Quantum Mechanical and Kinetic Studies of the Reaction of Methyl Radicals with Chlorine Molecules

Abstract

Quantum mechanical electronic structure calculations were carried out to determine equilibrium geometries, energetics, and normal-mode frequencies for stationary points along the minimum energy reaction path for the reaction of methyl radicals with chlorine molecules. The results are used to calculate the rate coefficient, employing both extended RRKM theory and quasi-classical trajectory techniques. The results of both methods agree well with each other and with the experimental measurements. The reactivity is investigated and is shown to be greatly enhanced by increasing the initial translational energy of the reactants. A large part of the total available energy is shown to be directed into product vibration, in good consistency with the experimental observations.

Kinetics of the reactions of vinyl radicals with molecular oxygen and chlorine at temperatures 200–362 K

“…These reaction steps appear to be exothermic and have little or no activation energy. [15][16][17][18][19][20][21][22][23] Together they constitute an efficient cyclic process for the production of chlorine-containing hydro-carbons. Much is known about the reactions of chlorine atoms with saturated hydrocarbons, step (2a), 17 but the reactions of saturated hydrocarbon free radicals with molecular chlorine, step…”

“…, [17][18][19][20] are less known. A few rate coefficient measurements exist for the reactions of chlorine atoms with unsaturated hydrocarbons, when both steps (2a) and (2b) are available.…”

Quantum Mechanical and Rice−Ramsperger−Kassel−Marcus Investigation of the Thermal Unimolecular Decomposition of CF2BrO and CF2ClO Radicals

“…Thus, suitably adjusted parameters for the present fluorinated radicals had to be estimated in analogy with the values employed for the corresponding chlorinated and brominated methoxy analogues. [24][25][26] The estimated data, ) 250 K and σ ) 5.2 Å, have been used in the falloff calculations.…”

“…Bratom elimination, on the other hand, has always been found to be the preferred decomposition pathway. 10,17,[25][26] C-Br bond breaking in CH 2 BrO presents a much lower energy barrier of only 2.6 kcal mol -1 , while HBr elimination exhibits a higher barrier, 11.5 kcal mol -1 . 26 Thus, the chloromethoxy and bromomethoxy radicals present varying patterns of unimolecular decomposition, and it would be interesting to examine and compare with the dissociation pathways of the corresponding brominated and chlorinated fluorine containing species.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Because of the capacity of many such radicals to release active bromine and chlorine atoms in the stratosphere, many experimental and theoretical studies have been devoted to their chemical activity including various unimolecular decomposition pathways and reactions with molecular oxygen and nitrogen monoxide. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] The brominated and chlorinated fluoromethoxy radicals in particular [4][5]11,15 have been shown in all experimental studies investigating their atmospheric fate to decompose by releasing Cl and Br atoms They thus show interesting peculiarities compared to corresponding hydrogenated species. For instance, CH 2 ClO, in several studies of the unimolecular dissociation pathways, has shown no evidence for loss of Cl atoms, 18 which is consistent with the rather significant barrier of the order of 11 kcal mol -1 24 calculated for the C-Cl bond scission.…”