Supplementary information for Direct ab-initio Dynamics Calculations of Thermal Rate Constants for the CH4 + O2 = CH3 + HO2 reaction (original) (raw)
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Thermal rate constants of the CH 4 ? O 2 = CH 3 ? HO 2 reaction were calculated from first principles using both the conventional transition state theory (TST) and canonical variational TST methods with correction from the explicit hindered rotation treatment. The CCSD(T)/ aug-cc-pVTZ//BH&HLYP/aug-cc-pVDZ method was used to characterize the necessary potential energy surface along the minimum energy path. We found that the correction for hindered rotation treatment, as well as the re-crossing effects noticeably affect the rate constants of the title process. The calculated rate constants for both forward and reverse directions are expressed in the modified Arrhenius form as k
The Journal of Chemical Physics, 1995
We present direct ab initio dynamics studies of vibrational-state selected reaction rates of the OH+H2→H+H2O reaction. Rate constants for both the OH+H2(v=1) and OH(v=1)+H2 reactions were calculated based on a full variational transition state theory plus multidimensional semiclassical tunneling approximations within a statistical diabatic model. The potential energy surface information was calculated at an accurate level of molecular orbital theory. In particular, geometries and frequencies along the minimum energy path were calculated at the quadratic configuration interaction level including all single and double excitations (QCISD) with the 6-311+G(d,p) basis set. Energies along the minimum energy path were further improved by a series of single point projected fourth-order Möller–Plesset perturbation theory (PMP4) calculations using the 6-311++G(2df,2pd) basis set. Our present results of vibrational excited state rate enhancements agree very well with previous experimental data...
Direct ab Initio Dynamics Study of the OH + HOCO Reaction
The Journal of Physical Chemistry A, 2005
The reaction between OH and HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The energetics are refined using the coupled-cluster method with basis set extrapolation to the complete basis set (CBS) limit. Results show that the OH + HOCO reaction produces H 2 O + CO 2 as final products and the reaction passes through an HOC(O)OH intermediate. In addition, the OH + HOCO reaction has been studied using a direct dynamics method with a dual-level ab initio theory. Dynamics calculations show that hydrogen bonding plays an important role during the initial stages of the reaction. The thermal rate constant is estimated over the temperature range 250-800 K. The OH + HOCO reaction is found to be nearly temperature-independent at lower temperatures, and at 300 K, the thermal rate constant is predicted to be 1.03 × 10 -11 cm 3 molecule -1 s -1 . In addition, there may be an indication of a small peak in the rate constant at a temperature between 300 and 400 K. *
The Journal of Chemical Physics, 2000
The potential energy surface for the gas-phase CH 4 ϩOH→CH 3 ϩH 2 O reaction and its deuterated analogs was constructed with suitable functional forms to represent vibrational modes, and was calibrated by using the experimental thermal rate constants and kinetic isotope effects. On this surface, the forward and reverse thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K, finding reasonable agreement with the available experimental data. We also calculated six sets of kinetic isotope effects and, in general, the theoretical results underestimate the few available experiments, with exception of the C-13 isotopic effect values which are overestimated. Finally, this surface is also used to analyze dynamical features, such as reaction-path curvature and coupling between the reaction coordinate and vibrational modes.
Theoretical study of the CH4+F→CH3+FH reaction. I. Ab initio reaction path
The Journal of Chemical Physics, 1996
Using ab initio information, the reaction path for the CH 4 ϩF→CH 3 ϩFH reaction was traced and the coupling between the reaction coordinate and normal modes was analyzed along it. The FH product may be vibrationally excited due to the nonadiabatic flow of energy between the reaction coordinate and this bound mode, manifest in the large peak in the coupling term after the saddle point. It was concluded that the variational effects were due only to entropic effects. The rate constants were calculated for the temperature range 100-500 K using the variational transition state theory with different levels of calculation to calibrate the reaction path. Agreement was found with the experimental values when using the QCI/b3 shifted curve, avoiding the errors associated with the use of the single-point calculation.
Ab initio calculation on the rate constants of the reaction H 2 O 2 + Cl
Ab initio calculations followed by transition state theory (TST) treatment are performed to investigate the gas phase elementary bimolecular reaction of hydrogen peroxide with chlorine atom H 2 O 2 + Cl. The electronic structure calculations are done using the post Hartree–Fock MCQDPT2//CASSCF approach and the aug-cc-pVTZ basis set. Two reaction pathways are considered: the H-abstraction path (denoted as path1) and the OH-abstraction one (denoted as path2). For each path, the activated complex is located. The zero point vibrational energy corrected values of the classical barrier heights were predicted to be 3.2 and 10.4 kcal/mol for path1 and path2, respectively. Reaction rate constants are calculated for the temperature range 200–2500 K using the transition state theory (TST) incorporating a Zero-Curvature Tunneling correction. The results of theoretical rate constants for path1 are in good agreement with the available experimental data. The calculated branching ratios show that the H-abstraction path is the most probable process occurring for temperatures below 1500 K confirming hence the experimental findings. The contribution of path2 to the reaction dominates at higher temperatures.
