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.