6 | ESTIMATING THE FIRST DISSOCIATION CONSTANT VALUES WITH PERRIN – DEMPSREY – SERJEANT (PDS) METHODS
The PDS method19 is also called a pencil and paper method which estimates the first dissociation constant values at 298.15K of chemical compounds based on group contributions. The method simply collects pKa1 values at 298.15K and divides the compounds into multiple functional groups. The dissociation constant of the main amino groups (primary, secondary and tertiary amines) and the constant shift (∆pKa ) of each functional group will be calculated by minimizing the difference between the experimental and estimated values. Therefore, the method will be more accurate if more data is available in the database. To improve the accuracy of the method, Sumon et al.20 collected more experimental data and reoptimized the constants and the constant shifts. Furthermore, Qian et al.21 added more data as well as contribution groups to improve the accuracy further. Qian’s method is referred to as the QSSG method while Sumon’s method is simply called the modified PDS method in this work. The suggested constant values of the three methods were summarized in Table S6. For applying the methods to determine the value of the first dissociation constants at 298.15 K, n-2AOE13PDA amine was chosen as an example for demonstration purposes. For the original PDS method, the secondary amino has a value of 11.15 and with the two primary amino groups at γ and δ positions. Therefore, the final value was determined as 11.15 – 0.8‧0.4 – 0.4‧0.4‧0.8 = 10.70, compared to the experimental value of 10.25. The modeling results are presented in Table 12.
Table 12 Estimation of the first pKa of the studied amines by using original PDS, Sumon’s and Qian’s methods at 298.15 K