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

Abstract

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 À1 with an activation energy of 5.0 kcal mol À1 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 À1 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 À1 at 0 K; to hydroperoxyacetaldehyde with a calculated activation energy of 22.8 kcal mol À1 at 0 K; and to oxirane excited molecular oxygen ( 1 D 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 À1 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 À1 with an energy barrier of 48.7 kcal mol À1, whereas the tautomerization of SOZ leading to hydroxymethyl formate is highly exothermic (72.2 kcal mol À1) and has an activation energy of 32.6 kcal mol À1 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 À1) and has a low energy barrier (3.0 kcal mol À1 ); isomerization to formic acid, also highly exothermic (101.9 kcal mol À1 ) with a low activation energy (2.2 kcal mol À1 at 0 K); and radical fragmentation into H HCOO, in a slightly endothermic process (4.9 kcal mol À1) with an activation energy of 18.4 kcal mol À1 at 0 K. The dissociation of formic acid into H 2 , CO 2 , CO and H 2 O and the decomposition of HCOO into H radicals CO 2 are also discussed.

Perspective: Spectroscopy and kinetics of small gaseous Criegee intermediates

“…56,93 The enhanced rate coefficient of the reaction CH 2 OO + (H 2 O) 2 was theoretically predicted 111,112 before the experimental realization of this important reaction. 80,86,89 …”

“…First-order decay rate coefficients of CH 2 OO have been reported to be in the range 0-200 s −1 , depending on experimental conditions. 65,66 Anglada et al investigated with theory the ozonolysis of ethene 86 and the unimolecular isomerization and loss of an O-atom from CH 2 OO and CH 3 CHOO. 87 They concluded that energetic CH 2 OO can decompose to HCO + OH and O( 3 P) + H 2 CO or isomerize to dioxirane that isomerizes further to methylenebis(oxy).…”

Observation of the Simplest Criegee Intermediate Ch2oo in the Gas-Phase Ozonolysis of Ethylene

“…The first step of the O 3 + C 2 H 4 reaction is highly exothermic (∆H ≅ −50 kcal/mol) (24,25), but the overall reaction rate coefficient is relatively small (1.45 × 10 −18 cm 3 molecule…”

Matrix isolation FTIR spectroscopical study of ethene secondary ozonide

“…This is also true for the dissociation of the secondary ozonide. Two different paths of the ethene SOZ dissociation are theoretically predicted [11]. One path includes the direct formation of dioxymethane and formaldehyde (1).…”

“…These values are much higher than 134-159 kJ/mol (activation energy for ethene SOZ decomposition [20]) and therefore are too high for these potentials to be a reaction coordinate of dissociation of ethene SOZ. Theoretically calculated barrier for ethene SOZ dissociation to dioxymethane (dioxirane) and formaldehyde is 204 kJ/mole, while dissociation to hydroxymethylformiate is 136.5 kJ/mol [11]. Our experimental results support these theoretical predictions.…”