a Energies in kcal mol-1. Selected
bond lengths and bond angles in Å and degrees, respectively.
The total energy density, Hb , is sum of the
“gradient” kinetic energy density, Gb , and the
potential energy density, Vb ,
(Hb = Gb + Vb ).
From Table S1, it is clear that Hb is negative
for several complexes, implying that they conceive medium-to-strongly
bound interactions. For instance, it is most negative for the S···O bond
(Hb = –0.166 a.u.) in
Br2O···SCN–. Both this and the
∇2ρ b of –0.258 a.u. at the
S···O bcp signify the formation of a covalent bond between the S and O
atoms in the complex. This conclusion is consistent with the
recommendation of Cremer and Kraka,78 who have
suggested that if Vb is dominant overGb at the bcps (hence, Hb< 0), this indicates that the region has a “charge density
concentration”. On the other hand, and for the weakly bound
interactions in the complexes shown in 5-6, 11, 13-14, 22, 25, 29-30 and
33-34 of Fig. 1, the Hb values are all positive.
This is not unexpected since the intermolecular distances in these
complexes are relatively longer (Table 1) and the nature of these
interactions are closed-shell type. This occurs when there is an
appreciable depletion in the charge density at the bcps of bonded atomic
basins. A similar insight into the ionic and covalent nature of the
aforesaid interactions can be gained from the ratio
–Vb /Gb (values not
shown).79,80