J. Phys. Chem. A 2005, 109, 2207-2216
2207
IR Kinetic Spectroscopy Investigation of the CH4 + O(1D) Reaction Hong-bing Chen, W. D. Thweatt, Jinjin Wang, Graham P. Glass,* and R. F. Curl* Chemistry Department and Rice Quantum Institute, Rice UniVersity, Houston, Texas 77005 ReceiVed: October 1, 2004; In Final Form: January 10, 2005
The branching of the title reaction into several product channels has been investigated quantitatively by laser infrared kinetic spectroscopy for CH4 and CD4. It is found that OH (OD) is produced in 67 ( 5% (60 ( 5%) yield compared to the initial O(1D) concentration. H (D) product is produced in 30 ( 10%(35 ( 10%). H2CO is produced in 5% yield in the CH4 system (it was not possible to measure the CD2O yield in the CD4 case). D2O is produced in 8% yield in the CD4 system (it was not feasible to measure the H2O yield). The ratio of the overall rate constant of the CD4 reaction to the overall rate constant of the O(1D) + N2O reaction was determined to be 1.25 ( 0.1. A measurement of the reaction of O(1D) with NO2 gave 1.3 × 10-10 cm3 molecule-1 s-1 relative to the literature values for the rate constants of O(1D) with H2 and CH4. Hot atom effects in O(1D) reactions were observed.
Introduction Methane is the most abundant hydrocarbon in the atmosphere and consequently one of the most important atmospheric species. The reaction of methane with O(1D) is a source for stratospheric OH, and that oxidation is also a source of a portion of stratospheric H2O, which itself is a source of OH through reaction with O(1D). These reactions occur in the region of high ozone concentration at altitudes from about 20 to 50 km, where hydroxyl radical (OH) can cause the catalytic destruction of very large quantities of ozone via the HOx cycles.1,2 Since the reaction of O(1D) with CH4 is important in the atmosphere, it has been studied many times previously using various experimental and theoretical methods. The reaction is extremely fast: most reported3-5 rate constants cluster around 1.5 × 10-10 cm3 molecule-1 s-1 except for one measurement6 of 3.8 × 10-10 cm3 molecule-1 s-1 and another7 of 2.2 ( 0.2 × 10-10 cm3 molecule-1 s-1. The JPL compilation8 recommends a value of the total reaction rate constant of 1.5 × 10-10 cm3 molecule-1 s-1. The reaction has numerous exothermic channels, some of which are listed below:
O(1D) + CH4 f CH3 + OH f CH2OH + H f CH3O + H
∆H ) -179 kJ/mol
(1a)
∆H ) -172 kJ/mol (1b) ∆H ) -128 kJ/mol
(1c)
f H2CO + H2
∆H ) -473 kJ/mol (1d)
f H2CO + 2H
∆H ) -37 kJ/mol
f 1CH2 + H2O
∆H ) -176 kJ/mol (1f)
f O(3P) + CH4
∆H ) -439 kJ/mol (1g)
(1e)
Here, the reaction enthalpies were calculated using data from Atkinson et al.9 The branching ratios for the major channels have been measured by a number of groups, and it is now well established * To whom correspondence should be addressed.
that the dominant products of O(1D) + CH4 are CH3 + OH. Lin and DeMore10 analyzed the final products of photolyzed mixtures of N2O/CH4 and concluded that (1a) accounted for 90% and (1d) accounted for 9% of the observed products. Addison et al.11 reported an OH yield of 80%. Casavecchia et al.12 used a molecular beam apparatus to observe H and CH2OH (or CH3O) products, and reported that the yield of H2 was
