π-system radicals may interact with each other in a special way. An actual recombination of the radicals is not possible due to the delocalization in the π-system. But the binding between two such radicals is still a lot stronger than would be expected from pure van-der-Waals interactions. This phenomenon is called "pancake bonding".
There is a new paper out by us, studying this: "Study of the Diradicaloid Character in a Prototypical Pancake-Bonded Dimer: The Stacked Tetracyanoethylene (TCNE) Anion Dimer and the Neutral K2TCNE2 Complex" in ChemPhysChem.
Pancake bonding is quite demanding from a computational point of view, since you need to describe the static correlation of the radicals as well as dynamic correlation, which determines the dispersion interaction. We used the multireference averaged quadratic coupled cluster (MR-AQCC) method, which should be able to deal with these effects. MR-AQCC is very similar to multireference configuration interaction only that there is an additional correction for size-extensivity.
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