Theoretical Study of Oxygen Isotope Exchange and Quenching in the O(~1D) + CO_2 Reaction

摘要

Ab initio multireference configuration interaction calculations have been carried out for the CO_3 system in singlet and triplet electronic states to investigate the mechanism of the O(~1D) + CO_2 reaction.The reaction has been shown to occur through the formation of an O-CO_2 complex s0,which then isomerizes to a cyclic O=(CO_2) structure s1 over a barrier at s-TSO located approx 0.3 kcal/mol below the reactants.The cyclic isomer s1,48.8 kcal/mol lower in energy than O(~1D) + CO_2,can in turn rearrange to a D_(3h) 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 tl,which can dissociate to O(~3P) + CO_2.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 ~(16)O(~1D) + ~(44)CO_2 reaction and for the isotope-labeled ~(18)O(~1D) + ~(44)CO_2 reaction.For the latter,the calculated relative branching ratios of the ~(16)O(~1D) + ~(46)CO_2 and ~(16)O(~3P) + ~(46)CO_2 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 E_(col).The overall relative yields computed for ~(44)CO_2 and ~(46)CO_2 show that the attacking O(~1D) atom can be incorporated into the product CO_2 molecule with a near-statistical probability of 2/3.