How Long Can You Make an Oxygen Chain?

Abstract

This paper reports theoretical gas-phase structures and energetics using G2(MP2) theory for saturated oxygen chains of the general formula HOnH. Structural trends are discussed using a simple hyperconjugation model which is capable of giving a qualitative explanation for trends in bond lengths and dihedral angles. Bond dissociation energies (BDEs) are calculated for chains of increasing length, giving 49.9, 33.9, and 17.8 kcal/mol for H2O2, H2O3, and H2O4, respectively. From an analysis of the radical stabilization energy of the fragments remaining after dissociation, it is shown that a minimum value for the BDE for any hydrogen polyoxide is 6.4 kcal/mol, which occurs for the center bond in H2O6, and that longer chains will have a higher BDE. Decomposition pathways responsible for the observed instability of the polyoxides higher than hydrogen peroxide are discussed and results are given for three low-barrier dissociation paths: a solvent-assisted path, a base-catalyzed path, and a proton relay mechanism. These mechanisms are probably general and account for the instability of polyoxide chains in proton-containing solvents. Preventing proton transfer, e.g. by perfluoroalkylation, would therefore be expected to increase chain stability, in agreement with experimental observations.