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

Abstract

Time-resolved infrared emission of CO2 and OCS was observed in reactions O(P-1) + OCS and O(D-1) + OCS with a step-scan Fourier transform spectrometer. The CO2 emission involves Delta nu(3) = -1 transitions from highly vibrationally excited states, whereas emission of OCS is mainly from the transition (0, 0 degrees, 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(D-1) + OCS. For O(P-3) + OCS, weak emission Of CO2 diminishes when Ar is added, indicating that O(P-3) 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 Dunharn 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* congruent to 18 000 +/- 500 cm(-1) into all vibrational modes of CO2. For the reaction of O(D-1) + OCS, approximately 51% of the available energy is converted into vibrational energy of CO2, and a statistical prediction using E* congruent to 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(P-3) + OCS(X-1 Sigma(+)) surface proceeds via direct abstraction and substitution channels with barriers of 27.6 and 36.4 kJ mol(-1), respectively, to produce SO(X-3 Sigma(-)) + CO(X-1 Sigma(+)) and S(P-3) + CO2(X(1)A(1)), whereas two intermediates, OSCO and SC(O)O, are formed from the singlet O(D-1) + OCS(X-1 Sigma(+)) surface without barrier, followed by decomposition to SO(a(1)Delta) + CO(X-1 Sigma(+)) and S(D-1) + CO2(X(1)A(1)), respectively. For the ground-state reaction O(P-3) + OCS(X-1 Sigma(+)), the singlet-triplet curve crossings play important roles in the observed kinetics and chemiluminescence.