An ab initio study of the hydrogen abstraction reaction of methane by bromine atoms and bromine monoxide radicals (original) (raw)
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The Journal of Physical Chemistry A, 2004
Theoretical calculations were carried out on the H-, Cl-, and F-atom abstraction reactions from a series of seven substituted halogenated methanes (CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 4 , CHF 3 , CHF 2 Cl, and CHFCl 2 ) by H atom attacks. Geometry optimizations and vibrational frequency calculations were performed using unrestricted Møller-Plesset second-order perturbation theory (UMP2) with the 6-311++G(d,p) basis set. Single-point energy calculations were performed with the highly correlated ab initio coupled cluster method in the space of single, double and triple (pertubatively) electron excitations CCSD(T) using the 6-311++G(3df,3pd) basis set. Canonical transition-state theory with a simple tunneling correction was used to predict the rate constants as a function of temperature (700-2500 K), and three-parameter Arrhenius expressions were obtained by fitting to the computed rate constants for elementary channels and overall reaction.
An ab initio study of H abstraction in halogen-substituted methanes by the -OH radical
Journal of Molecular Structure: THEOCHEM, 1993
The H-abstraction reaction by the OH radical from methane and various halomethanes (CH,Cl, CH,F, CH,Cl,, CH,ClF, CH,F,, CHCl,F and CHF,) was studied using ab initio unrestricted Hartree-Fock (UHF) and multiconfigurational self-consistent field (MCSCF) computational methods. It was found that the structure of the transition state determined at the UHF and MCSCF levels is very similar and that the topology of these reaction surfaces is satisfactorily described using a UHF wavefunction. It is pointed out that to obtain values of the activation energies in good agreement with experiment it is essential to take into account the correlation energy contribution on reactants and transition states. The MP2/6-31G*//3-21G* approach (6-31G* single-point second-order Moller-Plesset (MP2) computations on the 3-21G* optimized geometries) is proposed as the minimal computational level needed to obtain a description of these reactions in reasonable agreement with the experimental evidence.
Chemical Physics, 2009
A new analytical potential energy surface is presented for the reaction of hydrogen abstraction from methane by a hydrogen atom. It is based on an analytical expression proposed by Jordan and Gilbert [J. Chem. Phys. 102, 5669 (1995)], and its fittable parameters were obtained by a multibeginning optimization procedure to reproduce high-level ab initio electronic structure calculations obtained at the CCSD(T)/cc-pVTZ level. The ab initio information employed in the fit includes properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, points on the reaction path, and points on the reaction swath. No experimental information is used. By comparison with the reference results we show that the resulting surface reproduces well not only the ab initio data used in the fitting but also other thermochemical and kinetic results computed at the same ab initio level, such as equilibrium constants, rate constants, and kinetic isotope effects, which were not used in the fit. In this way we show that the new potential energy surface is correctly fitted and almost as accurate as the CCSD(T)/cc-pVTZ method in describing the kinetics of the reaction. We analyze the limitations of the functional form and the fitting method employed, and suggest some solutions to their drawbacks. In a forthcoming communication, we test the quality of the new surface by comparing its results with experimental values.
Australian Journal of Chemistry, 2012
The reactions of the HBr molecule with CH2CH2Cl (reaction R1), CH2CHCl2 (R2), CH2CH2Br (R3) and CH2CHBr2 (R4) are investigated by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points were calculated at the MPW1K/6–311+G(d,p) and BMK/6–311+G(d,p) levels. To refine the reaction enthalpy and energy barrier height of each reaction, single-point energy calculations were carried out by the G2M(RCC5) method based on the geometries optimized at the above-mentioned two levels. Using the canonical variational transition state theory or the canonical variational transition state theory with the small-curvature tunneling correction, the rate constants of HBr with CH2CH2Cl (R1), CH2CHCl2 (R2), CH2CH2Br (R3), and CH2CHBr2 (R4) were calculated over a wide temperature range of 200–2000 K at the G2M(RCC5)//MPW1K/6–311+G(d,p) level. The effect of chlorine or bromine substitution on the ethyl radical reactivity is discussed. Finally, the total rate con...
