Kinetic Isotope Effect in Photochlorination of H2, CH4, CHCl3, and C2H6

Accurate Quantum Dynamics Calculations for the Cl + CH4/CHD3/CD4 Reaction Rates

Full-dimensional quantum dynamics simulations of the reaction of Cl with methane and its isotopomers are reported. Thermal rate constants are computed for the Cl + CH4 → HCl + CH3, Cl + CHD3 → HCl + CD3, and Cl + CD4 → DCl + CD3 reactions. Temperatures between 200 and 500 K are considered. In this temperature range, excellent agreement with the experiment is obtained. A detailed analysis of the kinetic isotope effect reveals the crucial importance of the CH3/CD3 umbrella motion. Comparison with approximate ring-polymer molecular dynamics simulations shows significant differences depending on the isotope studied.

An accurate potential energy surface and ring polymer molecular dynamics study of the Cl + CH4→ HCl + CH3reaction

Thermal rate coefficients for the Cl + CH₄/CD₄reactions were studied on a new full-dimensional accurate potential energy surface with the spin–orbit corrections considered in the entrance channel.

Full-dimensional analytical potential energy surface describing the gas-phase Cl + C2H6 reaction and kinetics study of rate constants and kinetic isotope effects

Within the Born–Oppenheimer approximation a full-dimensional analytical potential energy surface, PES-2017, was developed for the gas-phase hydrogen abstraction reaction between the chlorine atom and ethane, which is a nine body system.

Temperature dependence of the Cl atom reaction with deuterated methanes

Kinetic isotope effect (KIE) and reaction rate coefficients, k1-k4, for the gas-phase reaction of Cl atoms with (12)CH3D (k1), (12)CH2D2 (k2), (12)CHD3 (k3), and (12)CD4 (k4) over the temperature range 223-343 K in 630 Torr of synthetic air are reported. Rate coefficients were measured using a relative rate technique with (12)CH4 as the primary reference compound. Fourier transform infrared spectroscopy was used to monitor the methane isotopologue loss. The obtained KIE values were (12)CH3D: KIE1(T) = (1.227 ± 0.004) exp((43 ± 5)/T); (12)CH2D2: KIE2(T) = (1.14 ± 0.20) exp((191 ± 60)/T); (12)CHD3: KIE3(T) = (1.73 ± 0.34) exp((229 ± 60)/T); and (12)CD4: KIE4(T) = (1.01 ± 0.3) exp((724 ± 19)/T), where KIEx(T) = kCl+(12)CH4(T)/kx(T). The quoted uncertainties are at the 2σ (95% confidence) level and represent the precision of our data. The following Arrhenius expressions and 295 K rate coefficient values (in units of cm(3) molecule(-1) s(-1)) were derived from the above KIE using a rate coefficient of 7.3 × 10(-12) exp(-1280/T) cm(3) molecule(-1) s(-1) for the reaction of Cl with (12)CH4: k1(T) = (5.95 ± 0.70) × 10(-12) exp(-(1323 ± 50)/T), k1(295 K) = (6.7 ± 0.8) × 10(-14); k2(T) = (6.4 ± 1.3) × 10(-12) exp(-(1471 ± 60)/T), k2(295 K) = (4.4 ± 0.9) × 10(-14); k3(T) = (4.2 ± 1.0) × 10(-12) exp(-(1509 ± 60)/T), k3(295 K) = (2.53 ± 0.6) × 10(-14); and k4(T) = (7.13 ± 2.3) × 10(-12) exp(-(2000 ± 120)/T), k4(295 K) = (0.81 ± 0.26) × 10(-14). The reported uncertainties in the pre-exponential factors are 2σ and include estimated systematic errors in our measurements and the uncertainty in the reference reaction rate coefficient. The results from this study are compared with previously reported room-temperature rate coefficients for each of the deuterated methanes as well as the available temperature dependent data for the Cl atom reactions with CH3D and CD4. A two-dimensional atmospheric chemistry model was used to examine the implications of the present results to the atmospheric lifetime and vertical variation in the loss of the deuterated methane isotopologues. The relative contributions of the reactions of OH, Cl, and O((1)D) to the loss of the isotopologues in the stratosphere were also examined. The results of the calculations are described and discussed.

Quantum rate coefficients and kinetic isotope effect for the reaction Cl + CH4 → HCl + CH3 from ring polymer molecular dynamics

Thermal rate coefficients and kinetic isotope effect have been calculated for prototypical heavy-light-heavy polyatomic bimolecular reactions Cl + CH4/CD4 → HCl/DCl + CH3/CD3, using a recently proposed quantum dynamics approach: ring polymer molecular dynamics (RPMD). Agreement with experimental rate coefficients, which are quite scattered, is satisfactory. However, differences up to 50% have been found between the RPMD results and those obtained from the harmonic variational transition-state theory on one of the two full-dimensional potential energy surfaces used in the calculations. Possible reasons for such discrepancy are discussed. The present work is an important step in a series of benchmark studies aimed at assessing accuracy for RPMD for chemical reaction rates, which demonstrates that this novel method is a quite reliable alternative to previously developed techniques based on transition-state theory.

