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

Abstract

Chemically accurate full-dimensional non-spin-orbit and spin-orbit (SO) ground-state potential energy surfaces (PESs) are obtained for the Br + CH4 → 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 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), 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).

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

“…24,25 Using the same strategy we also developed ab initio PESs for the Cl, O, and Br + CH 4 reactions in 2011, 2012, and 2013, respectively. [26][27][28] Reaction dynamics simulations on these PESs could be compared with experiments of prominent groups of Nesbitt, 29 Crim, 30 Zare, 31 Yang, 32 and Liu 33,34 and motivated PES developments by Manthe 35 and Zhang 36 and their co-workers. Our simulations 26 provided cold HCl rotational distributions in agreement with experiment 37 for the first time, new insights into the Polanyi rules for polyatomic processes, 26,38 rotational mode-specificity, 39,40 and angular dependence of a TS barrier height.…”

The Reaction between Bromine and the Water Dimer and the Highly Exothermic Reverse Reaction

“…The SO correction for Br( 2 P) of 1132 cm −1 obtained here is in reasonable agreement with experiment (1228 cm −1 ) . Recently, Czakó has noted that, for the Br + CH 4 reaction, the SO correction is quenched at the products (HBr + CH 3 ), the exit complex, and the late transition state (geometrically similar to the product HBr) . The Br + CH 4 reaction is different from the present system, as there is no strongly bound Br···CH 4 entrance complex.…”

Alapvetõ kémiai reakciók dinamikájának ésmechanizmusainak vizsgálata

“…2009-ben, amikor kifejlesztettem a F + CH 4 reakció nagypontosságú teljes-dimenziós PES-ét. 16 Azóta már a Cl, O és a Br + metán reakciókra is publikáltunk hasonlóan jó minõségû PES-eket, 1,17,18 amelyek minden korábbinál pontosabb dinamika szimulációkat tettek lehetõvé. Az alábbiakban csak a legfontosabb eredményeket mutatatom be: Egy kémiai reakció lejátszódásához általában egy energiagáton kell keresztül mennünk és a reaktivitást nagyban meghatározza a gát magassága és alakja.…”

“…11,12 • Teljes-dimenziós analitikus PES-ek fejlesztése a F, O, Cl és Br + metán reakciókra. 1,16,17,18 • A Polanyi-szabályok módosítása. 1 • A szögfüggõ energiagát meghatározása.…”