Negative bond dissociation energy derivatives

We must measure the AH° of these three reactions to derive the acidity of HA in the gas phase. The acidity is reported as a AH° value, not a pK, because it does not reflect the donation of the proton to any solvent. Instead, the value is a heteroly tic bond dissociation energy. In those cases where the base (anionic or neutral) is stable in the gas phase, one can determine the proton affinity of this conjugate base of the acid, which would be the negative of the heteroly tic bond dissociation energy (Eq. 5.26). The most pertinent number for any reaction is AG° not AH°, which would require knowledge of AS°. AS° can often be estimated, but for most gas phase acidities the entropies of the reactions are all similar, such that relative AG° is barely different than relative AH°. Therefore, Table 5.4 lists gas phase acidities solely as AH° values. 

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