1H NMR and thermodynamic study of self-association and complex formation equilibria by hydrogen bonding. Methanol with chloroform or halothane

Abstract

Mixtures of methanol with two strong proton donors, chloroform and halothane (2-bromo-2-chloro-1,1,1-trifluoroethane), were studied. The behaviour of these systems is governed by aggregate formation through H-bonding, where methanol self-association and its complex formation with the proton donors compete. In order to obtain information about these aggregate formation equilibria, ¹H NMR chemical shifts of the chloroform or halothane proton and of the hydroxy proton of methanol were measured as a function of concentration and temperature. The NMR data are expressed in the form of a new quantity, defined in this work, the relative change of the chemical shift. This quantity is convenient because it gives directly the extent of H-bonding without containing any NMR-specific parameter. The NMR data and the excess thermodynamic functions from the literature (G^E or ln γ_i, H^E and C_P^E) were analysed using simple models of athermal association, amended by physical or thermal terms estimated on the basis of coupled homomorph and solution-of-groups approaches. Three particular models were tested, two of continuous methanol association (from tetramers to infinite size species) and one model that considers only methanol tetramerization. For the three models, methanol self-association parameters were previously obtained from independent data. Using enthalpy of solvation values obtained from quantum mechanical calculations, the equilibrium constant for the formation of methanol–chloroform and methanol–halothane complexes was the only fitted parameter. The continuous association models failed to fit the present data even qualitatively, whereas the tetramerization model gave reasonable agreement with experiment both for NMR and the excess thermodynamic functions. In accordance with previously studied mixtures of chloroform and halothane with oxygenated compounds, the methanol–halothane complex is found to be stronger than the methanol–chloroform complex; this is due to a more acidic hydrogen atom in halothane than in chloroform.