Abstract
Ab initio calculations at the G3 level were used in a theoretical description of the kinetics and mechanism of the chlorine abstraction reactions from mono-, di-, tri- and tetra-chloromethane by chlorine atoms. The calculated profiles of the potential energy surface of the reaction systems show that the mechanism of the studied reactions is complex and the Cl-abstraction proceeds via the formation of intermediate complexes. The multi-step reaction mechanism consists of two elementary steps in the case of CCl4 + Cl, and three for the other reactions. Rate constants were calculated using the theoretical method based on the RRKM theory and the simplified version of the statistical adiabatic channel model. The temperature dependencies of the calculated rate constants can be expressed, in temperature range of 200–3,000 K as\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\begin{array}{*{20}c} {k\\left( {\\mathrm{C}{{\\mathrm{H}}_3}\\mathrm{C}\\mathrm{l}+\\mathrm{Cl}} \\right) = 2.08\ imes {10^{-11 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{1.63 }}\ imes\\exp \\left( {{-12780 \\left/ {\\mathrm{T}} \\right.}} \\right)\\ }{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ {k\\left( {\\mathrm{C}{{\\mathrm{H}}_2}\\mathrm{C}{{\\mathrm{l}}_2}+\\mathrm{Cl}} \\right) = 2.36\ imes {10^{-11 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{1.23 }}\ imes\\exp \\left( {{-10960 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ {k\\left( {\\mathrm{C}\\mathrm{HC}{{\\mathrm{l}}_3}+\\mathrm{Cl}} \\right) = 5.28\ imes {10^{-11 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{0.97 }}\ imes\\exp \\left( {{-9200 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ {k\\left( {\\mathrm{C}\\mathrm{C}{{\\mathrm{l}}_4}+\\mathrm{Cl}} \\right) = 1.51\ imes {10^{-10 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{0.58 }}\ imes \\exp \\left( {{-7790 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ \\end{array} $$\\end{document} The rate constants for the reverse reactions CH3/CH2Cl/CHCl2/CCl3 + Cl2 were calculated via the equilibrium constants derived theoretically. The kinetic equations\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\begin{array}{*{20}c} {k\\left( {\\mathrm{C}{{\\mathrm{H}}_3}+\\mathrm{C}{{\\mathrm{l}}_2}} \\right) = 6.70\ imes {10^{-13 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{1.51 }}\ imes \\exp \\left( {{270 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ {k\\left( {\\mathrm{C}{{\\mathrm{H}}_2}\\mathrm{C}\\mathrm{l}+\\mathrm{C}{{\\mathrm{l}}_2}} \\right) = 7.34\ imes {10^{-14 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{1.43 }}\ imes \\exp \\left( {{390 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ {k\\left( {\\mathrm{C}\\mathrm{HC}{{\\mathrm{l}}_2}+\\mathrm{C}{{\\mathrm{l}}_2}} \\right) = 6.81\ imes {10^{-14 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{1.60 }}\ imes \\exp \\left( {{-370 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ {k\\left( {\\mathrm{C}\\mathrm{C}{{\\mathrm{l}}_3}+\\mathrm{C}{{\\mathrm{l}}_2}} \\right) = 1.43\ imes {10^{-13 }}\ imes {{{\\left( {{{\\mathrm{T}} \\left/ {300 } \\right.}} \\right)}}^{1.52 }}\ imes \\exp \\left( {{-550 \\left/ {\\mathrm{T}} \\right.}} \\right)}{\\mathrm{c}{{\\mathrm{m}}^3}\\mathrm{molecul}{{\\mathrm{e}}^{-1 }}{{\\mathrm{s}}^{-1 }}} \\\\ \\end{array} $$\\end{document}allow a very good description of the reaction kinetics. The derived expressions are a substantial supplement to the kinetic data necessary to describe and model the complex gas-phase reactions of importance in combustion and atmospheric chemistry.
Highlights
Chlorinated alkanes are used widely in laboratory syntheses and in the chemical industry [1]
The main aim of the present study was to perform a theoretical analysis of the kinetics of chlorine abstraction from chlorinated methanes, CH3Cl, CH2Cl2, CHCl3 and CCl4 by chlorine atoms
Theoretical investigations based on ab initio calculations of the CH4−xClx+ Cl → CH4−xClx−1+ Cl2 (x= 1,2,3 and 4) reaction systems at the G3 level were performed to gain insight into the reaction mechanism
Summary
Chlorinated alkanes are used widely in laboratory syntheses and in the chemical industry [1]. As a consequence, they are penetrating into the environment in steadily increasing amounts. The chemical inertness and high volatility of chloromethanes mean that they can remain in the atmosphere for a very long time. The products of the atmospheric destruction of chloromethanes have been proven to have a significant impact on chlorine chemistry in the atmosphere and may be involved in various catalytic reaction cycles responsible for the depletion of the stratospheric ozone layer [1, 2]. Monochloromethane (CH3Cl) is regarded as the most abundant halocarbon in the troposphere, and the largest natural source of stratospheric chlorine [1, 3].
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