Theoretical study of the interconversion of O-2-binding dicopper complexes

The structures and interconversion pathway between the [Cu-2(mu-eta(2):eta( 2)-O-2)](2+) and [Cu-2(mu-O)(2)](2+) isomers of model systems with three am monia ligands per copper center are investigated using both density functio nal theory with a B3LYP functional (B3LYP-DFT) and multiconfigurational per turbation theory (CASSCF/CASPT2). Both methods lead to thoroughly different results for the relative energy of both isomers. The CASPT2 results reveal an intrinsic stabilization of the [Cu-2(mu-O)(2)](2+) isomer, thus indicat ing that the presence of a [Cu-2(mu-eta(2):eta(2)-O-2)](2+) core in the res piratory proteins must be brought back to the presence of bulky capping lig ands and/or to external effects caused by solvents and counterions. Both is omers are found to be diamagnetic. For the [Cu-2(mu-eta(2):eta(2)-O-2)](2+) isomer an antiferromagnetic coupling constant, -2J, of 4209 cm(-1) is calc ulated at the CASPT2 level. On the other hand, in the [Cu-2(mu-O)(2)](2+) i somer, the lowest B-3(u) state is calculated at 9316 cm(-1), but is found t o correspond instead to an oxygen pi* --> sigma* excitation.