Theoretical investigation of the reaction mechanisms and kinetics of CFCl2CH2O2 and ClO in the atmosphere

Abstract

The reaction between CFCl₂CH₂O₂ radicals and ClO was studied using the B3LYP and CCSD(T) methods associated with the 6-311++G(d,p) and cc-pVTZ basis sets, and subsequently RRKM-TST theory was used to predict the thermal rate constants and product distributions. On the singlet PES, the dominant reaction is the addition of the ClO oxygen atom to the terminal-O of CFCl₂CH₂O₂ to generate adduct IM1 (CFCl₂CH₂OOOCl), and then dissociation to final products P1 (CFCl₂CHO + HO₂ + Cl) occurs. RRKM theory is employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that the collision-stabilized IM1 (CFCl₂CH₂OOOCl) dominates the reaction at 200-500 K (accounting for about 60-100%) and the dominant products are P1 (CFCl₂CHO + HO₂ + Cl). The yields of the other products are very low and insignificant for the title reaction. The total rate constants exhibit typical "falloff" behavior. The pathways on the triplet PES are less competitive than that on the singlet PES. The calculated overall rate constants are in good agreement with the experimental data. The atmospheric lifetime of CFCl₂CH₂O₂ in ClO is around 2.04 h. TD-DFT calculations imply that IM1 (CFCl₂CH₂OOOCl), IM2 (CFCl₂CH₂OOClO) and IM3 (CFCl₂CH₂OClO₂) will photolyze under sunlight.

Fig. 1. Optimized geometries for all the intermediates and transition states at the B3LYP/6-311++G(d,p) level for the reaction between CFCl₂CH₂O₂ and ClO. Bond distances are given in Å.

Fig. 2. Optimized geometries (length in Å and angle in degree) for all the reactants and products at the B3LYP/6-311++G(d,p) level for the reaction between CFCl₂CH₂O₂ and ClO. Angles are given in °, and bond distances are given in Å. The values in italics are experimental data from ref. 29.

Fig. 3. Potential energy surface obtained at the CCSD(T)//B3LYP level for the CFCl₂CH₂O₂ + ClO reaction.

Scheme 1. The primary pathways of the CFCl₂CH₂O₂ + ClO reaction.

Fig. 4. Plots of the rate coefficients for total and primary reaction channels versus 1000/T (K⁻¹) at 200–1000 K associated with the available experimental value.

Fig. 5. Predicted rate coefficients for the total reaction and each individual product pathway of the reaction between CFCl₂CH₂O₂ and ClO at 200–3000 K and 10⁻¹⁰ to 10¹⁰ atm.

Fig. 6. Predicted branching ratios for the CFCl₂CH₂O₂ + ClO reaction at the low pressure limit, atmospheric pressure and high pressure limit.

Fig. 7. Pressure dependence of the total rate coefficients for the CFCl₂CH₂O₂ + ClO reaction at 298, 1000 and 3000 K.

Fig. 8. Branching ratios for the CFCl₂CH₂O₂ + ClO reaction at 10⁻¹⁴ to 10¹⁴ torr at 298, 1000 and 3000 K.