A qualitative failure of B3LYP for textbook organic reactions.

Depending on the selected DFT functional, two different mechanisms are found for two organic reactions (an intramolecular nucleophilic aromatic substitution and a nucleophilic addition on a carbonyl moiety). Indeed, B3LYP predicts a concerted mechanism whereas M06-2X foresees a multistep one. Calculations at the MP4(SDQ) level proved the mechanisms to be stepwise. We studied these reactions with a large panel of exchange-correlation functionals and demonstrated that the amount of exact exchange is of first importance. For some borderline cases, the form of the functional has also an impact, e.g. the Meisenheimer σ-adduct of the intramolecular nucleophilic aromatic substitution can be located with B3PW91 but not with B3LYP. These results stress the need to use recently proposed functionals to investigate chemical reactivity.