Reaction Dynamics of O(1D,3P) + OCS Studied with Time-Resolved Fourier Transform Infrared Spectroscopy and Quantum Chemical Calculations

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Abstract

Time-resolved infrared emission of CO2 and OCS was observed in reactions O(3P) + OCS and O(1D) + OCS with a step-scan Fourier transform spectrometer. The CO2 emission involves Δν3 = −1 transitions from highly vibrationally excited states, whereas emission of OCS is mainly from the transition (0, 0°, 1) → (0, 0°, 0); the latter derives its energy via near-resonant V−V energy transfer from highly excited CO2. Rotationally resolved emission lines of CO (v ≤ 4 and J ≤ 30) were also observed in the reaction O(1D) + OCS. For O(3P) + OCS, weak emission of CO2 diminishes when Ar is added, indicating that O(3P) is translationally hot to overcome the barrier for CO2 formation. The band contour of CO2 agrees with a band shape simulated on the basis of a Dunham expansion model of CO2; the average vibrational energy of CO2 in this channel is 49% of the available energy. This vibrational distribution fits with that estimated through a statistical partitioning of energy E* ≅ 18 000 ± 500 cm−1 into all vibrational modes of CO2. For the reaction of O(1D) + OCS, approximately 51% of the available energy is converted into vibrational energy of CO2, and a statistical prediction using E* ≅ 30 000 ± 500 cm−1 best fits the data. The mechanisms of these reactions are also investigated with the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) method. The results indicate that the triplet O(3P) + OCS(X1Σ+) surface proceeds via direct abstraction and substitution channels with barriers of 27.6 and 36.4 kJ mol−1, respectively, to produce SO(X3Σ−) + CO(X1Σ+) and S(3P) + CO2(X1A1), whereas two intermediates, OSCO and SC(O)O, are formed from the singlet O(1D) + OCS(X1Σ+) surface without barrier, followed by decomposition to SO(a1Δ) + CO(X1Σ+) and S(1D) + CO2(X1A1), respectively. For the ground-state reaction O(3P) + OCS(X1Σ+), the singlet−triplet curve crossings play important roles in the observed kinetics and chemiluminescence.

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