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Ab Initio Characterization of the Isomerism Between the mu-eta2:eta2-Peroxo- and Bis(mu-oxo)dicopper Cores

Cramer, C. J.; Smith, B. A.; Tolman, W. B.
J. Am. Chem. Soc. 1996, 118, 11283.

The interconversion of model compounds {[(NH3)3Cu]2 (mu-eta2:eta2-O2)]2+ (1) and {[(NH3)3Cu]2(mu-O)2]2+ (2) has been examined using multireference second-order perturbation theory with an 8-electron/8-orbital active space. At this level of theory, 1 and 2 are separated by only 0.3 kcal/mol, and the barrier to isomerization is predicted to be very low based on single-point energy calculations for intermediate structures. The flat nature of the potential energy surface along the interconversion coordinate derives from a balancing of Coulomb forces and non-dynamic electron correlation. The latter effect depends critically on the significant energy change experienced by the 13au sigma*OO virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1.

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