Based on Table 11, for n-2AOE13PDA and 2-MPMDA amines, the difference in
energies were all positive. It means that the electronic energies of the
primary protonated amines were lower than the alternative positions
which were at the nitrogen closer to the functional groups. For
n-2AOE13PDA and DMAPAPA amines, the difference in energies were all
negative. Therefore, the electronic energy of the secondary protonated
of the amines was lower than at the primary position. Similarly, for
DMAPAPA and DAOMDPA amines, the different energies were positive;
therefore, for the tertiary protonated structures, the electronic
energies were higher than the primary positions. As a result, the
secondary amino groups were probably responsible for the first
pKa of n-2AOE13PDA and DMAPAPA amines while the
primary amino which were away from functional groups were responsible
for the first pKa of 2-MPMDA and DAOMDPA amines.
As mentioned in the literature,19 the secondary amino
group was protonated before the primary amino group which was in turn
protonated before the tertiary amino group. The computational study was
in good agreement with the literature.19