a Eint (SAPT) =Eeles + Eexch +Eind + Edisp
The most dominant energy term competing withEeles and Eind is the exchange term, Eexch , which is as large as 146.91, 138.39 and 100.45 kcal mol-1 for F2O···Cl, F2O···Br and F2O···F, respectively. The sum of the terms Eeles , Eind andEexch results in negative values for most of the complexes, showing that the stability of these complexes may be explained at the Hartree-Fock level. The inclusion of the dispersion part of the interaction energy, Edisp , to the sum (Eeles + Eind + Eexch ) leads Eint (SAPT) to be somewhat comparable to the ΔE (BSSE) for each complex. This shows that the geometric stability of each ion-molecule complex system is the result of a delicate balance between all the four energy components. Clearly, one should not disregard any particular interaction energy term and focus on another in order to advance the argument that the stability of the interacting members in a complex is purely Columbic in orgin.1,89 We have previously shown that such an argument can be misleading since it alone cannot explain the stability of the F···F interactions found in the complexes of (C6HnFm)2(n, m = 0–6),30 as well as of the X···X and X···O weakly bound interactions in many other complex systems.8,9,29,31 A notable example in this study is the complex Cl2O···OCN in which the dispersion term dominates and the sum (Eeles + Eind + Eexch ) is positive.
Conclusion
This study examined a series of 34 molecule-anion complexes and has shown that the O-atom in molecules such as OX2 (X = F, Cl, Br), O2F2, OFCN, and (CN)2O does indeed conceive a positive σ-hole, described by the presence of a positive electrostatic potential along the R–O bond extension. These σ-holes have a significant ability to promote the formation of O-centered chalcogen bonds with various anions, which interactions may or may not be strictly directional. The interaction energies of the resulting ion-molecule complexes could be classified as weak, moderate, strong, very strong, or ultra-strong. The nature of stability is clearly controlled by the nucleophilicity of the anionic bases, and the extent of their attractive engagements with the electrophilic site the O atom in the molecules studied. The results of the MESP model explained the intermolecular interactions in the complexes as of coulombic origin (viz. a negative site attracts a positive one). However, the magnitude of VS,maxon the O atom of the monomers is shown to be inadequate to explain the trend in the ΔE (BSSE) values found for this series of complexes. This result led to the conclusion that VS,maxcannot be used as a universal descriptor of the energy of intermolecular interactions. The NBO’s second-order analysis results showed that the origin of the chalcogen bonding is predominantly of n → σ* type charge transfer delocalization, and the magnitude ofE(2) associated these delocalizations were unusually large for some complexes. The results of QTAIM showed that the great majority of the interactions, of whatever flavor, in these molecule-anion complexes, are not only ionic but also contain appreciable covalent character; this is consistent with the large polarization and dispersion energies computed for these complexes.
Author Contributions: Conceptualization, problem design, computation, data analysis, chemical system drawing, interpretation, paper writing and editing: PRV and AV; Editing and Review: PRV, AV and HMM.
Funding: This research received no external funding. Accordingly, the funders had no role in the design, conceptualization and investigation of the study; in the handling, analyses, or interpretation of data; in the writing of the manuscript; or in our decision to publish the results.
Acknowledgments: This work was entirely conducted using the various facilities provided by the University of Tokyo and the computing center of Institute of Molecular Sciences (Okazaki). P.R.V. and A.V. thank Prof. K. Yamashita for support. A.V. is currently affiliated with AIST. H.M.M. acknowledges the financial support of the University of the Witwatersrand.
Conflicts of Interest: The authors declare no conflicts of interest.