Scales of Bronsted Basicity and Affinity in the Gas Phase

Various mass spectrometric techniques permit the study of proton transfer reactions in the gas phase and the definition of Bronsted basicity scales free of solvent effects [26], 

The gas-phase basicity GB and the proton affinity PA of a base B are defined as the standard Gibbs energy change and the standard enthalpy change, respectively, of the formal deprotonation reaction 1.12  

Unfortunately, this terminology, currently in use, is not completely correct since an affinity is a chemical potential (a AG value) whereas the proton affinity is an enthalpy. An alternative terminology for AH° might be enthalpy of basicity , but it seems unrealistic to propose a change of terminology now considering the accepted practice. 

The absolute basicity and affinity cannot be obtained directly because the gas-phase reaction 1.12 is extremely endoergic and endothermic. It is common practice to resort to thermodynamic cycles, involving enthalpies of formation and dissociation thresholds, to calculate absolute PAs. The transformation of absolute PAs into absolute GBs (Equation 1.13) requires the evaluation of the entropy of basicity (Equation 1.14) (mainly through quantum chemical calculations today)  

The number of absolute PA and GB values that can be accurately evaluated is very limited. In fact, most parts of the scales are obtained by measuring the relative basicity of an unknown using a reference base B° of known GB. Relative basicities, designated AGB, are obtained from equilibrium constants K of the proton exchange reaction 1.15 between 

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