engleski

X-ray charge density study of face-to-face stacking of quinoid rings

Stacking of quinoid rings which we recently observed is unlike "π-interactions" of aromatic rings: quinoid rings stack face-to-face without offset and with very short interplanar distance, interatomic distances being some 0.3 Å shorter than the sum of van der Waals radii. This indicates unusually strong interactions, which may be comparable to hydrogen bonds, and therefore may be applied in crystal engineering and materials science. The nature of this strong interaction remains to be defined. Therefore, we extended our research of quinoid stacking to a detailed in-depth study of interactions in a well-known model system, potassium hydrogen chloranilate dihydrate using a combination of X-ray charge density determination, quantum chemical methods and electrical conductivity. X-ray charge density study provided us with detailed high-resolution map of electron density. It revealed not just interesting data of electron delocalisation in hydrogen chloranilate anion, but also its electrostatic potential: as expected, single C-C bonds are electron-deficient (positive potential), while delocalised and double ones are electron-rich (negative potential). The mutual orientation of contiguous rings is such that positive and negative parts of the molecules come in a close contact, maximising electrostatic attraction, while minimising repulsion. Therefore, face-to-face stacking of quinoid rings is a dominantly electrostatic interaction. Quantum chemical calculations indicate that the stacking interaction between adjacent rings is notably influenced by dispersion forces ; apparently the dispersion component amounts to several kcal/mol and is slightly dependent on the torsional alignment between the rings. Electric conductivity was measured to assess the degree of charge transfer between the rings. As expected, the conductivity is very low, confirming the electrostatic nature of the interactions.

X-ray charge density; pi-interactions; quinoid rings

Djelomično financirano preko Zaklade HAZU (za 2013), slovensko-hrvatske bilateralne suradnje (za 2011-2012) te stipendije IRB-a i francuskog veleposlanstva