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