An ab initio study of the Cl(P)+C2H6→C2H5+HCl abstraction reaction

Electronic energies, geometries, and harmonic vibration frequencies for the reactants, products, and transition state for the Cl(³P)+C₂H₆→C₂H₅+HCl abstraction reaction were evaluated at the HF and MP2 levels using several correlation consistent polarized-valence basis sets. Single-point calculations at PMP2, MP4, QCISD(T), and CCSD(T) levels were also carried out. The values of the forward activation energies obtained at the MP4/cc-pVTZ, QCISD(T)/cc-pVTZ, and CCSD(T)/cc-pVTZ levels using the MP2/cc-pVTZ structures are equal to −0.1, −0.4, and −0.3 kcal/mol, respectively. The experimental value is equal to 0.3±0.2 kcal/mol. We found that the MP2/aug-cc-pVTZ adiabatic vibration energy for the reaction (−2.4 kcal/mol) agrees well with the experimental value −(2.2–2.6) kcal/mol. Rate constants calculated with the zeroth-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. In general, the theoretical rate constants differ from experiment by, at most, a factor of 2.6.     

Ab initio molecular orbital study of potential energy surface for the H2NO(B1)→NO(Π)+H2 reaction

The mechanism of the H₂NO(²B₁)→NO(²Π)+H₂ reaction has been examined using ab initio molecular orbital methods. Ground-state and first-excited-state potential surfaces were plotted at the FOCI/cc-pVTZ level of theory as functions of two appropriate internal degrees of freedom. A conical intersection was found on the C_s pathway that is symmetric with respect to the plane perpendicular to the molecular plane of C_2v H₂NO(²B₁). It is therefore considered that trajectories that start from H₂NO(²B₁) towards the product region detour around the conical intersection, pass through the neighborhood of the transition state that is located at the saddle point on the C_s pathway, and finally reach the products, NO(²Π)+H₂. Thus we can explain the mechanism of the H₂NO(²B₁)→NO(²Π)+H₂ reaction, which has remained unclear to date.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

The effect of reagent translational energy in the reaction O(D2) + H2 → OH(Π) + H

Quasiclassical trajectory calculations have been performed to determine the effect of reactant collision energy on product state distributions in the reaction O(¹D) + H₂ → OH(²Π) + H. The product vibrational distribution becomes more excited as the collision energy is increased. This is not due to an increase in the cross section for collinear abstraction. A detailed analysis has shown that strong O---H₂ repulsion, which occurs during the insertion of the O into the H---H bond, converts the kinetic energy of the reacting system to vibrational motion of the intermediate.     

Direct dynamics studies on the hydrogen abstraction reactions CF3O+CH4(CD4)→CF3OH(CF3OD)+CH3(CD3)

The dynamics properties of the hydrogen abstraction reaction CF₃O+CH₄→CF₃OH+CH₃ are studied by dual-level direct dynamics method. Optimization calculations are preformed by B3LYP and MP2 with the 6-311G(d,p) basis set, and the single-point calculations are done at the multi-coefficient correction method based on quadratic configuration interaction with single and double excitations (MC-QCISD) method. The rate constants are evaluated by canonical variational transition-state theory with a small-curvature tunneling correction over a wide range of temperature 200–2000 K. The agreement between theoretical and experimental rate constants is good in the measured temperature range. The calculated results show that the variational effect is small and almost neglected over the whole temperature range, whereas, the tunneling correction plays a role in the lower temperature range. The kinetic isotope effect for the reaction is ‘normal’. The value of k_H/k_D is 2.38 at room temperature and it decreases with the temperature increasing.     

Direct dynamic study on the hydrogen abstraction reaction CH3CN + OH-->CH2CN + H2O

The reaction of acetonitrile with hydroxyl has been studied using the direct ab initio dynamics methods. The geometries, vibrational frequencies of the stationary points, as well as the minimum energy paths were computed at the BHandHLYP and MP2 levels of theory with the 6-311G(d, p) basis set. The energies were further refined at the PMP4/6-311+G(2df, 2pd) and QCISD(T)/6-311+G(2df, 2pd) levels of theory based on the structures optimized at BHandHLYP/6-311G(d, p) and MP2/6-311G(d, p) levels of theory. The Polyrate 8.2 program was employed to predict the thermal rate constants using the canonical variational transition state theory incorporating a small-curvature tunneling correction. The computed rate constants are in good agreement with the available experimental data.     

