Abstract
The forward and reverse rate constants for nine proton transfer reactions have been measured as a function of relative kinetic energy using a selected-ion flow drift tube (SIFDT). In all but two cases van't Hoff plots of the equilibrium constant against reciprocal centre of mass collision energy were linear and values of the enthalpy and entropy changes were obtained from slope and intercept respectively. As Δ H is a measure of the difference between the proton affinities (PAs) of the two neutral species, the data can be used to provide a PA difference ladder. This ladder agrees extremely well with the established PA scale. The experimental values of entropy change agree well with values calculated from the entropies of the individual ions and neutrals. The agreements of the values of Δ H and Δ S so determined establish the validity, and utility of a SIFDT apparatus for PA studies, when linear pseudo van't Hoff plots are obtained. In the present study two N 2OH + measurements gave non-linear Arrhenius and van't Hoff plots, and had to be rejected, in agreement with the earlier work. Some speculations on why drift tube measurements lead to reliable thermodynamic data, in spite of the lack of thermodynamic equilibrium between internal and translational modes, are presented.
Published Version
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