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Semiempirical MNDO calculations of a symmetry-allowed dimerization of benzene are reported. The chemically bound C/sub s/ point-group dimer (o,p'-dibenzene) studies has the appearance of a 1,4-cyclohexadiene molecule bonded to 1,3-cyclohexadiene. The calculations augment results given earlier for four symmetry-forbidden dimerizations of benzene. The character of six stationary points on the C/sub 12/H/sub 12/ energy hypersurfrace is examined by use of the Hessian matrix. These points are (1) two benzenes 1, (2) the stable C/sub s/ symmetry benzene dimer 2, (3) a stable C/sub 1/ symmetry biradical 3, (4) two C/sub 1/ symmetry transition states 4 and 5, and a C/sub s/ symmetry hill 6. MNDO predicts that the formation of the two new sigma bonds on the lowest energy reaction path leading from 1 to 2 is asynchronous. The stationary point on a synchronous C/sub s/ symmetry path leading to 2 from 1 is a hill; i.e., there are two negative vibrational eigenvalues at this point. The points 3, 4, and 5 are shown to have biradical character by use of unrestricted Hartree-Fock (UMNDO) and 3 x 3 configuration interaction MNDO calculations; similar calculations on 1, 2, and 6 show that for these points a biradical description is less necessary.more » The activation barrier on the path from 1 to 2 via 4 and 5, corrected for zero-point vibrational effects, is 55 kcal/mol. This is 40 kcal/mol less than that found for any of the symmetry-forbidden dimers. The MNDO energy of metastable 2 and 3, corrected for zero-point effects, is 41 and 18 kcal/mol, respectively, higher than that of 1. At the stationary points, some comparisons of the MNDO energies with ab initio minimal basis set SCF and group additivity energies are made.« less |
