Molecular interactions in 2-hydroxyethyl-trimethylammonium acetate (choline acetate) ion pair
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Ó Indian Academy of Sciences Sadhana (0123456789().,-volV)FT3](0123456 789().,-volV)
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Molecular interactions in 2-hydroxyethyl-trimethylammonium acetate (choline acetate) ion pair UJWALA N PATILa,b,* a Department
of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India address: Department of Chemistry, Fergusson College (Autonomous), Fergusson College Road, Shivajinagar, Pune 411 004, India E-mail: [email protected] b Present
MS received 23 January 2020; revised 11 May 2020; accepted 19 May 2020
Abstract. The molecular structure, electron density topography, vibrational frequencies and molecular interactions of the choline acetate ion pair were investigated using M05-2X based density functional approach. Different conformers of choline acetate ion pair based on molecular interactions were obtained in the gas phase and also in a solvent environment. The lowest energy conformer showed the presence of both O–HO and C–HO interactions. A strong binding between the cation and anion was observed in the lowest energy conformers through ion pairing free energies. The O–HO and C–HO interactions were weakened from the gas phase to the condensed phase. Keywords. Molecular graph; critical point; intermolecular interaction; frequency shift.
1. Introduction The ionic liquids (ILs) have several unique physicochemical properties, such as nonvolatility, nonflammability, thermal stability, recyclability and good solvating capabilities which shows that they can be useful in catalysis and electrochemistry.1,2 ILs are used in various fields, such as solar cells, sensors, thermal fluids and lubricants.3,4 Recent studies have shown that some ILs can be toxic5 and nonbiodegradable.6,7 Alternatives for toxic and nonbiodegradable ILs are composed of cations and anions based on biomolecules such as lactic acid,8 choline, etc.9,10 Choline acetate [ChOAc] is a completely bioderived IL. It is reported to be less expensive, more biocompatible, more biodegradable and biorenewable in comparison with the imidazolium ILs.11 Eutectic solvents derived from choline acetate or choline chloride with glycerol are reported as highly biodegradable with excellent compatibility with a commercially immobilized Candida antarctica lipase B.12 According to the Hofmeister series, the chaotropic choline cation when combined with acetate anion should stabilize the lipase.13 Ai Asakawa et al. used choline acetate IL in the pre-
treatment of sugarcane bagasse.14,15 Choline’s (2-hydroxyethyl-trimethylammonium) environment-friendly nature and its structural aspects, like lack of aromaticity, presence of hydroxyl hydrogen and reduced nonpolar character16 make it interesting for this study. Wilcox et al. studied the structural and dynamic properties of choline acetate using molecular dynamic simulations. They observed that the hydroxyl group of choline shows hydrogen bonding with oxygen atoms of acetate anion.16 To study the physicochemical properties of ILs composed of different cations and anions, we need to
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