5 | PREDICTING THE ORDER OF PROTONATION
As observed by considering the molecular structures of the compounds,
two of the amines are diamines while the three of them are triamines.
The structure of Bis[2-(n,n-dimethylamino)ethyl]ether is symmetrical
while the other structures were not symmetrical with the presence of
primary, secondary and tertiary amino groups. Therefore, it is important
to study the order of protonation to better understand the reaction
mechanism of the amines. In this study, the amine structures were
optimized by commercial computational chemistry software [Gaussian (09
Revision B.01 SMP)] with different models and basis sets. These
molecular structures were built using GaussView version 5.0.9. Water was
used as a solvent with a polarizable continuum model (PCM). After
structural optimization, the energy (E) of different protonated amine
scenarios could be used for predicting the protonation locations.
Equation 9 was used to calculate the difference in energy levels. The
main chain of the organic compounds is the longest chain with additional
groups attached to the chain, and are called functional groups. In the
equation, EPrimary Protonated* is that of the amino
group which is closer to the functional group. In the case of
n-2AOE13PDA, the primary* group is
the one close to the secondary amino group because its structure is
straight. Table 11 shows the different energy levels for the scenarios.
ΔE(kJ·mol-1) =
ESecondary/Tertiary/Primary Protonated* -
EPrimary Protonated (9)
Table 11 Differences in energies between the different
protonated positions