Solvent Effects on Acidity and Basicity from Gas Phase Ion Equilibria Measurements

Chemistry Dept., University of Alberta, Edmonton, Canada 

If the solvent effect on the acidity or basicity of given molecules is to be assessed, the intrinsic or molecular acidities or basicities in the absence of the solvent must be known. Until recently no satisfactory general method existed for such determinations. However, the situation has drastically improved with the introduction of proton transfer equilibria measurements in the gas phase. The measurements already available (see Section 4) include hundreds of compounds and it is certain that within the next few years most important compounds will have been examined. However, gas phase ion equilibria measurements can do even more things, they can also provide information on how the acidity or basicity of a given compound changes in the presence of a controlled small number of molecules solvating the ions formed by the acid-base reactions. In this way gas phase ion equilibria measurements not only provide the normative acidities and basicities for isolated molecules but also throw a bridge to the behaviour in solution. 

Pullman (ed.). Environmental Effects on Molecular Structure and Properties, 81-94. All Rights Reserved Copyright 1976 by D. Reidel Publishing Company, Dordrecht-Holland 

Since ions are unstable in the gas phase because of positive-negative ion recombination or discharge on the wall, they must be created by ionizing radiation. The ion solvent molecule interactions or other ion molecule equilibria must be observed within the limited lifetime of the ions before their disappearance. Of interest here are two types of ion equilibria. Proton transfer equilibria involving bases B or acids AH as illustrated by reactions (1) and (2) and clustering equilibria as illustrated by reaction (3) written for the negative ion A and water molecules. 

As an actual example we can consider the proton transfer equilibrium (2) involving the two differently substituted benzoic acids. The measurement can be done by using a gas mixture containing 5 torr of a buffer gas like methane, containing a small amount ( 0.050 torr) of the weaker acid A2H and an even smaller amount of the stronger acid AjH. In addition to this a small quantity (0.1 torr) of SO2F2 is added and the mixture is irradiated with a burst of electrons e. The following major reactions occur  

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SOLVENT EFFECTS ON ACIDITY AND BASICITY FROM GAS PHASE ION EQUILIBRIA MEASUREMENTS... 

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