Bonding in mixed halogen and hydrogen peroxides
The geometries and vibrational frequencies of the hydrogen and halogen peroxides XOOX′ and the XOO and XO fragments (X, X′ = H, F, Cl, Br or I) have been studied using non-local density functional theory. The X–O, X′–O and O–O bond energies have been calculated and likely dissociation paths for these atmospherically important or potentially important molecules suggested. The sulfur analogues have also been examined. A unified model for these chemically diverse species is presented based on the interaction between O2 and X· · ·X fragments. The correlation between their electronic structures is outlined. The antibonding nature of the interaction between the halogen lone pairs and the π electrons of the O2 fragments causes lengthening and weakening of the halogen–oxygen bonds. The electronegativity of X and X′ determines the extent and direction of the electron transfer between the O2 and X· · ·X fragments. The O–O bond order is thus sensitive to the nature of the substituents and the multiple bond character decreases steadily as the electronegativity of X and X′ decreases. The O–O bond strengths, though, are also affected by steric interactions between the halogen `lone pairs'. The O–O bonds in the HO–OX′ species are thus much stronger than the bond orders and lengths suggest.