Ab initio studies of ClO_x reactions. I. Kinetics and mechanism for the OH + ClO reaction

摘要

The reaction of OH with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces predicted by the G2M method are presented. The reaction was shown to take place primarily over the single surface by two main channels producing H_2O + Cl and HCl _ O_2 (~1#DELTA#), with the former being dominant. The predicted total rate constant, k_ t = 5.27 X 10~(-9) T~1.03 exp (-40/T) cm~3 molecule~(-1) s~(-1), and product branching ratios in the temperature range 200-500 K at P < 200 atm agree satisfactorily with experimental values. The computed branching ratios, K_2/(k_1 + k_2) = 0.073 for HCl +~1 O_2 and 0.045-0.048 for DCl + ~1O_2 in the temperature range 200-500 K based on the recent experimental heat of formation for HO_2 ( 4.0 (+-) 0.8 kcal/mol) compare closely with the experimental values, 0.07 (+-) 0.03 and 0.05(+-) 0.02, respectively. At higher temperatures (1000-2500 K), the branching ratios increase slightly to 0.084-0.137 and 0.061-0.111 for the OH and OD reacations, respectively. The rate constant for HO_2 + Cl and HCl + O_2 production from OH + ClO in the temperature range, 500-2500 K, can be given by k_1 = 3.4 X 10~(-13) T~0.3 exp (725/T) and k_ 2 = 5.85 X 10~(-19) T~1.67 exp (1926/T) cm~3 molecule~(-1) s~(-1), respectively.