Kinetic and mechanistic study of the reaction of atomic chlorine with methyl bromide over an extended temperature range

Benchmark ab initio characterization of the multi-channel Cl + CH3X [X = F, Cl, Br, I] reactive potential energy surfaces

We determine benchmark geometries and relative energies for the stationary points of the Cl + CH3X [X = F, Cl, Br, I] reactions. We consider four possible reaction pathways: hydrogen abstraction, hydrogen substitution, halogen abstraction, and halogen substitution, where the substitution processes can proceed via either Walden inversion or front-side attack. We perform geometry optimizations and obtain harmonic vibrational frequencies at the explicitly-correlated UCCSD(T)-F12b/aug-cc-pVTZ level of theory, followed by UCCSD(T)-F12b/aug-cc-pVQZ single-point computations to make finite-basis-set error negligible. To reach chemical (<1 kcal mol-1), or even subchemical (<0.5 kcal mol-1) accuracy, we include core-correlation, scalar relativistic, post-(T), spin-orbit-splitting and zero-point-energy contributions, as well, in the relative energies of all the stationary points. Our benchmark 0 K reaction enthalpies are compared to available experimental results and show good agreement. The stationary-point structures and energetics are interpreted in terms of Hammond's postulate and used to make predictions related to the dynamical behavior of these reactive systems.

Theoretical study of the kinetics of reactions of the monohalogenated methanes with atomic chlorine

Ab initio calculations at the G2 level were used in a theoretical description of the kinetics and mechanism of the hydrogen abstraction reactions from fluoro-, chloro- and bromomethane by chlorine atoms. The profiles of the potential energy surfaces show that mechanism of the reactions under investigation is complex and consists of two - in the case of CH₃F+Cl - and of three elementary steps for CH₃Cl+Cl and CH₃Br+Cl. The heights of the energy barrier related to the H-abstraction are of 8–10 kJ mol⁻¹, the lowest value corresponds to CH₃Cl+Cl and the highest one to CH₃F+Cl. The rate constants were calculated using the theoretical method based on the RRKM theory and the simplified version of the statistical adiabatic channel model. The kinetic equations derived in this study\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \begin{array}{*{20}c} {k\left( {\mathrm{C}{{\mathrm{H}}_3}\mathrm{F}+\mathrm{Cl}} \right)=6.75\times 1{0^{-12 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{2.12 }}\times\exp (-900/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ {k\left( {\mathrm{C}{{\mathrm{H}}_3}\mathrm{Cl}+\mathrm{Cl}} \right)=6.97\times 1{0^{-12 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{1.73 }}\times\exp (-795/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ {k\left( {\mathrm{C}{{\mathrm{H}}_3}\mathrm{Br}+\mathrm{Cl}} \right)=6.26\times 1{0^{-12 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{1.82 }}\times\exp (-795/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ \end{array} $$\end{document}and\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \begin{array}{*{20}c} {k\left( {\mathrm{C}{{\mathrm{H}}_2}\mathrm{F}+\mathrm{HCl}} \right)=2.88\times 1{0^{-13 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{2.02 }}\times\exp (-1255/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ {k\left( {\mathrm{C}{{\mathrm{H}}_2}\mathrm{Cl}+\mathrm{HCl}} \right)=2.42\times 1{0^{-13 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{1.57 }}\times\exp (-2100/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ {k\left( {\mathrm{C}{{\mathrm{H}}_2}\mathrm{Br}+\mathrm{HCl}} \right)=2.21\times 1{0^{-13 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{1.69 }}\times\exp (-1485/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ \end{array} $$\end{document}allow a description of the kinetics of the reactions under investigation in the temperature range of 200–3000 K. The kinetics of reactions of the entirely deuterated reactants were also included in the kinetic analysis. Results of ab initio calculations show that D-abstraction process is related with the energy barrier of 5 kJ mol⁻¹ higher than the H-abstraction from the corresponding non-deuterated reactant molecule. The derived analytical equations for the reactions, CD₃X+Cl, CH₂X+HCl and CD₂X+DCl (X = F, Cl and Br) are a substantial supplement of the kinetic data necessary for the description and modeling of the processes of importance in the atmospheric chemistry.

Kinetics, Mechanism, and Thermochemistry of the Gas Phase Reaction of Atomic Chlorine with Dimethyl Sulfoxide

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of chlorine atoms with dimethyl sulfoxide (CH3S(O)CH3; DMSO) as a function of temperature (270-571 K) and pressure (5-500 Torr) in nitrogen bath gas. At T = 296 K and P > or = 5 Torr, measured rate coefficients increase with increasing pressure. Combining our data with literature values for low-pressure rate coefficients (0.5-3 Torr He) leads to a rate coefficient for the pressure independent H-transfer channel of k1a = 1.45 x 10(-11) cm3 molecule(-1) s(-1) and the following falloff parameters for the pressure-dependent addition channel in N2 bath gas: k(1b,0) = 2.53 x 10(-28) cm6 molecule(-2) s(-1); k(1b,infinity) = 1.17 x 10(-10) cm3 molecule(-1) s(-1), F(c) = 0.503. At the 95% confidence level, both k1a and k1b(P) have estimated accuracies of +/-30%. At T > 430 K, where adduct decomposition is fast enough that only the H-transfer pathway is important, measured rate coefficients are independent of pressure (30-100 Torr N2) and increase with increasing temperature. The following Arrhenius expression adequately describes the temperature dependence of the rate coefficients measured at over the range 438-571 K: k1a = (4.6 +/- 0.4) x 10(-11) exp[-(472 +/- 40)/T) cm3 molecule(-1) s(-1) (uncertainties are 2sigma, precision only). When our data at T > 430 K are combined with values for k1a at temperatures of 273-335 K that are obtained by correcting reported low-pressure rate coefficients from discharge flow studies to remove the contribution from the pressure-dependent channel, the following modified Arrhenius expression best describes the derived temperature dependence: k1a = 1.34 x 10(-15)T(1.40) exp(+383/T) cm3 molecule(-1) s(-1) (273 K < or = T < or = 571 K). At temperatures around 330 K, reversible addition is observed, thus allowing equilibrium constants for Cl-DMSO formation and dissociation to be determined. A third-law analysis of the equilibrium data using structural information obtained from electronic structure calculations leads to the following thermochemical parameters for the association reaction: delta(r)H(o)298 = -72.8 +/- 2.9 kJ mol(-1), deltaH(o)0 = -71.5 +/- 3.3 kJ mol(-1), and delta(r)S(o)298 = -110.6 +/- 4.0 J K(-1) mol(-1). In conjunction with standard enthalpies of formation of Cl and DMSO taken from the literature, the above values for delta(r)H(o) lead to the following values for the standard enthalpy of formation of Cl-DMSO: delta(f)H(o)298 = -102.7 +/- 4.9 kJ mol(-1) and delta(r)H(o)0 = -84.4 +/- 5.8 kJ mol(-1). Uncertainties in the above thermochemical parameters represent estimated accuracy at the 95% confidence level. In agreement with one published theoretical study, electronic structure calculations using density functional theory and G3B3 theory reproduce the experimental adduct bond strength quite well.

