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A: Kinetics, Dynamics, Photochemistry, and Excited States
Computational Study on the Mechanisms and Rate Constants for the O(P,D) + OCS Reactions 3
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Hsin-Tsung Chen, Tien Van Pham, and Ming-Chang Lin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05720 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019
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The Journal of Physical Chemistry
Computational Study on the Mechanisms and Rate Constants for the O(3P,1D) + OCS Reactions Hsin-Tsung Chen 1,2, Tien V. Pham 3,4, and M.C. Lin 2,3* 1 Department 2
of Chemistry, Chung Yuan Christian University, Chung Li District, Taoyuan City, 32023, Taiwan
Department of Applied Chemistry and Center for Interdisciplinary Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan
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Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan
4 School
of Chemical Engineering, Hanoi University of Science and Technology, Vietnam
ABSTRACT The mechanisms and kinetics of O(3P,1D) + OCS(X1Σ+) reactions have been studied by the high-level G2M(CC2) and CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) methods in conjunction with the transition state theory (TST) and variational Rice-Ramsperger-KasselMarcus (RRKM) theory calculations. The result shows that the triplet surface proceeds directly abstraction and substitution channels to produce SO(3P) + CO(X1Σ+) and S(3P) + CO2(1A1) by passing the barriers of 7.6 and 9.1 kcal/mol at G2M(CC2)//B3LYP/6-311+G(3df) level, respectively, while two stable intermediates, LM1 (OSCO1) and LM2 (SC(O)O1), are formed barrierlessly from O(1D) + OCS(X1Σ+) in the singlet surface, which lie at -40.5 and -50.1 kcal mol-1 relative to O(3P) + OCS(X1Σ+) reactants and decompose to CO(X1Σ+) + SO(a1Δ) and S(1D) + CO2(X1Σg+). LM1 and LM2 may also be produced by singlet-triplet surface crossings via MSX1 and MSX2; the predicted total rate constant for O(3P) + OCS(X1Σ+) reaction including the crossings, 9.2 × 10-11 exp(-5.18 kcal mol-1/RT) cm3 molecule-1 s-1, is in good agreement with available experimental data. The branching ratio of the CO2 product channel, 0.22 – 0.32, between 1200 and 1600 K, is also in excellent agreement with the value of 0.2 – 0.3 measured by Isshiki et al. (J. Phys. Chem. A. 2003, 107, 2464). Keywords: OCS(X1Σ+), O(3P,1D), MSX, singlet-triplet surface, non-adiabatic TST. *Corresponding authors: M. C. Lin, email address:
[email protected] 1 ACS Paragon Plus Environment
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1. INTRODUCTION Combustion chemistry involving sulfur has been getting much attention in a variety of modeling and kinetic studies;1-6 however, the kinetic database for compounds containing sulfur is considerably immature compared to those of hydrocarbons and nitrogen compounds. For example, recent works using a shock tube ARAS (atomic resonance absorption spectrometry) technique,7-13 which found several new product channels and different kinetic rates in the elementary reactions of H2S oxidation process, suggested that the general combustion models of sulfur-containing systems need to be corrected. As a continuation of the series study of hightemperature sulfur chemistry,14 this paper studies computationally the reactions COS with O (3P) and O(1D) atoms: O (3P,1D) + COS(X1Σ+) → SO(X3Σ-, a1Δ) + CO(X1Σ+)
(a)
O (3P,1D) + COS(X1Σ+) → S(3P,1D) + CO2(X1Σg+)
(b)
The two reactions may be inter-connected via their long-lived intermediates and also by surface crossings. The singlet reaction, O(1D) + COS(X1Σ+) → SO(1D) + CO(X1Σ+), has been investigated by Gauthier and Snelling.15 They estimated the rate constant to be about 3.0 × 1010
cm3 molecule-1 s-1 at 300 K by Vis-UV absorption spectrometry. In addition, the overall rate
constant for the triplet reaction, O (3P) + COS(X1Σ+) → Products, has been determined as k = 7.80 × 10-11 exp(-21.8 kJ mol-1/RT) cm3 molecule-1 s-1 by Singleton and Cvetanovic16 in the temperature range of 239 – 1900 K. The triplet reaction has also been investigated by Kruger and Wagner at around 1900 K in a shock tube study17 and by Homann et al. (T
