+-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic NMP]Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic Chl]Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H2O center dot center dot center dotChl](+)center dot center dot center dot H2O center dot center dot center dotPro](-)center dot center dot center dot H2O and H2O center dot center dot center dotNMP](+)Pro](-)center dot center dot center dot H2O, which implied preferable solvent-shared ion-pair (SIP) configurations for Chl]Pro]-H2O systems, whereas the contact ion-pair (CIP) state prevails for the NMP]Pro]-H2O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

Item Type:	Journal Article
Publication:	PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Publisher:	ROYAL SOC CHEMISTRY
Additional Information:	Copyright of this article belongs to ROYAL SOC CHEMISTRY
Keywords:	MOLECULAR-INTERACTIONS; INTERMOLECULAR INTERACTIONS; N-METHYLPYRROLIDONE; HYDROGEN-BONDS; WATER; MIXTURES; CHOLINE; NMR; IR; TEMPERATURE
Department/Centre:	Division of Chemical Sciences > Inorganic & Physical Chemistry
Date Deposited:	28 Nov 2019 11:19
Last Modified:	28 Nov 2019 11:19
URI:	http://eprints.iisc.ac.in/id/eprint/63834

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