engleski

Stacking of planar organic radicals: strong non-covalent or weak covalent interactions?

Stacking of planar organic radicals involves some of the closest interplanar and intermolecular C-C contacts, of ca. 2.9 Å. Quantum chemical models indicate a considerable covalent component in the total interaction, and this type of weak multicentric covalent bonding has been dubbed 'pancake bonding'. Three types of stacks of planar radicals are known: 1) stacks of 'pancake-bonded' dimers, separated by weaker stacking contacts, [3] 2) stacks of equidistant radicals with long-range ordering and 3) stacks of 'pancake-bonded' trimers of partially charged radicals. We have recently commenced the first X-ray charge density study of these unusual intermolecular interactions, and have for the first time experimentally confirmed their partial covalent character. Experimentally determined AIM-criteria for pancake bonding involve a presence of a (3, +3) critical point (i.e. a local minimum of electron density) between the rings, and presence of multiple (3, -1) (i.e. bonding) critical points with electron density close to 0.1 e Å-3. Also, the crystals are diamagnetic (stacks of types 1 and 3) or antiferromagnetic (type 2) due to coupled spins in the neighbouring radicals. To gain more insight into their nature, we have extended the study to variable-temperature (in the range 80 - 400 K) and high-pressure single-crystal X-ray diffraction. Temperature-dependence of the stacking parameters reveals data about strength (estimated to be ≥ 15 kcal mol-1) and dynamics of 'pancake bonding', while study under high pressure reveals compressibility and other parameters ; it also allows us to study this type of intermolecular interaction under extreme conditions.

pi-interactions ; multicentric bonding ; semiquinone radicals ; charge density ; high pressure

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