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.