Addition Reactions of ¹D and ³P Atomic Oxygen with Acetylene. Potential Energy Surfaces and Stability of the Primary Products. Is Oxirene Only a Triplet Molecule? A Theoretical Study

Abstract

The potential energy surfaces (PES) of the primary addition reactions of atomic oxygen O(1D) and O(3P) on acetylene were done using Density Functional Theory with the standard B3LYP hybrid functional. The crucial areas of these PES were then refined using the UMP2 and CCSD(T) schemes and, in few cases, CASPT2, MCSCF, and MR-AQCC ones. The stability of these primary products was studied. The reactivity of C2H2 and O, especially in low-temperature matrix conditions, is discussed on these bases. The singlet surface shows that the insertion of the oxygen into the C−H bond, resulting in ethynol HC⋮COH, is the main reaction. An addition onto the C⋮C bond, leading to oxirene and/or formylcarbene HC−CHO is nevertheless to be considered. A particular attention was given to the oxirene-formylcarbene region and MR-AQCC calculations predict an extremely flat area in agreement with previous CCSD(T) results. Secondary transposition of formylcarbene into ketene H2CCO could be avoided at very low temperature. From the triplet state of the oxygen atom, the triplet formylcarbene is expected as the major product. Nevertheless, a cyclic nonplanar species, the triplet state of oxirene, is stable on the triplet surface and could be trapped at low temperature, provided an intersystem triplet singlet crossing does not occur, which would lead to singlet oxirene, if stable. If not, this process would lead to singlet formyl carbene and finally to its triplet ground state. Indeed, singlet and triplet surfaces are very close to one another in this part of the PES. Hydrogen transposition giving triplet ketene can be ruled out in matrix conditions. The formation of cyclic three-member carbenes −C−O−CH2−, as well singlet as triplet, is unlikely though both species are rather stable on their respective PES.

Oxygen atom reactions with C2H6, C2H4, and C2H2 in ices

“…Ethynol, the direct insertion product of O( 1 D) + acetylene, is not observed in the IR spectrum in our experiments (Hochstrasser & Wirz 1989). Theoretical studies show that ethynol could convert to formylcarbene and then to ketene (Girard & Chaquin 2003), and so if an insertion chemistry is active it could be contributing to the observed ketene formation. However, we expect that the overall contribution will be minor since, as with ethylene, addition to the multiple bond should occur faster than insertion.…”

Formation of nitric oxide and nitrous oxide in electron-irradiated H218O/N2 ice mixtures—evidence for the existence of free oxygen atoms in interstellar and solar system analog ices

“…This in turn assists to derive reaction mechanisms on the decomposition of water and also on the reactivity of the energetic oxygen atoms released in this process. Since water is the dominating component of interstellar grains and of icy objects in the outer Solar System, these findings would provide evidence on a hitherto neglected route to generate highly reactive oxygen atoms from the most dominating ice componentwater-which in turn can add to unsaturated bonds 34 and insert into single bonds [35][36][37] of astrobiologically important molecules embedded inside these ices.…”

Dissection of the Multichannel Reaction O(3P) + C2H2: Differential Cross-Sections and Product Energy Distributions

“…Due to its vital significance in combustion chemistry, this reaction has attracted great attention. The kinetic and dynamic properties for this reaction have been measured and calculated over a wide temperature range using various techniques [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. Until recently, the experimental rate constants and branching ratios (BRs) have been confirmed by quasi-classical trajectory calculations based on the global potential energy surface (PES) at UCCSD(T)-F12b/VTZ-F12 level developed by our group [ 29 ].…”