Diradical character of the bond breaking in the reaction of Br2 with benzene: Reliable barriers using the CR-CC(2,3) method

Abstract

Abstract The transition state for the substitution reaction involving one Br2 species with benzene as a model system has a long Br–Br distance, leading to high multiconfigurational character of the wavefunction in apolar medium. Single reference methods can become erratic on this condition and even the CCSD(T) fails for this system. To describe accurately this reaction, the CR-CC(2,3) method was used because predicts a reliable dissociation curve for the Br2 when compared to MRCISD\xa0+\xa0Q calculations. The activation barriers for the reaction of one and two Br2 molecules with benzene were calculated with the CR-CC(2,3) method and used as benchmark to evaluate the performance of the functionals B3LYP, M06-2X, M08-HX, ωB97X-D and B2GP-PLYP. The M08-HX and the B2GP-PLYP are the best functionals, with mean absolute deviation close to 3\xa0kcal\xa0mol−1.