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Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Br(2P, 2P3/2) + CH4 → HBr + CH3 reaction
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10.1063/1.4797467
/content/aip/journal/jcp/138/13/10.1063/1.4797467
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/13/10.1063/1.4797467

Figures

Image of FIG. 1.
FIG. 1.

Potential energy curves for the entrance channel of the Br + CH4 reaction as a function of the C–Br distance along the C 3 axis with fixed CH4(eq) geometry. The left panels show the direct ab initio results obtained at the AE-ECP-MRCI+Q/aug-cc-pwCVDZ-PP level of theory, whereas the right panels show the one-dimensional cuts of the non-SO and SO PESs. A1 and E denote the non-SO ground and excited electronic states, respectively, and SO1, SO2, and SO3 are the three SO states.

Image of FIG. 2.
FIG. 2.

Schematics of the non-SO and SO PESs of the Br + CH4 → HBr + CH3 reaction showing the accurate electronic energies and the corresponding PES values relative to Br(2P3/2) + CH4(eq). The accurate benchmark data (upper red numbers) are all-electron relativistic CCSDT(Q)/complete-basis-set quality results obtained from the present focal-point analysis.

Image of FIG. 3.
FIG. 3.

Accuracy of the different frozen-core and all-electron ab initio correlation methods and bases based on computations at 15 representative configurations along the reaction coordinate of the Br + CH4 → HBr + CH3 reaction. The 15 energies are relative to Br + CH4(eq) and span a range from 0 to 21 000 cm−1. The RMS errors are relative to high-quality reference results obtained at the AE-ECP-UCCSD(T)/aug-cc-pwCVQZ-PP level of theory. All the computations employed ECP for Br and the corresponding aug-cc-pVnZ-PP and aug-cc-pwCVnZ-PP [n = D, T, Q] basis sets for frozen-core and all-electron computations, respectively. The all-electron composite energy is defined as E[AE-ECP-UCCSD(T)/aug-cc-pwCVDZ-PP] + E[AE-ECP-UMP2/aug-cc-pwCVTZ-PP] – E[AE-ECP-UMP2/aug-cc-pwCVDZ-PP].

Image of FIG. 4.
FIG. 4.

Accuracy of the composite method based on computations at 15 representative configurations for the Br + CH4 → HBr + CH3 reaction. E = 0 corresponds to the Br + CH4(eq) asymptote and the errors are relative to high-quality reference results obtained at the AE-ECP-UCCSD(T)/aug-cc-pwCVQZ-PP level of theory. All the computations used ECP for Br and all the electrons were correlated. The composite energy is defined as E[AE-ECP-UCCSD(T)/aug-cc-pwCVDZ-PP] + E[AE-ECP-UMP2/aug-cc-pwCVTZ-PP] – E[AE-ECP-UMP2/aug-cc-pwCVDZ-PP].

Image of FIG. 5.
FIG. 5.

Spin-orbit correction curves for the entrance channel of the Br + CH4 reaction as a function of the C–Br distance along the C 3 axis keeping CH4 at equilibrium with H3CH–Br and HCH3–Br orientations. The SO correction is defined as the difference between the SO and non-SO ground-state electronic energies. The curves were obtained by the AE-ECP-MRCI+Q/aug-cc-pwCVDZ-PP level of theory and the PES values, i.e., differences between the SO and non-SO PESs, are also shown for comparison.

Image of FIG. 6.
FIG. 6.

Cross sections as a function of collision energy for the ground-state (v = 0), bending-excited (v 4 and v 2), and stretching-excited (v 1 and v 3) Br(2P3/2) + CH4(v k = 0, 1) [k = 1, 2, 3, 4] reactions obtained by considering (a) all trajectories without ZPE constraint or weighting, (b) soft, (c) hard, and (d) CH3-based ZPE constraints, in which trajectories are discarded if (b) the sum of the product vibrational energies is less than the sum of their ZPEs, (c) either product has less vibrational energy than its ZPE, and (d) the CH3 product has less vibrational energy than its ZPE. (e) Gaussian binning was also employed using the 1GB procedure as described in Ref. 57 .

Image of FIG. 7.
FIG. 7.