Barrier height for the abstraction reaction of methylene with hydrogen,CH2(3B1) + H2 → CH3 + H

The abstraction reaction of methylene with hydrogen reinvestigated with a double zeta plus polarization basis at the configuration interaction level. The results are found to be very similar to those previously obtained without polarization functions, in line with previous findings at the SCF level. The barrier is computed to be 11.8 kcal/mole when an estimate of the effect of quadruple excitations is included.     

Direct dynamics study of the hydrogen abstraction reaction of CF3CH2Cl + Cl → CF3CHCl + HCl

The hydrogen abstraction reaction of Cl atoms with CF₃CH₂Cl (HCFC‐133a) is investigated by using density function theory and ab initio approach, and the rate constants are calculated by using the dual‐level direct dynamics method. Optimized geometries and frequencies of reactants, transition state, and products are computed at the B3LYP/6‐311+G(2d,2p) level. To refine the energetic information along the minimum energy path, single‐point energy calculations are carried out at the G3(MP2) level of theory. The interpolated single‐point energy method is employed to correct the energy profiles for the title reaction. The rate constants are evaluated by using the canonical variational transition state theory with a small‐curvature tunneling correction over a wide range of temperature, 200–2000 K. The variational effect for the reaction is moderate at low temperatures and very small at high temperatures. However, the tunneling correction has an important contribution in the lower temperature range. The agreement between calculated rate constants and available experimental values is good at lower temperatures but diverges significantly at higher temperatures. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 661–667, 2012     

Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.     

An ab initio study of the potential surface for the reaction H2COH → HCO + H2

The potential surface for the reaction H₂CO⁺H → HCO⁺ + H₂ has been studied by ab initio SCF calculations, using gaussian-type basis functions. A saddle point on the surface has been found, and a reaction path is proposed to explain the observed release of kinetic energy. The energy of activation and ΔE for the reaction have been estimated.     

2D and 3D quantum scattering calculations on the CH4 + OH → CH3 + H2O reaction

The effect on the thermal rate constant and the differential cross-sections of varying the dimensionality of quantum scattering calculations of a polyatomic reaction is investigated. The rotating bond approximation (RBA; 3D) and a rotating line approximation (RLA; 2D) are used for the CH₄ + OH → CH₃ + H₂O reaction. It is found that the RBA and RLA results are in close agreement when an adiabatic treatment is used for the degree of freedom which is treated explicitly in the RBA but not in the RLA.     

Quantum chemical study of hydrogen abstraction reactions of the ethynyl radical with hydrogen compounds (C2H+HX)

We have theoretically investigated the hydrogen abstraction reactions of ethynyl radical with simple hydrogen compounds, C₂H+HX, using quantum chemical computations. Computations have been performed using the density functional theory with the recently proposed MPW1K functional and the 6-311++G(3df,2p) basis set. An analysis of the resulting energy barriers for hydrogen abstraction reactions has been carried out using the bond dissociation energy of the breaking X–H bond and DFT-based reactivity parameters to rationalize the reaction behavior.     

The exchange reaction H2 (ν = 1) + H = H + H2

The classical trajectory method is applied to calculate the total cross section for the exchange reaction H₂(ν = 1) + H = H + H₂. The vibrational excitation is shown to influence efficiently the threshold value. Partial reaction rate-constants calculated on the basis of the cross sections obtained are in good agreement with those measured in H-maser experiments.     

Lifetime measurements of electronically excited C3(Πu) radicals in different vibrational states

The radiative lifetimes of nine vibrational levels of the C₃(¹Π_u) radical were obtained from decay time studies of the C₃(¹Π_u → ¹Σ⁺_g) fluorescence induced by a tunable dye laser. The lifetimes of the different vibronic levels were found to be constant within the experimental error limits, namely, τ_o = (200 ± 10) ns. The collisional deactivation of the C₃(¹Π_u) states by helium gives rate constants between 2.5 and 4 in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ units.     