Reduced dimensionality quantum dynamics of Cl + CH4? HCl + CH3 on an ab initio potential

We study the reaction Cl + CH(4)--> HCl + CH(3) using a 2-D potential energy surface obtained by fitting a double Morse analytical function to high level (CCSD(T)/cc-pVTZ//MP2/cc-pVTZ)ab initio data. Dynamics simulations are performed in hyperspherical coordinates with the close-coupled equations being solved using R-matrix propagation. Quantum contributions from spectator modes are included via a harmonic zero-point correction to the ab initio data prior to fitting the potential. This is the first time this method has been applied to a heavy-light-heavy reaction and the first time it has been used to study differential cross sections. We find thermal rate constants and state-to-state differential cross sections which are in good agreement with experimental data. We discuss the applicability of our method to the study of kinetic isotope effects (KIEs), which we derive for the CH(4)/CD(4) substitution. The calculated KIE compares favourably with experiment. Finally, we discuss the sensitivity of the results of dynamics simulations on the accuracy of the fitted potential.

Potential energy surface, kinetics, and dynamics study of the Cl+CH4→HCl+CH3 reaction

A modified and recalibrated potential energy surface for the gas-phase Cl+CH4-->HCl+CH3 reaction is reported and tested. It is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to updated experimental and theoretical stationary point properties and experimental forward thermal rate constants. From the kinetics point of view, the forward and reverse thermal rate constants and the activation energies were calculated using the variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 150-2500 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of tunneling in this hydrogen abstraction reaction. A dynamics study was also performed on this PES using quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found a noticeable internal energy in the coproduct methyl radical, both in the ground-state [CH4 (v=0)] and vibrationally excited [CH4 (v=1)] reactions. This CH3 internal energy was directly precluded in some experiments or oversimplified in previous theoretical studies using pseudotriatomic models. Second, our QCT calculations give HCl rotational distributions slightly hotter than those in experiment, but correctly describing the experimental trend of decreasing the HCl product rotation excitation in going from HCl (v'=0) to HCl (v'=1) for the CH4 (v=1) reaction. Third, the state specific scattering distributions present a reasonable agreement with experiment, although they tend to make the reaction more forward and backward scattered than found experimentally probably because of the hotter rotational distribution and the deficiencies of the QCT methods.

Study of the Carbon-13 and Deuterium Kinetic Isotope Effects in the Cl and OH Reactions of CH4 and CH3Cl

Relative rate experiments have been carried out for three isotopologues of chloromethane and their reactions with Cl atoms and OH radicals. The OH and Cl reaction rates of CH2DCl and CHD2Cl were measured by long-path FTIR spectroscopy relative to CH3Cl at 298+/-2 K and 1013+/-10 hPa in purified air. The FTIR spectra were fitted using a nonlinear least squares spectral fitting method including measured high-resolution infrared spectra as references. The relative reaction rates defined by alpha=klight/kheavy were determined to be kOH+CH3Cl/kOH+CH2DCl=1.41+/-0.05, kOH+CH3Cl/kOH+CHD2Cl=2.03+/-0.05, kCl+CH3Cl/kCl+CH2DCl=1.42+/-0.04, and kCl+CH3Cl/kCl+CHD2Cl=2.27+/-0.04. The carbon-13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3Cl were investigated further using variational transition state theory, and the results were compared to similar calculations performed for the CH4+OH/Cl reaction systems. The calculations show that the order of magnitude difference for the carbon-13 kinetic isotope effect in the OH reaction of CH3Cl compared to CH4 reported by Gola et al. (Atmos. Chem. Phys. 2005, 5, 2395) can be explained by the lower barrier to internal rotation of the OH radical in the transition state of the CH4+OH reaction than in the CH3Cl+OH reaction. The deuterium kinetic isotope effects can be explained in terms of combined variational effects and tunneling.

Activation of C−H / H−H Bonds by Rhodium(II) Porphyrin Bimetalloradicals

Reactivity, kinetic, and thermodynamic studies are reported for reactions of a rhodium(II) bimetalloradical with H(2), and with the methyl C-H bonds for a series of substrates CH(3)R (R = H, CH(3), OH, C(6)H(5)) using a m-xylyl diether tethered diporphyrin ligand. Bimolecular substrate reactions involving the intramolecular use of two metalloradical centers and preorganization of the four-centered transition state (M*...X...Y*...M) result in large rate enhancements as compared to termolecular reactions of monometalloradicals. Activation parameters and deuterium kinetic isotope effects for the substrate reactions are reported. The C-H bond reactions become less thermodynamically favorable as the substrate steric requirements increase, and the activation free energy (DeltaG++) decreases regularly as DeltaG degrees becomes more favorable. An absolute Rh-H bond dissociation enthalpy of 61.1 +/- 0.4 kcal mol(-1) is directly determined, and the derived Rh-CH(2)R BDE values increase regularly with the increase in the C-H BDE.

Transition-state models and hydrogen-isotope effects. Kinetic isotope effects provide a sensitive test for detailed models of reacting systems

The transition-state theory of chemical kinetics, coupled with relatively crude methods of constructing potential-energy surfaces for reacting systems, has great utility in the forecasting of kinetic properties. In particular, it permits prediction of the effect of isotopic substitution on rate constants, and comparison of these predictions with experimental data provides a particularly sensitive test for the combination of potential-energy surface and transition-state theory. More rigorous tests of each of these factors depend on future developments in quantum chemistry, in studies of chemical reactions in molecular beams, and in detailed trajectory calculations of scattering processes.