Direct measurements of the reaction OH + CH 2 O → HCO + H 2 O at high temperatures

The reaction of hydroxyl [OH] radicals with formaldehyde [CH 2 O] was studied at temperatures ranging from 934 K to 1670 K behind reflected shock waves at an average total pressure of 1.6 atm. OH radicals were produced by shock‐heating tert‐butyl hydroperoxide [(CH 3 ) 3 COOH], while 1,3,5‐trioxane [(CH 2 O) 3 ] was used in the preshock mixtures to generate reproducible levels of CH 2 O. OH concentration time‐histories were inferred from laser absorption using the well‐characterized R 1 (5) line of the OH A‐X (0, 0) band near 306.7 nm. Detailed error analyses, taking into account both experimental and mechanism‐induced contributions, yielded uncertainty estimates of ±25% at 1595 K and ±15% at 1229 K for the rate of the reaction between CH 2 O and OH. These uncertainties are substantially lower than the factor of two uncertainty currently used for this reaction at high temperatures. The rate constants were fit with the recent low‐temperature measurements of Sivakumaran et al. (Phys Chem Chem Phys 2003,5,4821–4827) to the three‐parameter form shown below; this fit reconciles experimental data on the title reaction at low, intermediate, and high temperatures (200–1670 K). equation image The reaction of OH with CH 2 O was also studied using quantum chemical methods at the CCSD(T) level of theory using the 6‐311 ++ G(d,p) basis set. The transition state for the H‐atom metathesis reaction was located, and reaction rate coefficients were calculated. Reasonable agreement with the experimental measurements was obtained. The decomposition rate of tert‐butyl hydroperoxide to a tert‐butoxy radical and an OH radical was measured, and fit to the following form equation image Uncertainty limits for k 5 were estimated to be ±25% in the 900–1000 K temperature range, a marked reduction from the factor of 2–3 uncertainty currently recommended for this reaction in the literature.