Product Distributions of the C2H2 + O and HCCO + H Reactions. Rate Constant of CH2(X3B1) + H

Article abstract of DOI:10.1021/j100203a043

The branching ratios of the two dominant methylene sources in C2H2/O/H systems C2H2 + O -> CH2(³B₁) + CO (r2a) or HCCO + H (r2b) and HCCO + H -> CH2(¹A₁) + CO (r3a) or CH2(³B₁) + CO (r3b) have been determined, at 285 K, using discharge-flow/molecular beam mass spectrometry techniques (D-F/MBMS).The ratios were derived from the observed decrease of MBMS CH2 signals upon substituting 25percent of the helium bath gas by 0.5 Torr of methane, which selectively scavenges CH2(¹A₁); in the absence of CH4 the singlet CH2 is mainly collisionally converted to CH2(³B₁).In this way, the ratios of the total formation rates of singlet and triplet CH2 are obtained.As the rate of reaction r3 is linked to the rate of reaction r2b by the known fraction of HCCO reacting with H, in competition with O, both the ratios k_2a/k_2b and k_3a/k_3b can be extracted from the data.Thus, including also probably systematic errors, the HCCO yield of C2H2 + O is found to be k_2b/k₂ = 85 ⁺⁴_-9 percent and the CH2(¹A₁) yield of HCCO + H, k_3a/k₃ = 92 +/- 15percent (95percent confidence intervals).In addition, the rate constant of CH2(³B₁) + H -> CH(²Π) + H2 (r1) was derived relative to the known K(³CH2+O) from CH2 signals at different / ratios; k₁ = (1.6 +/- 0.6) * 10¹⁴ cm³ mol^-1 s^-1.