Simulations confirm that the use of electron withdrawing groups on radicals weakens the resulting C–C bond. The reason for this so-called captodative effect has now been discovered, according to a paper published in ChemistryEurope.

Radicals, molecules with one unpaired electron, are known to be highly reactive substances. But if you choose your substituents well, you can reduce this reactivity to some extent. If you use both electron donating and electron withdrawing groups, this is called the captodative effect. That it works is well known in organic chemistry, but how it works is another question. Eva Blokker, Matthias Bickelhaupt and colleagues from the TheoCheM group at VU Amsterdam have used orbital theory and simulations to map the mechanism behind the captodative effect. Their results are published in the new open access journal ChemistryEurope.

In the paper, they looked at para-substituted phenyl dicyanomethyl radicals, while varying the substituent on the phenyl group. If you put hydrogen or cyan (withdrawing group) at the para position, the C–C bond between the two radicals is stronger than if you use -OMe or -NMe₂ (both donating groups), which actually make the bond weaker.