Proton Affinity, Basicity, and Acidity

In addition to the concepts reviewed in the last two sections (appearance energy, ionization energy, and electron affinity), three others are relevant in gas-phase molecular energetics, namely, proton affinity, gas-phase basicity, and gas-phase acidity. 

The proton affinity of any species A in the gas phase, abbreviated by PA(A), is defined as the negative of the standard enthalpy of reaction 4.25 at 298.15 K. The minus sign ensures that proton affinities always have positive values. 

The gas-phase basicity of A, which we represent by GB A), is the standard Gibbs energy of reaction 4.25. It is also usually defined at T = 298.15 K and it is related to PA(A) by equation 4.26. 

Note that the standard enthalpy of this reaction, Aacid77°(AH), is equal to the proton affinity of the anion, PA(A ). As shown in figure 4.5, this quantity can be related to PA(A) by using the adiabatic ionization energy of AH and the adiabatic electron affinity of A. The result is also expressed by equation 4.28 (derived from equations 4.4 and4.9), where A = (TT g - o)ah+ ( 298 o)ah and A = ( 298 o )a- - ( 298— o )a These thermal corrections are often smaller than the usual experimental uncertainties of proton affinity data (ca. 4 kJ mol-1). 

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When comparing literature data for the quantities addressed in this section, it is therefore essential to check if those data are consistent, that is, if they are based on the same value for the anchor. On the other hand, note that proton affinity, basicity, and acidity values do not depend on whether we follow the electron convention, the ion convention, or the electron FD convention. This is clearly evidenced by reactions 4.25 and 4.27, which do not involve the electron as a reactant or product species. However, it is also obvious that the values of the standard enthalpies of formation of AH+ and A-, calculated from PA(A) and A acid-7/0 (AB), respectively, will vary with the convention used to derive the standard enthalpy of formation of the proton. 

Determination of proton affinities (basicities) and acidities in the gas phase provides a means of systematically representing a large number of ion-molecule reactions and of the relationships between these quantities and bond strengths, ionization potentials and electron affinities without disturbance by solvation phenomena 47). 

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