Probing the Hydroxyl in [2,2,2]-Cryptand/KF/Choline Chloride Non-Covalent Host-Guest Complex in Solution by 19F-NMR Spectroscopy: Structural Correlation and Thermodynamic Reversal of Host-Guest Configurations in Solution and in the Gas Phase

Abstract

We study the structures of the [2,2,2]-cryptand/KF/choline chloride ([2,2,2]/KF/ChCl) non-covalent host-guest complex in solution using ¹⁹F-NMR spectroscopy. We examine the environment around the hydroxyl group in the ChCl guest, to determine whether the functional group forms a hydrogen bond with the [2,2,2]/KF host or interacts with solvent molecules (dimethyl sulfoxide (DMSO), ethylene glycol (EG)). We find that the ¹⁹F peaks are characteristic of the structural features of the complex, showing that the –OH group in the ChCl guest is solvated away from the [2,2,2]/KF host in EG-d₆, and that it forms a hydrogen bond with F^- in DMSO-d₆ isolated from the solvent. By carrying out density functional theory (DFT) calculations, we find that a gas-phase [2,2,2]/KF/Ch⁺ conformer (with a ‘naked –OH group) of higher Gibbs free energy (by 13.6 kcal/mol) is structurally connected to the solution-phase configuration, where the –OH forms H-bonds with EG-d₆ molecules. On the other hand, the global minimum Gibbs energy gas-phase complex (featuring the [F^-…HO-] H-bond) is structurally correlated with the configuration observed in DMSO-d₆. These observations of ‘thermodynamic reversal’ and structural correlation of the [2,2,2]/KF/ChCl complexes in the two phases, connected by the electrospray ionization/mass spectroscopy (ESI/MS) procedure, are discussed in relation to the potential for explicating host-guest-solvent interactions in solution from gas-phase host - guest configurations.