Computational study of the rate constants and kinetic isotope effects for the CH3+HBr→CH4+Br reaction

Abstract

The reactions CH n D 4−n + OH → P and CH 4 + OD → CH 3 + HOD as a test of current direct dynamics computational methods to determine variational transition-state rate constants. I. Potential energy surface for a seven-atom reaction. Thermal rate constants and kinetic isotope effects for CH 4 +OH Ab initio direct dynamics methods have been used to study the title reaction. The electronic structure information including geometries, gradients and force constants ͑Hessians͒ is calculated at the QCISD/6-31ϩG(d) level. With the aid of intrinsic reaction coordinate theory, the minimum energy path ͑MEP͒ is obtained at the same level, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4/6-311ϩϩG(3d f ,3pd) level. For this reaction, the theoretical rate constants by using improved canonical variational transition state theory incorporating small-curvature tunneling correction are in excellent agreement with the more recent experimental values. Our calculated results show that for the CH 3 ϩHBr reaction, the rate constants have a strong nonlinear Arrhenius behavior, i.e., the CH 3 ϩHBr reaction has negative temperature dependence at TϽ536 K, but clearly shows positive temperature dependence at T Ͼ536 K. The current work predicts that the kinetic isotope effect for the title reaction is normal in the temperature range 200-482 K, i.e., k HBr /k DBr Ͼ1. This result is in good agreement with the experimental one.

A Chemical Kinetic Mechanism for 2‐Bromo‐3,3,3‐trifluoropropene (2‐BTP) Flame Inhibition

“…Note that no rate expressions are given in that paper—we estimated the temperature dependence from the figures provided in the paper. Computational work by Espinosa‐Garcia , Sheng et al , and Krasnoperov et al also confirm the positive temperature dependence of the rate constants at high temperatures. The rate constants from our fitted rate expression with A = 2.10 × 10 7 cm 3 /mol/s, b = 1.57, and E = –6.0 kJ/mol agree with the low‐temperature data to within 6% (roughly the experimental uncertainties).…”

Flame Inhibition Chemistry: Rate Coefficients of the Reactions of HBr with CH3 and OH Radicals at High Temperatures Determined by Quasiclassical Trajectory Calculations

“…The rate coefficients obtained with the TST and QCT methods between 600 and 3000 K are shown in Figures and , together with the available experimental data for reaction and reaction , − respectively. The error bars on the QCT points represent ±1 standard deviation.…”

“…Temperature dependence of the calculated rate coefficients of reaction CH 3 + HBr → CH 4 + Br (reaction ). The experimental data are taken from refs − . Black dots, QCT results; blue line, fit to QCT data according to eq ; black line, TST results; red line, rate coefficient estimated by Burgess et al, (eq ).…”

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Br(2P, 2P3/2) + CH4 → HBr + CH3 reaction

“…There are a lot of contradictions in the literature about the sign of the saddle-point energy relative to HBr + CH 3 and the existence/stability of a CH 3 -HBr complex, which may or may not affect the dynamics and kinetics of the HBr + CH 3 reaction. Theoretical studies based on Gaussian-1 theory, 29 all-electron UMP2/6-311G(3df,d,p), 26 and frozen-core CCSD(T)/cc-pVTZ + BSSE 30 found a positive vibrationally adiabatic ground state barrier height, whereas a slightly negative barrier was predicted by QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311G(2df,2p), 31 MP4/6-311 ++G(3df,3pd)//QCISD/6-31+G(d), 27 and frozen-core ECP-RCCSD(T)/cc-pVnZ-PP//QCISD/6-311G(d,p) extrapolated to the complete basis set (CBS) using n = 2−4. 28 In the present study we finally resolve the contradictions by employing the state-of-the-art ab initio focal-point analysis (FPA) technique, 32,33 thereby improving the accuracy and reducing the uncertainty concerning the energetics of the title reaction.…”

“…The Br + CH 4 reaction has the unique feature that the saddle point and the HBr + CH 3 asymptote have nearly the same energy. Many theoretical and experimental studies show that the rate constants of the HBr + CH 3 reaction have a strong nonlinear Arrhenius behavior and negative temperature dependence at lower temperatures, [26][27][28] but the detailed explanation of the kinetics is still an open question. There are a lot of contradictions in the literature about the sign of the saddle-point energy relative to HBr + CH 3 and the existence/stability of a CH 3 -HBr complex, which may or may not affect the dynamics and kinetics of the HBr + CH 3 reaction.…”