Cross sections as a function of total energy [sum of the collision energy and the vibrational energy relative to CH4(v = 0)] for the ground-state (v = 0), bending-excited (v 4 and v 2), and stretching-excited (v 1 and v 3) Br(2P3/2) + CH4(v k = 0, 1) [k = 1, 2, 3, 4] reactions obtained by considering all trajectories without ZPE constraint or weighting.

Image of FIG. 8.
FIG. 8.

Cross sections as a function of collision energy for the Br(2P) + CH4(v = 0) and Br(2P3/2) + CH4(v = 0) reactions obtained by using the non-SO and SO PESs, respectively, considering all trajectories without ZPE constraint or weighting.

Image of FIG. 9.
FIG. 9.

Reaction probabilities as a function of impact parameter for the ground-state (v = 0), bending-excited (v 4 and v 2), and stretching-excited (v 1 and v 3) Br(2P3/2) + CH4(v k = 0, 1) [k = 1, 2, 3, 4] reactions at different collision energies obtained by considering all trajectories without ZPE constraint or weighting.

Image of FIG. 10.
FIG. 10.

Differential cross sections for the ground-state (v = 0), bending-excited (v 4 and v 2), and stretching-excited (v 1 and v 3) Br(2P3/2) + CH4(v k = 0, 1) [k = 1, 2, 3, 4] reactions at different collision energies obtained by considering all trajectories without ZPE constraint or weighting.

Image of FIG. 11.
FIG. 11.

Differential cross sections as in Fig. 10 , but each panel corresponds to different state-specific Br(2P3/2) + CH4(v k = 0, 1) [k = 1, 2, 3, 4] reactions.

Tables

Generic image for table
Table I.

Structure (in Å and degrees; C 3v symmetry) and non-SO classical barrier height (V SP, cm−1) for the saddle point (CH3–Hb–Br)SP at different levels of theory.

Generic image for table
Table II.

Equilibrium structures (in Å and degrees) and dissociation energies (D e, cm−1) of the complexes CH3–HBr and CH3–BrH at different levels of theory.

Generic image for table
Table III.

Equilibrium structures (in Å and degrees) of the reactants and products and the vibrationless enthalpy (ΔE e, cm−1) of the Br(2P) + CH4 → HBr + CH3 reaction at different levels of theory.

Generic image for table
Table IV.

Focal-point analysis of the classical barrier height (V SP, cm−1) of the Br + CH4 → HBr + CH3 reaction based on all-electron effective core potential (ECP) as well as all-electron Douglas−Kroll (DK) computations. a

Generic image for table
Table V.

Focal-point analysis of the dissociation energy (D e, cm−1) of the CH3–HBr complex based on all-electron effective core potential (ECP) as well as all-electron Douglas−Kroll (DK) computations. a

Generic image for table
Table VI.

Focal-point analysis of the dissociation energy (D e, cm−1) of the CH3–BrH complex based on all-electron effective core potential (ECP) as well as all-electron Douglas−Kroll (DK) computations. a

Generic image for table
Table VII.

Focal-point analysis of the vibrationless endoergicity (ΔE e, cm−1) of the Br + CH4 → HBr + CH3 reaction based on all-electron effective core potential (ECP) as well as all-electron Douglas−Kroll (DK) computations. a

Generic image for table
Table VIII.

Summary of the focal-point analysis results (in cm−1) showing the complete basis set (CBS) results at ROHF and all-electron RMP2, UCCSD, and UCCSD(T) levels as well as the effects of the post-CCSD(T) electron correlation (T(Q)), the spin-orbit (SO) couplings, and zero-point vibrational energy (ZPE) for the barrier height and enthalpy of the Br + CH4 → HBr + CH3 reaction as well as for the dissociation energies of CH3–HBr and CH3–BrH.

Generic image for table
Table IX.

Properties of the stationary points of the potential energy surface (PES).

Generic image for table
Table X.

Properties of the potential energy surface (PES) for the reactants and products.

Generic image for table
Table XI.

Harmonic vibrational frequencies (in cm−1) for (CH3–H–Br)SP, CH3–HBr, and CH3–BrH.

Generic image for table
Table XII.

Harmonic vibrational frequencies (in cm−1) for CH4, CH3, and HBr.

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/content/aip/journal/jcp/138/13/10.1063/1.4797467
2013-04-01
2014-04-17
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Br(2P, 2P3/2) + CH4 → HBr + CH3 reaction
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/13/10.1063/1.4797467
10.1063/1.4797467
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