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