3.5 Analysis of the MESP model results
The results of the MESP model shown in Fig. 2. The electrophilic
character and the extent of electrophilicity of the O atom in the
monomers can be rationalized by the positive sign and magnitude ofVs,max on O, respectively. For example, theVs,max values were calculated to be 34.2, 32.4,
16.4, 3.5, 1.6 and 12.4 kcal mol-1 on the C–O, F–O,
F–O, Cl–O, Br–O and O–O bond extensions in (CN)2O,
NCFO, F2O, Cl2O, Br2O,
and F2O2, respectively (Fig. 2). These
are all positive, and hence represent the O-centered σ-holes on
respective monomers. Evidently, the intermolecular association between
the interacting monomers shown in Fig. 1 can be rationalized by
Coulomb’s law. For instance, the positive site on the O atom along the
X–O (X = O, F, Cl, Br, CN) bond extensions in
F2O2, X2O (X = F, Cl,
Br), NCFO, and (CN)2O attracts the electron rich anion
species X– (X = F, Cl, Br), SCN–,
OCN–, NO3– and
CN– by Coulombic interaction, leading to the
formation of the ion-molecule complexes of Fig. 1. Similarly, the O atom
along the F–O extension in NCFO has a positiveVS,max of 32.4 kcal mol-1 and
is responsible for the formation of very strong complexes with
Cl– and Br– ((ΔE (BSSE) for
NCFO···Cl– (16) and NCFO···Br–(17) were found to be –48.74 and –51.46 kcal mol-1,
respectively).
On the other hand, the O atom on the F–O extension in
F2O has a relatively small positiveVS,max of 16.4 kcal mol-1, yet
it is responsible for the formation of yet more stable complexes with
the same anions ((ΔE (BSSE) for
F2O···Cl– (8) and
F2O···Br– (9) were –54.10 and
–54.93 kcal mol-1, respectively). The complexes of
F2O2 with X– are
comparatively weaker than those of
Cl2O···X– and
Br2O···X– (X = F, Cl, Br) (see Table
1 for ΔE (BSSE) values). The above-mentioned preference in the
stabilization energy is also unusual since VS,maxon the O atom in F2O2(VS,max = 12.4 kcal mol-1) is
larger than that in Cl2O (VS,max= 3.5 kcal mol-1) and in Br2O
(VS,max = 1.6 kcal mol-1).
The latter results unequivocally demonstrate that the local mostVS,max values associated with O’s σ-hole in
isolated monomers cannot be used as a universal descriptor of
intermolecular interactions, or a measure of complex stability. This is
arguably because the dependence between VS,maxand ΔE has previously been demonstrated to be linear in some
systems,22,84-86 but observed not to be so in the
complexes reported in this study. This suggests that the Coulombic view
of explaining the origin of intermolecular interactions in the complexes
examined in this study is incomplete, and such a view is not applicable
to all systems as is not universal.30,31,87,88 The
potential limitations of the MESP model has been
discussed;17 it fails to explain Type-I halogen
bonding and Type-III halogen-centered noncovalent interactions in
complex systems. These interactions are the consequence of attraction
between bonded atoms that have similar electrostatic potential; the
former unusually does have a bent geometry and the latter is linear or
quasi-linear. Another limitation of the model is that it fails to
provide any insight into the origins of the attraction persisting
between the interacting monomers of complexes of Fig. 1, which can be
revealed using NBO’s second-order analysis (see above, 3.4). Clearly,
the standalone use of the MESP model may lead to erroneous
interpretations of the nature and origin of intermolecular interactions
in systems where multi-fold interaction topologies of van der Waal type
play a crucial role in determining the stability to the equilibrium
geometries.8,9,88