The Ozonolysis of Ethylene: A Theoretical Study of the Gas-Phase Reaction Mechanism

摘要

The gas-phase reaction mechanism of ethylene ozonolysis has been investigated from a theoretical point of view. The formation of the ethylene primary ozonide (1, 2, 3-trioxolane, POZ) from the ozone-ethylene reaction is calculated to be exothermic by 49.2 kcal mol~(-1) with an activation energy of 5.0 kcal mol~(-1) at 0 K, in agreement with experimental estimates. We have found two different paths for the cleavage of POZ, namely a concerted and a stepwise mechanism. The concerted path leads to the formation of Criegee intermediates (carbonyl oxide-formaldehyde pairs), for which we have calculated an activation energy of 18.7 kcal mol~(-1) at 0 K. The non-concerted mechanism involves three different routes for the POZ decomposition, leading to the formation of Criegee intermediates with a computed activation energy of 21.6 kcal mol~(-1) at 0 K; to hydroperoxyacetaldehyde with a calculated activation energy of 22.8 kcal mol~(-1) at 0 K; and to oxirane + excited molecular oxygen (~1 #DELTA#_g) with a higher activation energy. Moreover, hydroperoxyacetaldehyde is formed with an excess of energy, so that it can decompose yielding OH radicals. The reaction of carbonyl oxide and formaldehyde produces the ethylene secondary ozonide (1, 2, 4-trioxolane, SOZ) and involves the formation of a van der Waals complex on the reaction coordinate, prior to the transition state. The process is calculated to be exothermic by 46.0 kcal mol~(-1) and the energy of the transition state is computed to be lower than that of the reactants: this process can therefore be considered barrierless. SOZ cleaves by a stepwise mechanism and we have found two different fates for its decomposition: dioxymethane + formaldehyde, and hydroxymethyl formate. The former is calculated to be endothermic by 33.3 kcal mol~(-1) with an energy barrier of 48.7 kcal mol~(-1), whereas the tautomerization of SOZ leading to hydroxymethyl formate is highly exothermic (72.2 kcal mol~(-1)) and has an activation energy of 32.6 kcal mol~(-1) at 0 K. The unimolecular decomposition of dioxymethane, following three different paths, is also reported: dissociation into CO_2 + H_2, which is highly exothermic (111.6 kcal mol~(-1)) and has a low energy barrier (3.0 kacl cm~(-1)); isomerization to formic acid, also highly exothermic (101.9 kcal mol~(-1)) with a low activation energy (2.2 kcal mol~(-1) at 0 K); and radical fragmentation into H + HCOO, in a slightly endothermic process (4.9 kcal mol~(-1)) with an activation energy of 18.4 kcal mol~(-1) at 0 K. The dissociation of formic acid into H_2, CO_2, CO and H_2O and the decomposition of HCOO into H radicals + CO_2 are also discussed.