A partial open shell FSGO calculation on the triplet methylene abstraction reaction with the hydrogen molecule: 3CH2 + H2 = CH3 + H

A partial open shell FSGO method is used to study the reaction path for the triplet methylene abstraction reaction with the hydrogen molecule, and the results obtained compare favourably with those from larger ab initio calculations.     

The A Πu-X Πg emission spectrum of I2

The A ²Π_u-X ²Π_g electronic emission spectrum of I₂⁺ has been recorded at a low rotational temperature in a crossed molecular beam/electron beam apparatus. Six vibrational sequences with five or more members have been assigned to progressions in ν′, giving ω′_e = 122±8 cm⁻¹, but a full vibrational analysis has not been possible. It is not known whether this is due to the relatively poor resolution (≈5 cm⁻¹) at which the spectrum has been recorded or because the A ²Π_u state is perturbed in one or both spin-orbit components. Existing data on the A state of I₂⁺ are reviewed.     

Direct dynamic study on the hydrogen abstraction reaction of H2CO with NCO

A direct ab initio dynamics method is used to investigate the hydrogen‐abstraction reaction of H₂CO with NCO. The potential energy surface information is obtained at the MP2/6‐311G(d,p) level. More accurate single‐point energy is refined at the G3(MP2)//MP2/6‐311G(d,p) level. Furthermore, the rate constants of reaction H₂CO + NCO are evaluated by using the canonical variational transition state theory with small‐curvature tunneling contributions over a wide temperature range of 200–2000 K. The calculated reaction enthalpy and rate constants are in good agreement with the available experimental values. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 394–400, 2009     

On the 6Πu state of Na2

The 6¹Π_u state of sodium dimer has been observed up to v = 53 in excitation spectra of the system, recorded by polarisation labelling spectroscopy technique. The Dunham coefficients are derived and the potential energy curve constructed by the inverted perturbation approach method. Equilibrium constants for the 6¹Π_u state of Na₂ are: T_e = 35446.06 ± 0.04 cm⁻¹ (with respect to the minimum of the electronic ground state), Y₁₀ = 111.388 ± 0.019 cm⁻¹, Y₀₁ = 0.112122 ± 0.000017 cm⁻¹.     

Direct dynamics study on hydrogen abstraction reaction of CF3CHOHCF3 with OH radical

Dual-level direct dynamics method is employed to investigate the H-abstraction reaction CF₃CHOHCF₃ with OH radical. Two hydrogen-abstraction reaction channels are possible: one from the methylene (–CH–) position and the other from the hydroxyl (–OH) position. The minimum energy path is calculated at the B3LYP/6-311G(d,p) level, and the energetic information is further refined by a new powerful and inexpensive BMC-CCSD method. To testify the accuracy of the structures and the energies, the recently developed hybrid density functional theory BB1K and higher level MC-QCISD are applied to this system. Hydrogen-bonded complexes are presented at both reactants and products sides of these two channels, which indicating that the reaction may proceed via an indirect mechanism. The rate constants for each reaction channel are evaluated by canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide range of temperatures from 200 to 2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values in the temperature region 250–430 K. The present results indicate that the two channels are competitive. At lower temperature, the reaction occurs mainly via the hydroxyl-H-abstraction channel, while the methylene-H-abstraction channel is preferred when the temperature is higher than 273 K.     

Self-consistent-field potential energy surface in three dimensions for the Cl + H2 → ClH + H chemical reaction

Non-empirical self-consistent-field calculations have been carried out for 38 points on the potential surface for the Cl + H₂ → ClH + H chemical reaction. A basis set of seven s, five p, and one d functions on chlorine and three s and one p on each hydrogen atom was used. The least energy path occurs for the linear Cl---H---H arrangement. A much higher barrier is found for the approach of Cl along the H---H perpendicular bisector. The linear barrier height is predicted to be 26.2 kcal/mole and the saddle point occurs for R(Cl---H) ≈ 1.46 Å, R(H---H) ≈ 0.94 Å. The experimental activation energy is 5.5 kcal/mole. It seems likely that a general feature of the Hartree-Fock approximation is an overestimation of barrier heights. The exothermicity is calculated to be −6.7 kcal/mole, compared to the near Hartree-Fock result −2.3 kcal/mole and experiment −3.0 kcal/mole.