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Questions on Non-Covalent Bonding Interactions in Theoretical Chemistry

The terms non covalent bond and non covalent interaction are widely used in almost all domains of chemistry as well as solid state physics and biochemistry. Non covalent interactions are particularly important in supramolecular chemistry where they play an important role in the organization and architec- ture of supramolecular assemblies. They are weaker than covalent, ionic and metallic bonds, often referred to as primary bonds, and are classified into sub- classes of bonds and interactions according to the nature of the element of the electrophilic centre. The review article of Alkorta et al.[1] distinguishes hy- drogen bonds, alkali bonds (group I), alkaline earth bonds (group II), regium bonds (groups X and XI), spodium bonds (group XII), triel bonds (group XII), tetrel bonds (group XIV), pnictogen bonds (group XV), chalcogen bonds group (XVI), halogen bonds (group XVII) and aerogen bonds (group XVIII). In ad- dition to this classification, an orbital based terminology, i; e. :σ-hole π-hole, π ´ π stacking, may be used in addition. The terms “secondary bonding” and “secondary interactions” are also used as an alternative to “non covalent” (see for example Echeverria and Alvarez[2] or Landrum and Hoffmann[3]. The concept of secondary bond, introduced in 1972 by Alcock[4] covers a large range of inter- atomic/intermolecular interactions in which the internuclear distance between the involved non metal atoms is longer than standard one and less than the sum of the van der Waals radii of the latter atoms. The stabilization energy brought by the formation of a secondary bond is expected to be less than 100 kJ/mol. Neither non-covalent bonds nor secondary bonds are considered in the IU- PAC Compendium of Chemical Terminology[5]. Although the definition of bond still relies on the chemist’s freedom of choice to consider an aggregate as an in- dependent molecular species, the definition of molecule, namely “An electrically neutral entity consisting of more than one atom (n ą 1). Rigorously, a molecule, in which n>1 must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state”, since it prescribes a discriminant property (i.e. that at least one bound-bound transition is ob- served in the vibrational spectrum). In the case of the rare gas dimers in the ground state, the spectra of Ar2 , Kr2 and Xe2 display bound-bound transitions bands whereas internuclear distances are of close to the sum of the van der Waals radii and even larger. This implies that the internuclear distance upper bound criterion of Alcock is weak. The experimental evidence of non covalent bond or interaction in molecu- lar aggregates is a sensitive topic, especially for intramolecular interactions and quantum chemists have developed dedicated tools during the two last decades such as the Non Covalent Interaction (NCI) index[6] and the Density Overlap Region Indicator (DORI) [7] which both provide convincing visual representa- 1tions. In spite, or maybe as a result, of a large number of scientific meetings and publications devoted to this subject, the concepts and the vocabulary of chemists working in this field often lack unity and sometimes clarity as a same or expres- sion can be used for somewhat different concepts and reciprocally. This fact can be due to the superposition of ideas rooted to different approaches such as perturbation theory, energy decomposition techniques, molecular orbital the- ory, Molecular Electrostatic Potential (MESP), quantum chemistry as well as experimental chemistry. Moreover, chemical concepts seldom have scientifically rigorous definitions compared to those Mathematics and Physics.

References

[1] I. Alkorta, J. Elguero and A. Frontera, Crystals, 10, 180 (2020).

[2] J. Echeverría and S. Alvarez, Chem. Sci., 14, 11647 (2023).

[3] G. A. Landrum and R. Hoffmann, Angewandte Chemie International Edi- tion, 37, 1887 (1998).

[4] N. Alcock, vol. 15 of Adv. Inorg. Chem. and Radiochem., (pp. 1 – 58), Academic Press (1972).

[5] A. D. McNaught and A. Wilkinson, Compendium of Chemical Terminology The Gold Book , Blackwell Science, Oxford, 2nd edn. (1997).

[6] E. R. Johnson, S. Keinan, P. Mori-Sánchez, J. Contreras-Garciá, A. J. Cohen and W. Yang, J. Am. Chem. Soc., 132, 6498 (2010).

[7] P. de Silva and C. Corminboeuf, J. Chem. Theory Comput., 10, 3745 (2014).