Physical Chemistry Research, 2014
The hydrogen abstraction reaction by OH radical from CH2BrCH2Br (R1) and CH3CHBr2 (R2) is investigated theoretically by semiclassical transition state theory. The stationary points for both reactions are located by using ωB97X-D and KMLYP density functional methods along with cc-pVTZ basis. Single-point energy calculations are performed at the QCISD(T) and CCSD(T) levels of theory with different basis sets. The results show that the activation energies are very sensitive to the effects of electron correlation and basis set. In order to correct basis set effects on the calculated energetic, a correction factor (CF) is determined from the energy difference between the MP2/cc-pVTZ and MP2/aug-cc-pVTZ levels. xij vibrational anharmonicity coefficients, needed for semi-classical transition state theory, are calculated at the KMLYP/cc-pVTZ level of theory. Thermal rate coefficients are computed over the temperature range from 200 to 3000 K and they are shown to be in accordance with avail...
Chemical Physics, 2004
The direct dynamics of the hydrogen abstraction reactions of CH 2 O with CH 3 /OH are studied using ab initio molecular orbital theory. Both the geometry optimizations of all the stationary points and the vibrational frequency calculations are carried out at the UQCISD/6-311G(d,p) level. The single-point energy is obtained by the multicoefficient Gaussian 3-version 3s (MCG3/3) method. The analysis to the changes of the interatomic distances on the minimum energy paths show that, the breaking of C-H bonds of CH 2 O and (i) the forming of C-H bond of CH 4 in the reaction of CH 2 O with CH 3 , (ii) the forming of O-H bond of H 2 O in the reaction of CH 2 O with OH, are both concerted. For each reaction, there exists a reactive vibrational normal-mode, and its frequencies change is relevant to the forming and breaking of the above covalent bond. Furthermore, the theoretical forward reaction rate constants in the temperature range 300-3000 K are computed by canonical variational transition state theory with small-curvature tunneling correction (CVT/SCT) method. The computed values of the rate constants are in good agreement with the available experimental data in the measured temperature range. Moreover, the tunneling effects are found to contribute significantly to the rate constants at low temperatures.
Industrial & Engineering Chemistry Research, 2003
Two empirical methods have been developed to estimate the activation energy (E a) of hydrogenabstraction reactions involving hydrocarbons by using ground-state thermochemical properties: reaction enthalpy (∆H), broken bond energy (D b), and formed bond energy (D f). E a) 12.67 + 2.98V c + 0.50∆H + 0.00604∆H 2 kcal/mol (method I); E a) 54.47 + 4.21V c-1.00D f + 0.00581D b D f + 0.00681(D b-D f) 2 kcal/mol (method II). The development of the methods is based on a fundamental understanding of the transition state structures (TSSs) and multiple regression analysis of a test set of 71 hydrogen-abstraction reactions involving the abstraction of alkane, allylic, and benzylic hydrogens. There is a significant effect of the π-conjugate TSS on the correlation, which is discussed in terms of quantum chemical understanding. The effect of the π-conjugate TSS is considered by adding an indicator variable V c into the methods. The average absolute error and standard error are 0.47 and 0.64 kcal/mol for method I and 0.40 and 0.56 kcal/mol for method II for the test set. A comparison of the two proposed methods with previous empirical methods was performed statistically. The results show that the two developed methods significantly improve the estimation accuracy relative to the previous empirical methods.