Rates and Mechanisms for the Reactions of Chlorine Atoms with Iodoethane and 2-Iodopropane

The reaction of Cl atoms with iodoethane has been studied via a combination of laser flash photolysis/resonance fluorescence (LFP-RF), environmental chamber/Fourier transform (FT)IR, and quantum chemical techniques. Above 330 K, the flash photolysis data indicate that the reaction proceeds predominantly via hydrogen abstraction. The following Arrhenius expressions (in units of cm3 molecule(-1) s(-1)) apply over the temperature range 334-434 K for reaction of Cl with CH3CH2I (k4(H)) and CD3CD2I (k4(D)): k4(H) = (6.53 +/- 3.40) x 10(-11) exp[-(428 +/- 206)/T] and k4(D) = (2.21 +/- 0.44) x 10(-11) exp[-(317 +/- 76)/T]. At room temperature and below, the reaction proceeds both via hydrogen abstraction and via reversible formation of an iodoethane/Cl adduct. Analysis of the LFP-RF data yields a binding enthalpy (0 K) for CD3CD2I x Cl of 57 +/- 10 kJ mol(-1). Calculations using density functional theory show that the adduct is characterized by a C-I-Cl bond angle of 84.5 degrees; theoretical binding enthalpies of 38.2 kJ/mol, G2'[ECP(S)], and 59.0 kJ mol(-1), B3LYP/ECP, are reasonably consistent with the experimentally derived result. Product studies conducted in the environmental chamber show that hydrogen abstraction from both the -CH2I and -CH3 groups occur to a significant extent and also provide evidence for a reaction of the CH3CH2I x Cl adduct with CH3CH2I, leading to CH3CH2Cl formation. Complementary environmental chamber studies of the reaction of Cl atoms with 2-iodopropane, CH3CHICH3, are also presented. As determined by relative rate methods, the reaction proceeds with an effective rate coefficient, k6, of (5.0 +/- 0.6) x 10(-11) cm3 molecule(-1) s(-1) at 298 K. Product studies indicate that this reaction also occurs via two abstraction channels (from the CH3 groups and from the -CHI- group) and via reversible adduct formation.

Direct Observation of Adduct Formation of Alkyl and Aromatic Iodides with Cl Atoms Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with RI (R = n-C3H7, n-C4H9, cyclo-C6H11, C6H5, C6F5, and p-CH3C6H4) have been studied using cavity ring-down spectroscopy at a temperature range of 233-313 K and at 100 Torr total pressure of N2 diluent. Visible absorption spectra of the RI-Cl adducts were recorded at 440-520 nm at 263 K. The yields of the adducts were temperature-dependent. There was no discernible reaction of the adducts in the presence of 100 Torr of O2 at 263 K. Theoretical calculations were performed for C4H9I-Cl and C6H5I-Cl for quantitative explanation of the absorption spectra and the strength of the I-Cl bonds in the charge-transfer complexes. Evidence for the adduct formation following the reaction of Cl with C6H5Br was sought but not found at 440 and 520 nm.

Observation of Adducts in the Reaction of Cl Atoms with XCH2I (X = H, CH3, Cl, Br, I) Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) have been studied using cavity ring-down spectroscopy in 25-125 Torr total pressure of N2 diluent at 250 K. Formation of the XCH2I-Cl adduct is the dominant channel in all reactions. The visible absorption spectrum of the XCH2I-Cl adduct was recorded at 405-632 nm. Absorption cross-sections at 435 nm are as follows (in units of 10(-18) cm2 molecule(-1)): 12 for CH3I, 21 for CH3CH2I, 3.7 for CH2ICl, 7.1 for CH2IBr, and 3.7 for CH2I2. Rate constants for the reaction of Cl with CH3I were determined from rise profiles of the CH3I-Cl adduct. k(Cl + CH3I) increases from (0.4 +/- 0.1) x 10(-11) at 25 Torr to (2.0 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) at 125 Torr of N2 diluent. There is no discernible reaction of the CH3I-Cl adduct with 5-10 Torr of O2. Evidence for the formation of an adduct following the reaction of Cl atoms with CF3I and CH3Br was sought but not found. Absorption attributable to the formation of the XCH2I-Cl adduct following the reaction of Cl atoms with XCH2I (X = H, CH3, Br, I) was measured as a function of temperature over the range 250-320 K.