Computational Study on the Mechanisms and Rate Constants for the O(³P,¹D) + OCS Reactions

Area of Science:

• Chemical Kinetics
• Atmospheric Chemistry
• Quantum Chemistry

Background:

• The reaction between oxygen atoms and carbonyl sulfide (OCS) is crucial for understanding atmospheric sulfur and carbon cycles.
• Previous studies have explored aspects of this reaction, but detailed mechanistic insights and accurate rate constants were lacking.

Purpose of the Study:

• To elucidate the reaction mechanisms and kinetics of O(3P, 1D) + OCS reactions.
• To accurately calculate rate constants and product branching ratios using high-level computational methods.

Main Methods:

• High-level ab initio calculations using G2M(CC2) and CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) methods.
• Application of transition-state theory and variational Rice-Ramsperger-Kassel-Marcus (VRKMM) theory.
• Investigation of both triplet and singlet potential energy surfaces, including intersystem crossings.

Main Results:

• The triplet surface proceeds via direct abstraction and substitution, yielding SO + CO and S + CO2 with low energy barriers.
• The singlet surface forms two stable intermediates, LM1 (OSCO) and LM2 (SC(O)O), barrierlessly, which can decompose to different products.
• Calculated total rate constant and CO2 branching ratio show excellent agreement with experimental data, particularly between 1200-1600 K.

Conclusions:

• The study provides a comprehensive mechanistic understanding of the O + OCS reaction system.
• Computational results accurately reproduce experimental observations, validating the theoretical approach.
• The findings contribute to a more precise modeling of atmospheric processes involving OCS.