An atoms-in-molecules characterization of the nature of the O-O bond in peroxides and nitroxide dimers

The quantum theory of atoms-in-molecules (QTAIM) method is used to examine the O-O bond in peroxides (RO-OR) and nitroxide dimers (R2NO-ONR2), including Fremy's salt. The electron density (rho), electron kinetic energy density [K(rho)], and Laplacian of the electron density (del(2)rho) at bond critical points characterize the nature of the O-O bond. The data distinguish O-O bonding of two kinds. Large values of rho and positive backward difference del(2)rho and K(rho) suggest that simple peroxides have charge-shift bonds. Nitroxide dimers, with smaller rho, positive backward difference del(2)rho, and near-zero K(rho), show a lack of shared electron density, suggesting there is no conventional O-O bonding in these molecules. QTAIM analysis at the B3LYP/6-311+G(d,p) level of theory gives results in agreement with valence-bond theory and X-ray diffraction characterizations of peroxide O-O bonds as charge-shift bonds. In contrast, CCSD/cc-pVDZ calculations fail to agree with previous results because of an insufficient, single-determinant treatment of the charge-shift bond.