Neutron Diffraction Study of Indole Solvation in Deep Eutectic Systems of Choline Chloride, Malic Acid, and Water

Oliver S. Hammond, Ria Atri, Daniel T. Bowron, Karen J. Edler

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6 Citations (SciVal)


Deep eutectic systems are currently under intense investigation to replace traditional organic solvents in a range of syntheses. Here, indole in choline chloride-malic acid deep eutectic solvent (DES) was studied as a function of water content, to identify solute interactions with the DES which affect heterocycle reactivity and selectivity, and as a proxy for biomolecule solvation. Empirical Potential Structure Refinement models of neutron diffraction data showed [Cholinium]+ cations associate strongly with the indole π-system due to electrostatics, whereas malic acid is only weakly associated. Trace water is sequestered into the DES and does not interact strongly with indole. When water is added to the DES, it does not interact with the indole π-system but is exclusively in-plane with the heterocyclic rings, forming strong H-bonds with the -NH group, and also weak H-bonds and thus prominent hydrophobic hydration of the indole aromatic region, which could direct selectivity in reactions.

Original languageEnglish
Article numbere202200566
JournalChemistry - A European Journal
Issue number41
Early online date13 Jun 2022
Publication statusPublished - 20 Jul 2022

Bibliographical note

Funding Information:
The authors thank the EPSRC for a PhD studentship in the CSCT CDT at the University of Bath for O.S.H and R.A. (EP/L016354/1). ISIS Neutron & Muon Source are thanked for co-funding O.S.H. (STFC Studentship Agreement 3578) and neutron diffraction access and support on NIMROD under beamtime allocation RB1720287. Raw experimental neutron diffraction data are available through ISIS-ICAT, DOI: 10.5286/ISIS.E.RB1720287. Processed diffraction datasets are available through the University of Bath research data archive system, DOI:

Publisher Copyright:
© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.


  • deep eutectic solvents
  • green chemistry
  • heterocycles
  • neutron diffraction
  • pi interactions

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry


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