Organic modifier effect energy values

The capacity ratio of partially ionized compounds can be predicted using eqn (9). For evaluation by the above approach, and ki were replaced with molecular interaction energy values. The p Ta values predicted from the atomic partial charge, related to the measured pXa values in liquid chromatography, were used for the calculation, and the concentration effect of an organic modifier (0.022 pH units per % methanol from ref. 36) was added for the calculation. The predicted p Ta values derived from Hammett s equation were used for further evaluation. The results are shown in Figure 6.26. 

Extensive collections of pK values are available in the literature, e.g., [98-101]. It is also possible to predict pK values for a broad range of organic acids and bases using linear free energy relationships based on a systematic treatment of electronic (inductive, electrostatic, etc.) effects of substituents which modify the charge on the acidic and basic center. Quantitative treatment of these effects involves the use of the Hammett Equation which has been a real landmark in mechanistic organic chemistry. A Hammett parameter (a), defined as follows ... 

---