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

Abstract

The quantum theory of atoms-in-molecules (QTAIM) method is used to examine the OO bond in peroxides (RO-OR) and nitroxide dimers (R2 NO-ONR2 ), including Fremy's salt. The electron density (ρ), electron kinetic energy density [K(ρ)], and Laplacian of the electron density (∇2 ρ) at bond critical points characterize the nature of the OO bond. The data distinguish OO bonding of two kinds. Large values of ρ and positive ∇2 ρ and K(ρ) suggest that simple peroxides have charge-shift bonds. Nitroxide dimers, with smaller ρ, positive ∇2 ρ, and near-zero K(ρ), 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.