Electronic structure and bonding in crystalline peroxides

Abstract

Hartree-Fock and density-functional PW91 theories as realized in the CRYSTAL95 code have been applied to investigate the structural and electronic properties of Ba, Sr, and Ca peroxide materials with the calcium carbide crystal structure, results for which are compared with those for the corresponding oxides. Special attention is paid to the stabilization of the peroxide molecular ion ${\mathrm{O}}_{2}^{2\mathrm{\ensuremath{-}}}$ in the ionic environment provided by the lattice, and to chemical bonding effects. In order to describe the covalent bonding within the ${\mathrm{O}}_{2}^{2\mathrm{\ensuremath{-}}}$ ion and the polarization of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion in the crystal electrostatic field, it is essential to include an account of the effects of electron correlation. The PW91 density functional has allowed us to reproduce the crystallographic parameters within a 3% error. The chemical bonding within the peroxide molecular ion has a complex nature with a balance between the weak covalent bond of ${\ensuremath{\sigma}}_{z}$ type and the strong electrostatic repulsion of the closed-shell electron groups occupying $\mathrm{O}2s$ and $\mathrm{O}{2p}_{x}$ and ${2p}_{y}$ states. Compression of the peroxide ion in the ionic crystals gives rise to an excessive overlap of the $\mathrm{O}2s$ closed shells of the two ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ions of a peroxide molecular ion ${\mathrm{O}}_{2}^{2\mathrm{\ensuremath{-}}},$ which in turn determines the antibonding character of the interaction and chemical bonding in the ${\mathrm{O}}_{2}^{2\mathrm{\ensuremath{-}}}$ molecular ion.