Thermal Rate Constants for the O(³P) + HBr and O(³P) + DBr Reactions: Transition-State Theory and Quantum Mechanical Calculations

Abstract

The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

Energy Disposal and Thermal Rate Constants for the OH + HBr and OH + DBr Reactions: Quasiclassical Trajectory Calculations on an Accurate Potential Energy Surface

“…However, the modified Clary potential yields a reversed division of energy between translation and rotation when compared to our results and to Persky’s observations . The energy disposal in the OH + HBr reaction is very similar to that in the O( 3 P) + HBr reaction, which has an early transition state with a bending frequency of 213 cm –1 and a barrier height of 5.01 kcal mol –1 and positive activation energy. , The fractional energy release for the O + HBr reaction in vibration and rotation was measured to be 0.51 and 0.24, respectively. , This fact suggests that the energy disposal in the OH + HBr reaction has two major determinant factors, the kinematic constraints imposed by the mass combination of reactants and products, as in many other abstractions of light atoms between two heavy atoms (H + LH′ → HL + H′), that lead to the characteristic large release to product vibration energy and the low bending frequencies at the transition state that relate to the rotational excitation of the products.…”

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

“…The rate constant used in our modeling work for HBr + O → Br + OH was taken from Nava et al and has been utilized by many others in modeling work. We have revaluated this rate expression using all the experimental values by Nicovich and Wine , Nava et al , Singleton and Cvetanovic , and Brown and Smith and also taking into consideration the computational values by de Oliveira‐Filho et al and Broida et al . Fits to the experimental data and the computed values yielded nonlinear Arrhenius expressions ( T b ) with b ranging from 1.92 to 2.53 and activation energies E = 5.6–7.3 kJ/mol.…”

How To Arrive at Accurate Benchmark Values for Transition Metal Compounds: Computation or Experiment?

“…Furthermore, the recent progress in the development of linear-scaling coupled-cluster methods has pushed the limits of application to quite large molecules. − To summarize, the situation for closed-shell molecules is such that the well-known CCSD­(T) method (CC theory with singles, doubles, and perturbative triples clusters) used with sufficiently large basis sets is believed to provide accurate thermochemical data within 1 kcal mol –1 accuracy. More advanced methods that, in particular, take care of correlation effects beyond CCSD­(T) by additive schemes can even reach the 1 kJ mol –1 regime and may even be used to challenge the validity of experimentally derived results. − Examples are the high accuracy extrapolated ab initio thermochemistry approach (HEAT), the Weizmann-4 protocol (W4 , ), focal point analysis (FPA − ), the Feller–Peterson–Dixon method (FPD ,, ), and the correlation consistent composite approach (ccCA , ) and its multireference version (MR-ccCA).…”