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

Abstract

The interconversion of model compounds {[(NH 3 ) 3 Cu] 2 (μ-η 2:η 2 -O 2 )} 2+ ( 1 ) and {[(NH 3 ) 3 Cu] 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 13a u virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1 .