What Are the Reasons for the Kinetic Stability of a Mixture of H₂ and O₂?

Abstract

Calculations at the (14,10)CASSCF/6-31G** and the MR-(S)DCI/cc-pVTZ levels are employed to answer the title question by studying three possible modes of reaction between dioxygen and dihydrogen molecules at the ground triplet state and excited singlet state of O2. These reaction modes, which are analogous to well-established mechanisms for oxidants such as transition metal oxene cations and mono-oxygenase enzymes, are the following: (i) the concerted addition, (ii) the oxene-insertion, and (iii) the hydrogen abstraction followed by hydrogen rebound. The “rebound” mechanism is found to be the most preferable of the three mechanisms. However, the barrier of the H-abstraction step is substantial both for the triplet and the singlet states of O2, and the process is highly endothermic (>30 kcal/mol) and is unlikely to proceed at ambient conditions. The calculations revealed also that the lowest singlet state of O2 has very high barriers for reaction and therefore cannot mediate a facile oxidation of H2 in contrast to transition metal oxenide cation catalysts and mono-oxygenase enzymes. This fundamental difference is explained.

Synthesis of Metallo-Deuteroporphyrin Derivatives and the Study of Their Biomimetic Catalytic Properties

“…Thousands of such proteins are now known, such as 57 in the human genome (Guengerich, 2005), 20 in Mycobacterium tuberculosis (McLean & Munro, 2008), 272 in Arabidopsis (Ehlting et al, 2006), and the surprising number of 457 in rice (Schuler & Werck, 2003), and so on. Molecular oxygen, itself, is unreactive toward organic molecules at low temperatures either due to spin-forbiddenness or to high barriers (Filatov et al, 2000). Consequently, living systems mainly use enzymes that modify dioxygen to a form capable of performing the desired oxidation reaction.…”

Oxidase Catalysis via Aerobically Generated Hypervalent Iodine Intermediates

“…Development of chemical strategies to couple O2 reduction directly to substrate oxidation would enable sustainable synthetic methods. [1][2][3] Selective and efficient utilization of O2, however, is challenging due to the triplet ground state of O2, which imposes substantial kinetic barriers to O2 reduction and gives rise to poorly selective radical chemistry, 4,5 and due to the disparity between the electron inventories of four-electron O2 reduction and two-electron substrate oxidation. 6,7 Broadly, there are two approaches to O2 utilization: (1) oxygenase chemistry, in which O2 serves as both an oxidant and the source of oxygen content in the organic reaction products, and (2) oxidase chemistry, in which O2 serves as a proton and electron acceptor but is not incorporated into the organic reaction products ( Figure 1).…”

Physics and chemistry of the influence of excited molecules on combustion enhancement

“…As was revealed on the basis of ab initio calculations [7,39,40], the principal chain-initiation reaction in the H 2 −O 2 mixture is the reaction…”