130
M. M. Kabanda and K. P. Otukile
O3 atom is not yet complete (because the 1.264 Å O–H bond distance is quite large
in comparison to the standard 0.97 Å O–H bond distance). When the transfer of the
H11 proton from N10 to O3 is complete, it results in the formation of the H 2 O group,
which is an easy leaving group. The IRC results indicates that the protonation of the
carbonyl O2, the formation of the oxocyclic C1–N10, coupled with the release of the
H11 from N10 and its subsequent attachment to O3 (resulting in the formation of the
H 2 O bonded to C1) may occur as a concerted reaction step involving asynchronous
bond-formation and bond-breaking processes.
The final stage in the process is the formation of the product from TS3 by the
elimination of the leaving group (the water molecule) and the simultaneous restoration of the sp
2 character of the C1 atom (i.e., the simultaneous conversion of the C–O
back to C=O) as a result of the concomitant transfer of the proton from O2H13 to the
chloride (Cl5
− ) ion. The formation of the product from IM2 has an activation energy
barrier of 37.612 kcal/mol; however, the product is only 6.029 kcal/mol lower than
the starting isolated reactants.
An examination of the relative energies obtained with all the methods also suggest
that the products have lower energy than the starting reactant molecules, which
implies that the process is exothermic. The G values suggest that the reaction is
exergonic, which implies that it is thermodynamically feasible to occur. The energetic
values (reported in Table 1 and represented graphically in Fig. 4) also show that the
step connecting RC-t with TS1-t is the rate-determining step of the investigated
reaction, with energy lying about 60.776 kcal/mol above the energy of the free
reactants.
To obtain more information on the transition structures, intermediates and the
resulting complex product, NBO and QTAIM analyses were performed, which provide deeper insight into their charge and electron distributions. Figure 5 shows the
molecular graphs along with the NBO charges on the structures, while Table 3 reports
properties related to bond critical point data (i.e., the electron density (ρ) and the
Laplacian of the electron density, ∇
2
ρ). NBO charges on TS1 show a regular charge
distribution, in which the leaving hydrogen atom (H13) of the S atom has a positive
charge of 0.035 a.u. and the leaving chloride ion has a negative charge of −0.532
a.u. (which is higher than in RC-t). The S atom also attains a higher positive charge
than in the RC-t structure. In TS2, the electrostatic stabilisation contributes to the
lowering of the negative charge on the chloride ion and an increase in the positive
charge on the S atom.
In RC-t, the Laplacians of the electron density for all the bonds, with the exception of the O2···H11 and H6···N9 bonds, are covalent in nature. The Laplacians of
the electronic charge density of the O2···H11 and H6···N9 bonds are 0.1045 and
0.0795 a.u, respectively. The low and positive values of ∇
2
ρ for both the O2···H11
and H6···N9 bonds of the RC-t complex show that there is a weak intermolecular
hydrogen bond interaction between the carbonyl O atom of chloroacetic acid and the
hydrogen atom of the amino group and between the hydroxyl H atom of chloroacetic
acid and the imine N atom [45–47]. The electronic charge density (ρ) corresponding
to both O2···H11 and H6···N9 bonds are 0.0288 and 0.0669 au, respectively. These
M. M. Kabanda and K. P. Otukile
O3 atom is not yet complete (because the 1.264 Å O–H bond distance is quite large
in comparison to the standard 0.97 Å O–H bond distance). When the transfer of the
H11 proton from N10 to O3 is complete, it results in the formation of the H 2 O group,
which is an easy leaving group. The IRC results indicates that the protonation of the
carbonyl O2, the formation of the oxocyclic C1–N10, coupled with the release of the
H11 from N10 and its subsequent attachment to O3 (resulting in the formation of the
H 2 O bonded to C1) may occur as a concerted reaction step involving asynchronous
bond-formation and bond-breaking processes.
The final stage in the process is the formation of the product from TS3 by the
elimination of the leaving group (the water molecule) and the simultaneous restoration of the sp
2 character of the C1 atom (i.e., the simultaneous conversion of the C–O
back to C=O) as a result of the concomitant transfer of the proton from O2H13 to the
chloride (Cl5
− ) ion. The formation of the product from IM2 has an activation energy
barrier of 37.612 kcal/mol; however, the product is only 6.029 kcal/mol lower than
the starting isolated reactants.
An examination of the relative energies obtained with all the methods also suggest
that the products have lower energy than the starting reactant molecules, which
implies that the process is exothermic. The G values suggest that the reaction is
exergonic, which implies that it is thermodynamically feasible to occur. The energetic
values (reported in Table 1 and represented graphically in Fig. 4) also show that the
step connecting RC-t with TS1-t is the rate-determining step of the investigated
reaction, with energy lying about 60.776 kcal/mol above the energy of the free
reactants.
To obtain more information on the transition structures, intermediates and the
resulting complex product, NBO and QTAIM analyses were performed, which provide deeper insight into their charge and electron distributions. Figure 5 shows the
molecular graphs along with the NBO charges on the structures, while Table 3 reports
properties related to bond critical point data (i.e., the electron density (ρ) and the
Laplacian of the electron density, ∇
2
ρ). NBO charges on TS1 show a regular charge
distribution, in which the leaving hydrogen atom (H13) of the S atom has a positive
charge of 0.035 a.u. and the leaving chloride ion has a negative charge of −0.532
a.u. (which is higher than in RC-t). The S atom also attains a higher positive charge
than in the RC-t structure. In TS2, the electrostatic stabilisation contributes to the
lowering of the negative charge on the chloride ion and an increase in the positive
charge on the S atom.
In RC-t, the Laplacians of the electron density for all the bonds, with the exception of the O2···H11 and H6···N9 bonds, are covalent in nature. The Laplacians of
the electronic charge density of the O2···H11 and H6···N9 bonds are 0.1045 and
0.0795 a.u, respectively. The low and positive values of ∇
2
ρ for both the O2···H11
and H6···N9 bonds of the RC-t complex show that there is a weak intermolecular
hydrogen bond interaction between the carbonyl O atom of chloroacetic acid and the
hydrogen atom of the amino group and between the hydroxyl H atom of chloroacetic
acid and the imine N atom [45–47]. The electronic charge density (ρ) corresponding
to both O2···H11 and H6···N9 bonds are 0.0288 and 0.0669 au, respectively. These