Trimethylglycine complexes with carboxylic acids and HF: solvation by a polar aprotic solvent

A series of strong H-bonded complexes of trimethylglycine, also known as betaine, with acetic, chloroacetic, dichloroacetic, trifluoroacetic and hydrofluoric acids as well as the homo-conjugated cation of betaine with trifluoroacetate as the counteranion were investigated by low-temperature (120–160 K) liquid-state NMR spectroscopy using CDF3/CDF2Cl mixture as the solvent. The temperature dependencies of 1H NMR chemical shifts are analyzed in terms of the solvent–solute interactions. The experimental data are explained assuming the combined action of two main effects. Firstly, the solvent ordering around the negatively charged OHX region of the complex (X = O, F) at low temperatures, which leads to a contraction and symmetrisation of the H-bond; this effect dominates for the homo-conjugated cation of betaine. Secondly, at low temperatures structures with a larger dipole moment are preferentially stabilized, an effect which dominates for the neutral betaine–acid complexes. The way this second contribution affects the H-bond geometry seems to depend on the proton position. For the Be+COO−⋯HOOCCH3 complex (Be = (CH3)3NCH2–) the proton displaces towards the hydrogen bond center (H-bond symmetrisation, O⋯O contraction). In contrast, for the Be+COOH⋯−OOCCF3 complex the proton shifts further away from the center, closer to the betaine moiety (H-bond asymmetrisation, O⋯O elongation). Hydrogen bond geometries and their changes upon lowering the temperature were estimated using previously published H-bond correlations.