engleski

Stacking interactions: a broader perspective

Stacking of aromatic rings is a well-known type of intermolecular interaction. It is usually considered as rather weak (typically <1 kcal mol-1), and is assumed that it is an exclusive property of aromatic rings ; the fact that other types of planar conjugated rings also stack is usually overlooked. However, there is a growing amount of evidence that all rings stack, and that aromaticity is not a conditio sine qua non for formation of π-stacks. Stacking of planar organic radicals has been used in design of organic semiconductors and magnetic materials for more than a decade. Our detailed studies have shown that interactions between quinoid rings and semiquinone radicals are by an order of a magnitude stronger than interactions between aromatics. As a result of numerous X-ray charge density studies on different types of stacked rings, we propose a generalised model of stacking, which accounts for both aromatic and non-aromatic rings. It covers a wide range of interactions and energies ranging from weak dispersion (E ≤ 1 kcal mol-1) to non-localised two-electron multicentric covalent bonding ('pancake bonding', E ≥ 15 kcal mol-1). Thus, stacking can be compared to hydrogen and halogen bonding and can be categorised as weak, mainly dispersion (aromatic stacks) ; medium, mainly electrostatic (non-aromatic rings, such as quinones and metal-chelates) and strong, partially covalent (planar organic radicals).

pi-interactions ; stacking ; charge density ; radicals ; quinones

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