Thermodynamics of proton transfer reactions in the gas phase

This work is a study of proton transfer equilibria, with temperature dependence, in binary mixtures containing benzene, mono and disubstituted benzenes, using High Pressure Pulsed Electron Beam Source Mass Spectrometry. The measurements of equilibrium constants with temperature variation led to the determination of thermodynamic properties such as relative proton affinities (PAs), gas phase basicities (GBs) and entropy changes. It was also intended to infer the sites for proton attachment in the species under study through the entropy changes. For Proton Transfer Reactions it is generally accepted that the main contribution to the entropy changes arises from rotational symmetry changes which depend upon the protonation site. They are, therefore, usually very small. The existence of isomeric species can give rise to another entropy contribution designated as entropy of mixing which is, in most cases in proton transfer reactions, also very small. The sum of both contributions for the systems under study was in general expected to be less than 13 Jmol ¹K ˉ ¹. The ΔS° experimentally determined were found to be, for some systems, much higher (60Jmol ¹K ˉ ¹) than expected on the grounds of the classical contributions. Confirmation for these values was sought from ab initio calculations which were carried out at 4-31G basis set level. The conclusions drawn from this study indicate that a novel contribution to entropy changes must be taken into account in aromatics protonating on the ring if various adjacent sites are equally favoured for proton attachment. This is considered to be an internal translational contribution due to proton migration which is supposed to produce a dynamic structure for those protonated compounds. Substituted benzenes where, this contribution does not occur, seem to have a structure similar to the benzenium ion