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Title: Ab Initio Studies of ClOx Reactions. VI. Theoretical Prediction of Total Rate Constant and Product Branching Probabilities for the OH2 + ClO Reaction
Journal: J. Phys. Chem. A
Volume: 107
Page(s): 3841 - 3850
Year: 2003
Reference type: Journal article
Squib: 2003XU/ZHU3841-3850
Reaction:
HO2 + ClO → O2 + HOCl
Reaction order:
2
Temperature:
150 - 1000
K
Rate expression:
4.28x10-12 [cm3/molecule s] (T/298 K)-0.64 e890 [J/mole]/RT
Category: Theory
Data type:
Ab initio
Pressure dependence:
Rate constant is low pressure limit
Comments:
This rate expression is for lower pressures (< 1 torr) where stabilization channels are unimportant.
Quantum chemical study of reaction of HO2 + ClO. Used ab initio G2M(CC) method to calculated energetics of products, intermediates, and pathways for the different possible reaction channels. Computed channels on both singlet and triplet energy surfaces. Used the VARIFLEX program to calculated rate expressions for barrierless reactions and ChemRate program for reactions with barriers. The dominant pathway is the barrierless reaction HO2 + ClO -> HOCl + O2. At high pressures (>100 torr) and low temperatures (<300 K), the formation of the intemediates HOOOCl and HOOClO (about 20 and 7 kcal/mol more stable than reactants, respectively) can have a non-negligible contribution due to stabilization. At high temperatures (1000-2000 K) the product channel HO2 + ClO -> OH + ClOO can contribute (1-10%). All other channels are essentially insignificant and contribute less than 0.1% even at the highest temperatures. Because of stabilization processes the overall rate constant displays a negative temperature coefficient. Both the experimental data and the ab initio calculations demonstrate this. The calculated rate expression for the overall rate is in good agreement with the experimental data, which is generally in the 200-400 K range, and itself has some scatter to it (factor of 1.5-2). This scatter may be due to pressure dependence in this regime.
See companion paper Xu et al, JPCA 107, 1040 (2003)
View full bibliographic record.
Rate constant values calculated from the Arrhenius expression:
| T (K) | k(T) [cm3/molecule s] |
|---|---|
| 150 | 1.36E-11 |
| 200 | 9.43E-12 |
| 250 | 7.35E-12 |
| 300 | 6.09E-12 |
| 350 | 5.24E-12 |
| 400 | 4.63E-12 |
| 450 | 4.17E-12 |
| 500 | 3.81E-12 |
| 550 | 3.51E-12 |
| 600 | 3.27E-12 |
| 650 | 3.06E-12 |
| 700 | 2.89E-12 |
| 750 | 2.73E-12 |
| 800 | 2.60E-12 |
| 850 | 2.48E-12 |
| 900 | 2.38E-12 |
| 950 | 2.28E-12 |
| 1000 | 2.19E-12 |