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 HO n H. 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.

Competition Kinetics of the Nonbranched-Chain Addition of Free Radicals to Olefins, Formaldehyde, and Oxygen

“…The spectrum of the HO • 4 radical is similar to that of ozone, but the molar absorption coefficient ε(HO • 4 ) λmax of the former is almost two times larger [68]. The assumption about the cyclic structure of the HO • 4 radical can stem from the fact that its mean lifetime in water at 294 K, which is (3.6±0.4)×10 −5 s (as estimated [59] using the Gaussian-98 program confirmed that the cyclic structure of HO • 4 [73] is energetically more favorable than the helical structure [69] (the difference in energy is 4.8-7.3 kJ mol −1 , depending on the computational method and the basis set). 10 For example, with the MP2(full)/6-31G(d) method, the difference between the full energies of the cyclic and acyclic HO • 4 conformers with their zero-point energies (ZPE) values taken into account (which reduces the energy difference by 1.1 kJ mol −1 ) is −5.1 kJ mol −1 and the entropy of the acyclic-to-cyclic HO • 4 transition is ΔS • 298 =−1.6 kJ mol −1 K −1 .…”

“…The oxygen concentration attained in the liquid may be below the thermodynamically equilibrium oxygen concentration because of diffusion limitations hampering the establishment of the gas-liquid saturated solution equilibrium under given experimental conditions (for example, when the gas is bubbled through the liquid) or because the Henry law is violated for the given gasliquid system under real conditions. 4 radical with a helical structure were carried out using the G2(MP2) method [69]. The stabilization energies of HO • 2 , HO • 4 , and HO • 3 were calculated in the same work to be 64.5 ± 0.1, 69.5 ± 0.8, and 88.5 ± 0.8 kJ mol −1 , respectively.…”

Kinetics of Nonbranched-Chain Processes of the Free-Radical Addition to Molecules of Alkenes, Formaldehyde, and Oxygen with Competing Reactions of Resulting 1:1 Adduct Radicals with Saturated and Unsaturated Components of the Binary Reaction System

“…Nonbranched-chain oxidation of hydrogen and changes in enthalpy (ΔН˚2 98 , kJ mol -1 ) for elementary reactions 10 10 According to Francisco and Williams [49], the enthalpy of formation HO radical with a helical structure were carried out using the G2(MP2) method [68]. The stabilization energies of HO were calculated in the same work to be 64.5  0.1, 69.5  0.8, and 88.5  0.8 kJ mol -1 , respectively.…”

“…The O4 molecule was identified by NR mass spectrometry [74]. assumption about the cyclic structure of the 4 HO  radical can stem from the fact that its mean lifetime in water at 294 K, which is (3.6 ± 0.4) × 10 -5 s (as estimated [66] HO  radical [68,83] estimated in the same way [66] for the same conditions [84], (9.1 ± 0.9) × 10 -6 s. MP2/6-311++G** calculations using the Gaussian-98 program confirmed that the cyclic structure of 4 HO  [85] is energetically more favorable than the helical structure [68] (the difference in energy is 4.8-7.3 kJ mol -1 , depending on the computational method and the basis set). 11 For example, with the MP2(full)/6-31G(d) method, the difference between the full energies of the cyclic and acyclic HO  can exist in both forms, but the cyclic structure is obviously dominant (87%, K eq = 6.5) [85].…”

A New Mechanism of Free-Radical Nonbranched-Chain Oxidation: Oxygen as an Antioxidant

“…9 According to Francisco and Williams [20], the enthalpy of formation HO radical with a helical structure were carried out using the G2(MP2) method [39]. The stabilization energies of HO were calculated in the same work to be 64.5  0.1, 69.5  0.8, and 88.5  0.8 kJ mol -1 , respectively.…”