Theoretical Study of Oxygen Isotope Exchange and Quenching in the O(1D) + CO2 Reaction

Abstract

Ab initio multireference configuration interaction calculations have been carried out for the CO3 system in singlet and triplet electronic states to investigate the mechanism of the O(1D) + CO2 reaction. The reaction has been shown to occur through the formation of an O−CO2 complex s0, which then isomerizes to a cyclic O(CO2) structure s1 over a barrier at s-TS0 located ∼0.3 kcal/mol below the reactants. The cyclic isomer s1, 48.8 kcal/mol lower in energy than O(1D) + CO2, can in turn rearrange to a D3h structure s2, only slightly higher in energy. The isomers s1 and s2 formed in the reaction possess high internal energy and can decompose into the initial reactants or undergo singlet−triplet intersystem crossing to form the triplet isomer t1, which can dissociate to O(3P) + CO2. If the attacking oxygen atom is isotope-labeled, isotope exchange can occur due to symmetry properties of s1 and s2. The ab initio energies, molecular parameters, and spin−orbit coupling constants were employed in statistical calculations of various isomerization and dissociation reaction rates. For intersystem crossing rates, we used the theory of radiationless transitions, in which the rates are determined as a product of the overlap of electronic wave functions (spin−orbit coupling) and the overlap of vibrational wave functions (Franck−Condon factor). The calculated rate constants were then used to compute the product branching ratios both for the case of the 16O(1D) + 44CO2 reaction and for the isotope-labeled 18O(1D) + 44CO2 reaction. For the latter, the calculated relative branching ratios of the 16O(1D) + 46CO2 and 16O(3P) + 46CO2 products at collision energies of 4.2 and 7.7 kcal/mol, 17/83 and 42/58, agree reasonably well with the experimental values of 16/84 and 33/67 (ref 4), respectively, and reproduce the qualitative trend with Ecol. The overall relative yields computed for 44CO2 and 46CO2 show that the attacking O(1D) atom can be incorporated into the product CO2 molecule with a near-statistical probability of 2/3.