Symmetry, radical ions, and butadienes: Exploring the limits of density functional theory

Pure and hybrid density functional theory (DFT) methods are evaluated regarding their ability to correctly describe asymmetric transition structures for symmetric butadiene radical cations. The study covers unsubstituted butadiene radical cation as well as (2,3-X,X)-disubstituted butadiene radical cations, where X is -CH3, -OH, -F, or -SiH3. The DFT methods either converge toward an erroneous symmetric solution or describe the asymmetric transition structure qualitatively correctly. Whether or not the asymmetric transition structure is found depends on the amount of Hartree-Fock density exchange included in the method, as well as on the electronic characteristics of the substituents, as described by their ol values. The disparate behaviors encountered are rationalized by a two-state model. It is also shown that molecular symmetry does not automatically induce, errors, as reported by earlier studies. Instead, electronic symmetry is introduced as a factor to observe when organic radical cations are studied computationally.