a The percentage values in parentheses give the
contribution to the total attractive interactions
ΔEelstat + ΔEorb.b The percentage values in parentheses give the
contribution to the total orbital interactions ΔEorb.
S2N2 shows four bond critical points
(BCPs) for each of the four S‒N bonds and a ring critical point (RCP) at
the center (Figure 3a). The BCPs are relatively close to S-atoms,
indicating bond polarization. The electron density\(\rho\left(r\right)\) and the Laplacian of the electron density\(\nabla^{2}(r)\) at the BCPs indicate the accumulation of electrons and
thus a covalent nature of the bonds (Table 4).
The negative total energy densityH(r) at the BCP also supports covalent
nature.[64] A high ε value for S‒N bonds also
indicates multiple bond character due to the presence of
π-delocalization. Likewise, a substantial value of ρ(r) and\(\nabla^{2}(r)\) at the RCP indicates possible delocalization of
electrons,[65] which can be correlated with the
negative NICS(1)zz values (Table 2). Contour line
diagram of the Laplacian distribution of the electron density\(\nabla^{2}(r)\) of the S2N2is also shown in figure 3a. Solid lines represent the areas of
electronic charge concentration, while dashed lines show areas of charge
depletion. The shape of the Laplacian distribution clearly shows that N
atoms of the S2N2 ring have an
area of charge concentration and S atoms of theS2N2 ring have an area of
charge depletion along the extension of the S‒N covalent bonds.