Paradigm Shifts in the Theory of Chemical Structure and Reactivity

In my talk, I will address recent findings that fundamentally change our understanding of three different phenomena in molecular structure and reactivity.

The first one concerns the contraction of C–H and other bonds as the carbon atoms involved vary, in s–p hybridization, along sp³ to sp² to sp, e.g., the C–H bond in ethane, ethene and ethyne [1]. Based on quantitative MO and energy decomposition analyses (EDA), I will show that this well-known trend is not, as taught in textbooks, determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, it turns out to be caused by a diminishing steric repulsion between substituents around the pertinent carbon atom, as the coordination number decreases from 4 to 2, along sp³ to sp² to sp.

Another paradigm shift that I address has to do with Lewis-acid catalysis in various chemical reactions, such as Michael addition or cycloaddition [2]. The commonly accepted picture behind this catalysis is that the Lewis acid causes a lowering in energy of the LUMO in the reactant to which it coordinates and, in that way, induces more stabilizing HOMO–LUMO orbital interactions with the other reactant. This is the so-called "LUMO-lowering catalysis". However, quantitative MO and activation strain analyses [3] show that (and why) the orbital interactions in general do not become more stabilizing upon Lewis-acid catalysis. What happens instead is that, in most cases, Pauli repulsion between the reactants is reduced which stabilizes the net interaction between reactants, also if the bonding orbital interactions are unaffected or even weakened. We have dubbed this mechanism "Pauli-lowering catalysis". I will explain how this works. And I will provide a rare example in which LUMO-lowering catalysis is still operative.

Finally, I will address a topic from intermolecular interactions [4]. I will examine with you how hydrogen, halogen, chalcogen and pnictogen bonds work, and that they have a sizeable orbital-interaction component. In fact, the accepted term of "Non-Covalent Interactions" is, from a physical point of view, questionable. I propose to refer to these interactions as "Intermolecular Covalent Interactions" (ICI).

References

[1] P. Vermeeren, W.-J. van Zeist, T. A. Hamlin, C. Fonseca Guerra, F. M. Bickelhaupt, Chem. Eur. J. 2021, 27, 7074.

[2] a) T. A. Hamlin, I. Fernandez, F. M. Bickelhaupt, Angew. Chem. Int. Ed. 2019, 58, 8922. b) P. Vermeeren, T. A. Hamlin, I. Fernandez, F. M. Bickelhaupt, Angew. Chem. Int. Ed. 2020, 59, 6201. c) P. Vermeeren, T. A. Hamlin, I. Fernandez, F. M. Bickelhaupt, Chem. Sci. 2020, 11, 8105. d) T. A. Hamlin, F. M. Bickelhaupt, I. Fernandez, Acc. Chem. Res. 2021, 54, 1972.

[3] P. Vermeeren,S. van der Lubbe, C. Fonseca Guerra, F. M. Bickelhaupt, T. A. Hamlin, Nat. Protoc. 2020, 15, 649.

[4] a) L. de Azevedo Santos, S. C. C. van der Lubbe, T. A. Hamlin, T. C. Ramalho, F. M. Bickelhaupt, ChemistryOpen 2021, 10, 391. b) L. de Azevedo Santos, T. A. Hamlin, T. C. Ramalho, F. M. Bickelhaupt, Phys. Chem. Chem. Phys. 2021, 23, 13842.