Hydrogen bonding in the gas phase. Part 2.—Determination of thermodynamic parameters for amine–methanol systems from pressure, volume, temperature measurements

Abstract

Pressure, volume, temperature studies have been made of gas phase amine–methanol systems. An apparatus which allows such observations without the need for weighing of original samples to be mixed is described. Equilibrium constants for the formation of hydrogen bonded complexes over the temperature range 25–45°C are reported and thermodynamic parameters have been evaluated for systems formed by methanol with methylamine, dimethylamine, trimethylamine and triethylamine. The values obtained are discussed in comparison with solution values. Larger values of ΔH° are found for gas phase complexes than for the corresponding complexes in solution, and the effect is interpreted in terms of solvation. It is found that the electron donating powers of the amines, as measured by ΔH° values for hydrogen bond formation in the gas phase, increase with increasing methylation of the amine. Wavenumber displacement Δν of the O—H stretching frequency on formation of such hydrogen bonds, are found to correlate with ΔH° values, and it is suggested that Δν values may be used to estimate ΔH° values.