### 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 kcalmol. 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}.

Original language | English |
---|---|

Article number | 134301 |

Journal | The Journal of Chemical Physics |

Volume | 138 |

Issue number | 13 |

DOIs | |

Publication status | Published - Apr 7 2013 |

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### ASJC Scopus subject areas

- Physics and Astronomy(all)
- Physical and Theoretical Chemistry

### Cite this

**Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Br( ^{2}P, ^{2}P_{3/2}) CH_{4} → HBr CH_{3} reaction.** / Czakó, Gábor.

Research output: Contribution to journal › Article

}

TY - JOUR

T1 - Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Br(2P, 2P3/2) CH4 → HBr CH3 reaction

AU - Czakó, Gábor

PY - 2013/4/7

Y1 - 2013/4/7

N2 - 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 CH3 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 kcalmol. 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, De values of 867(799 ± 10) and 399(344 ± 10) cm-1 for the complexes CH3-HBr and CH3-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 CH4 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 CH4(vk= 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.

AB - 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 CH3 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 kcalmol. 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, De values of 867(799 ± 10) and 399(344 ± 10) cm-1 for the complexes CH3-HBr and CH3-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 CH4 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 CH4(vk= 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.

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U2 - 10.1063/1.4797467

DO - 10.1063/1.4797467

M3 - Article

VL - 138

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 13

M1 - 134301

ER -