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Accurate

*ab initio* potential energy surface, thermochemistry, and dynamics of the Br(

^{2}P,

^{2}P

_{3/2}) + CH

_{4} → HBr + CH

_{3} reaction

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10.1063/1.4797467

### Abstract

Chemically accurate full-dimensional non-spin-orbit and spin-orbit (SO) ground-state potential energy surfaces (PESs) are obtained for the Br + CH_{4} → HBr + CH_{3} reaction by fitting 21 574 composite * ab initio * energy points. The composite method considers electron correlation methods up to CCSD(T), basis sets up to aug-cc-pwCVTZ-PP, correlation of the core electrons, scalar relativistic effects via an effective core potential (ECP), and SO corrections, thereby achieving an accuracy better than 0.5 kcal/mol. Benchmark structures and relative energies are computed for the stationary points using the * ab initio * focal-point analysis (FPA) scheme based on both ECP and Douglas−Kroll approaches providing all-electron relativistic CCSDT(Q)/complete-basis-set quality energies. The PESs accurately describe the saddle point of the abstraction reaction and the van der Waals complexes in the entrance and product channels. The SO-corrected PES provides a classical barrier height of 7285(7232 ± 50) cm^{−1}, *D* _{e} values of 867(799 ± 10) and 399(344 ± 10) cm^{−1} for the complexes CH_{3}–HBr and CH_{3}–BrH, respectively, and reaction endothermicity of 7867(7857 ± 50) cm^{−1}, in excellent agreement with the new, FPA-based benchmark data shown in parentheses. The difference between the Br + CH_{4} asymptotes of the non-SO and SO PESs is 1240 cm^{−1}, in good agreement with the experiment (1228 cm^{−1}). Quasiclassical trajectory calculations based on more than 13 million trajectories for the late-barrier Br + CH_{4}(*v* _{ k } = 0, 1) [*k* = 1, 2, 3, 4] reactions show that the vibrational energy, especially the excitation of the stretching modes, activates the reaction much more efficiently than translational energy, in agreement with the extended Polanyi rules. Angular distributions show dominant backward scattering for the ground-state reaction and forward scattering for the stretching-excited reactions. The reactivity on the non-SO PES is about 3−5 times larger than that on the SO PES in a wide collision energy range of 8000−16 000 cm^{−1}.

© 2013 American Institute of Physics

Received 29 January 2013
Accepted 08 March 2013
Published online 01 April 2013

Acknowledgments: This work was supported by the Scientific Research Fund of Hungary (OTKA, NK83583) and the Magyary Fellowship of the European Union and Hungary (TÁMOP 4.2.4.A/1-11-1-2012-0001).

Article outline:

I. INTRODUCTION

II. BENCHMARK *AB INITIO* CHARACTERIZATION

A. Computational details

B. The Br–CH_{4} van der Waals region

C. Structures of the saddle point and the CH_{3}–HBr and CH_{3}–BrH complexes

D. Barrier height, dissociation energies, and reactionenthalpy

III. *AB INITIO* NON-SPIN-ORBIT AND SPIN-ORBIT GROUND-STATE POTENTIAL ENERGY SURFACES

A. The *ab initio* data

B. Fitting the *ab initio* energies

C. The properties of the non-SO and SO potential energy surfaces

IV. QUASICLASSICAL TRAJECTORY CALCULATIONS

A. Computational details

1. Initial conditions

2. Final conditions

B. Mode-selectivity for the Br + CH_{4}reaction

V. SUMMARY AND CONCLUSIONS

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2014-04-17

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