Figure 3. Behaviours of both MK and NPA charges on the Cl1, N2,
N3 and N4 atoms of S(-) and R(+) forms of CQ by using B3LYP/6-311++G**
level of theory.
Figure 3 shows important differences in the two types of charges
analyzed, thus, in the S(-) form, the MK charges on the four atoms
studied show the same values (see blue and red lines) with exception of
N2 atoms which show mainly negative values in solution while, on the
contrary, the MK charges on the four atoms for the R(+) form present
different behaviour and values, having these charges in solution the
most negative values. Note that the MK charge on N3 atom belonging to
N3-H30 bond in solution has the most negative value because the H30 atom
is more labile in this medium due to its low value. Moreover, the MK
charge on N4 in solution has more negative value than the other ones.
Analyzing now the NPA charges (see green and purple lines) similar
behaviour on the Cl1, N2 and N3 atoms is observed, however, the NPA
charges on the N4 show negative values in solution. As a consequence of
these studies, the most negative MK charges values on the four atoms of
R(+) form than the S(-) ones could justify their higher stability in
solution. The NPA charges on the N4 belonging to ring R1 show the only
differences between the two forms in both media.
The molecular electrostatic potentials (MEP) calculated for both forms
in the two media from the MK charges [60] show practically the same
values and few differences are observed, for this reason they are not
presented here. The tendency in the MEP values shows that the MEPs of Cl
> N > C > H where clearly higher
values are observed in both media on the Cl1 atom while the lower value
is observed on the H30 atom because this atom belongs to the N3-H30
bond. When the mapped MEO surfaces 3D for both forms in gas phase by
using the GaussView program [54] are represented inFigure S1, different positions of blue and red colours are
observed. Therefore, the strong red colours are observed on the N4 atoms
of R1 rings of S(-) and R(+) forms while most weak red colours are
observed on the chloroquinolin rings of both forms. Then, strong blue
colours can be seen on the N3-H30 bonds because the lowest MEPs are
observed on the H30 atoms. Moreover, soft light colours are observed on
the other H atoms of aromatic and CH3 groups, as it is
shown in Figure S1. Obviously, the red colours are indicative of
nucleophilic sites because there regions are acceptors of H bonds while
the blue colours are associated to electrophilic sites because they are
donors of H bonds regions, as observed in the donors N3-H30 groups which
are strongly donors of H bonds.
Other interesting properties studied here are the bond orders (BO),
expressed as Wiberg indexes which have been calculated for the two S(-)
and R(+) forms of CQ in both media by using the B3LYP/6-311++G** method.
These values for all atoms of both forms are presented in Table
S2 . A very important difference is observed in the BO values of C1, N2
and N4 atoms because the values for these three atoms decrease in
solution while the values for the N3 atoms of two forms increase in
solution, as it is expected because these atoms belong to N3-H30 bonds
which are strongly donors of H bonds in solution. For this latter
reason, higher BO values are observed for the N3 atoms of both forms
while the lower values in the corresponding H30 atoms.
From these three studies performed in this section, the MK charge and
the mapped MEP 3D surfaces have shown significant differences between
the S(-) and R(+) species of CQ in both media.