(Intermolecular) Covalent Interactions:

A Quantitative Molecular Orbital Picture

F. Matthias Bickelhaupt

Vrije Universiteit Amsterdam, The Netherlands

f.m.bickelhaupt@vu.nl

In my lecture, I will address three main topics: (1) The basis of quantitative molecular orbital (MO) theory as contained in Kohn-Sham DFT; (2) Intramolecular covalent element–element bonds; and (3) Intermolecular Covalent (ICI) bonds between molecular fragments as they occur in reactant complexes, supramolecular chemistry and in crystals.

        In part 1 of my talk, I will point out how various mechanisms and concepts in quantitative MO theory can be quantified through our canonical energy decomposition analysis (EDA), and I will stress the crucial role of causality in a good physical model [1].

        In part 2, I will bring the concepts of quantitative MO theory to life by showing how the good old "electronegativity model" (i.e., "more polar bonds are often more stable than apolar covalent bonds") is sometimes correct, but in some cases truly fails; ironically, this happens for the textbook example of bonds that is invoked to illustrate the electronegativity model, namely, carbon–halogen bonds [2]. If time permits, I can show how alkyl substituents destabilize (not stabilize) organyl radicals, despite the occurrence of hyperconjugation [3].

        Finally, in part 3, I make the step from "regular" covalent bonds (bonds within molecules) to intermolecular bonds, that is, bonds between molecular fragments as they occur, for example, in reactant complexes, supramolecular chemistry, and in crystals. I will argue that we are dealing here with a continuous spectrum of bonding. Interestingly, the bonding mechanism of intermolecular bonds is in many cases relatively similar to that of regular covalent bonds; it is only weaker than the latter. Thus, I propose to use the designation "Intermolecular Covalent Interactions" instead of "Noncovalent Interactions" [4]. Interestingly, the s-hole model is shown to be fundamentally flawed [4] from whatever perspective (MO, VB, NBO, ...) one may choose to look at the bonding mechanism.

Literature

[1]  F. M. Bickelhaupt, E. J. Baerends, In: Reviews in Computational Chemistry; K. B. Lipkowitz and D. B. Boyd, Eds.; Wiley-VCH: New York, 2000, Vol. 15, pp. 1-86.

[2]  E. Blokker, X. Sun, J. Poater, J. M. van der Schuur, T. A. Hamlin, F. M. Bickelhaupt, Chem. Eur. J. 2021, 27, 15616.

[3]  E. Blokker, W.-J. van Zeist, X. Sun, J. Poater, J. M. van der Schuur, T. A. Hamlin, F. M. Bickelhaupt,  Angew. Chem. Int. Ed. 2022, 61, e202207477.

[4]  L. de Azevedo Santos, T. C. Ramalho, T. A. Hamlin, F. M. Bickelhaupt, Chem. Eur. J. 2023, e202203791.