The Journal of Chemical Physics, 2009
On a new potential energy surface ͑PES-2008͒ developed by our group ͑preceding paper͒, we performed an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 250-2000 K and a dynamics study using quasiclassical trajectory ͑QCT͒ and quantum-mechanical ͑QM͒ calculations at collision energies between 0.7 and 2.0 eV for the title reaction and isotopically substituted versions. Kinetically, the H+CH 4 forward and reverse thermal rate constants reproduce the available experimental data, with a small curvature of the Arrhenius plot indicating the role of tunneling in this hydrogen abstraction reaction. Five sets of kinetic isotope effects are also calculated. In general, they reproduce the experimental information. Dynamically, we focused on the H + CD 4 reaction because there are more experimental studies for comparison. Most of the available energy appears as product translational energy ͑55%-68%͒, with the HD product being vibrationally cold ͑vЈ =0,1͒ in agreement with experiment, although rotationally hotter than experiment. The reaction cross section is practically negligible at 0.7 eV and still small at 1.5 eV, reproducing the experimental evidence, although our values are smaller. The product angular distribution is analyzed using QCT and QM methods. While at low energies ͑0.7 eV͒ both the QCT and the QM calculations yield forward scattered CD 3 product, i.e., a rebound mechanism, at high energy ͑1.2 eV͒ only the QM calculations reproduce the experiment. The agreement with this wide variety of kinetic and dynamic experimental data ͑always qualitative and in some cases quantitative͒ shows the capacity of the PES-2008 surface to describe the reaction system.
The Journal of Chemical Physics, 2001
Employing the newly developed high-resolution pulsed field ionization-photoelectron (PFI-PE)photoion coincidence (PFI-PEPICO) technique, we have examined the dissociation of energy-selected NH 3 + to form NH 2 + + H near its threshold. The breakdown curves for NH 2 + and NH 3 + thus obtained yield a value of 15.765±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for NH 2 + from NH 3. This value, together with the known ionization energy (IE=10.1864±0.0001 eV) and 0 K bond dissociation energy (D 0 =4.6017±0.0025 eV) for NH 3 , allows the determination of the D 0 (NH 2 +-H) and IE(NH 2), which are 5.5786±0.0010 and 11.1633±0.0025 eV, respectively. Using the known 0 K heats of formation (∆Η°f 0) for NH 3 and H and the AE(NH 2 +), we obtain the ∆Η°f 0 (NH 2 +) = 302.60±0.08 kcal/mol. The PFI-PE spectrum for NH 3 exhibits a step-like feature at the 0 K AE(NH 2 +), indicating that the dissociation of excited NH 3 in high-n (n≥100) Rydberg states at energies slightly above the dissociation threshold occurs on a time scale ≤10-7 s. This step confirms the AE(NH 2 +) value derived from the PFI-PEPICO measurements. Highly accurate energetic data with well-founded error limits, such as those obtained in the present and other studies using the PFI techniques, are expected to play an important role for the development of the next generation of ab initio quantum computation procedures. This experiment has stimulated a state-of-the-art ab initio quantum chemical calculation (Dixon et al., J. Chem. Phys., accepted). The comparison between theoretical predictions and the best experimental results for the NH 2 /NH 2 + and NH 3 /NH 3 + systems indicates that the accuracy of the computational scheme used is ≤0.4 kcal/mol.
Industrial & Engineering Chemistry Research, 2001
A semiempirical method, the PM3-family-correlation (PM3-FC) method, has been developed to estimate the activation energies for hydrogen abstraction reactions between hydrocarbon radicals and hydrogen donors. The method combines semiempirical PM3 calculations of transition state enthalpies [∆H°f(TS)] with family correlations between the PM3-calculated and experimental ∆H°f(TS) values on the basis of regression analysis. For a test set of 40 hydrogen abstraction reactions, including alkyl + alkyl-H, alkyl + allylalky-H R /arylalkyl-H R , and benzyl + allylalkyl-H R /arylalkyl-H R , it is found that the deviations between the PM3-calculated and experimental ∆H°f(TS) values are systematic and dependent on the transition state structures. The structurally homologous transition states show excellent linear correlation. Using the obtained linear regression parameters to scale the PM3-calculated ∆H°f(TS) values leads to a very significant increase in estimation accuracy. The activation energies for the whole test set are estimated by using the scaled ∆H°f(TS) values on the basis of the transition state theory. The average absolute deviation between the PM3-FC-estimated and experimental activation energies is 0.3 kcal/mol, with a standard deviation of 0.5 kcal/mol.