The Journal of Physical Chemistry A, 2014
The complex relationship of computed rate coefficients (k's) with different ab initio/DFT and TST levels was studied. The MEPs, gradients, and Hessians of the title reaction were computed using the MP2 and DFT methods. Electronic energies were improved to the UCCSD(T)-F12x/CBS level, and k's were calculated at the TST, CVT, and ICVT levels with various tunnelling corrections. Although computed microcanonical and tunnelling effects are small, computed k TST values are larger than computed k TST/ZCT and k TST/SCT values by 3 orders of magnitude at low temperatures, because computed κ (TST/CAG) values are as small as 6 × 10 −4 . In some cases, the maximum of the ΔG/s curves at a certain T is far away from the MEP maximum. This raises the question of the range of s to be considered in a VTST calculation and, of a possible scenario, where no maximum on the ΔG curve can be located and hence a breakdown of VTST occurs. For duallevel direct dynamics calculations, different entropic contributions from different lower levels can lead to computed k's, which differ by more than 1 order of magnitude. Matching computed and experimental k values leads to an empirical barrier of 1.34 kcal mol −1 for the title reaction. a All MP2 and DFT calculations have employed the 6-311++G** basis set. The F12x//MP2 calculations were carried out by employing MP2 geometries for all species involved, whereas F12x//B3LYP indicates that the B3LYP geometry of TS was used (all other species at MP2 geometries). The spin−orbit contributions for the 2 P state of Cl have not been included in the evaluation of ΔE e ‡ given in this table. The relative energies given are with the 2 P state of Cl (directly from ab initio/DFT calculations, using symmetry where appropriate; see also footnote a of ). b The 1/X 3 extrapolation formula was used for CBS extrapolation (see text). c The best estimates are averages of the CBS-F12a and CBS-F12b values. The estimated uncertainties (in parentheses) are the differences between the best and F12b/VTZ-F12 values. d Without SO contributions from Cl; computed MP2 vibrational frequencies were used for zero-point energy corrections (ΔZPE) and 10% of ΔZPE was added to the uncertainties.
Critical evaluation of the potential energy surface of the CH3 + HO2 reaction system
The Journal of chemical physics, 2015
The CH3 + HO2 reaction system was studied theoretically by a newly developed, HEAT345-(Q) method based CHEAT1 protocol and includes the combined singlet and triplet potential energy surfaces. The main simplification is based on the CCSDT(Q)/cc-pVDZ calculation which is computationally inexpensive. Despite the economic and black-box treatment of higher excitations, the results are within 0.6 kcal/mol of the highly accurate literature values. Furthermore, the CHEAT1 surpassed the popular standard composite methods such as CBS-4M, CBS-QB3, CBS-APNO, G2, G3, G3MP2B3, G4, W1U, and W1BD mainly due to their poor performance in characterizing transition states (TS). For TS structures, various standard DFT and MP2 method have also been tested against the resulting CCSD/cc-pVTZ geometry of our protocol. A fairly good agreement was only found in the cases of the B2PLYP and BHandHLYP functionals, which were able to reproduce the structures of all TS studied within a maximum absolute deviation o...
The Journal of Physical Chemistry A, 2009
Thermal rate constants are calculated for the H + CH 4 f CH 3 + H 2 reaction employing the potential energy surface of Espinosa-García (Espinosa-García, J. J. Chem. Phys. 2002, 116, 10664). Two theoretical approaches are used. First, we employ the multiconfigurational time-dependent Hartree method combined with flux correlation functions. In this way rate constants in the range 225-400 K are obtained and compared with previous results using the same theoretical method but the potential energy surface of Wu et al. Manthe, U. Science 2004, 306, 2227. It is found that the Espinosa-García surface results in larger rate constants. Second, a harmonic quantum transition state theory (HQTST) implementation of instanton theory is used to obtain rate constants in a temperature interval from 20 K up to the crossover temperature at 296 K. The HQTST estimates are larger than MCTDH ones by a factor of about three in the common temperature range. Comparison is also made with various tunneling corrections to transition state theory and quantum instanton theory. † Part of the "George C. Schatz Festschrift". * Corresponding authors, nyman@chem.gu.se and hj@hi.is.