Ab Initio Characterization of the Isomerism between the μ-η²:η²-Peroxo- and Bis(μ-oxo)dicopper Cores

The interconversion of model compounds {[(NH3)3Cu]2(μ-η2:η2-O2)}2+ (1) and {[(NH3)3Cu]2(μ-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 nondynamic electron correlation. The latter effect depends critically on the significant energy change experienced by the 13au virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1.