engleski

Two-electron multicentric covalent bonding: how many centres?

Charge density was studied for three model systems with strong interactions bordering intra- and intermolecular: hydrogen bonding, halogen interactions and π-stacking. Our studies revealed that in all three cases the interaction involves multicentric two-electron covalent bonding. Nature of the strong hydrogen bond was studied on the well-known Zundel cation. It has long been known that the strongest hydrogen bonds are in fact weak covalent 2-electron/3-centre bonds of order 0.5 ; our study indicated that the proton donor and acceptor can't be distinguished. Both O-H bonds have a maximum electron density of ca. 1 e Å-3, and the negative Laplacian indicates a dominantly covalent interaction. It is quite well known that hydrogen and halogen bonding are similar, and recent studies showed that they are also similarly strong. In a halogen bond D-X···A the covalent bond D-X is elongated and the halogen is displaced towards the acceptor. Therefore we studied three systems with varying strenght of the halogen bond. Pure N-bromosuccinimide was used as a standard, to evaluate charge density of an unperturbed N-Br bond. In a co-crystal of N-bromosuccinimide with 3, 5-dimethylpyridine, the Br atom is displaced towards the acceptor by almost 0.4 Å and the maximum electron density in the "intermolecular" Br···N contact is 0.4 e Å-3, indicating a significant covalent contribution. The strongest halogen bond was studied on a bromonium ion, whose central N-Br-N fragment is in fact a two-electron/three-centre covalent bond. π-stacking of planar radicals involves spin coupling and mixing of SOMO orbitals, so it also has a considerable covalent character. Our studies of semiquinone radicals involved three types of contact: closely interacting radical dimers (known as 'pancake bonding'), trimers of partially charged rings and stacks of equidistant radicals. X-ray charge density and quantum chemical modelling showed significant covalent contribution in all of them. Therefore, 'pancake bonding' can be regarded two-electron multicentric covalent bonding. Apparently, covalent bond can also be unlocalised.

multicentric bonding, hydrogen bonding, halogen bonding, pi-stacking, X-ray charge density

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