Mechanism for covalence bond benzene dimers formation: A DFT and MP2 investigation

Covalent dimerization mechanisms of benzene were studied by MP2 and three DFT methods with 6-311G(d,p) basis set. All the methods predicted similar meta- and para-cycloaddition mechanisms. MP2 results showed that (1) the activation enthalpies of both the endo- and exo-meta-cycloadditions are about 90 kcal/mol, twice as high as that of para-cycloaddition and (2) further transformation of para-cycloaddition adduct to D2h-(CH)12 and endo-meta-cycloaddition adduct to D3d-(CH)12 need to surmount high enthalpy barriers of 65.02 and 90.78 kcal/mol (relative to two molecules of benzene), respectively. Molecular orbital analysis revealed that meta-cycloaddition of benzene is ground state allowed reaction.