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Kinetics of the Reactions of Hydroxyl Radicals with Diacetylene and Vinylacetylene

Abstract Highly unsaturated hydrocarbons like diacetylene (C4H2) or vinylacetylene (C4H4) are important intermediates in combustion that can have impact on soot formation. One of their major loss channels is reaction with hydroxyl radicals (OH). We studied the reactions C4H2 + OH → products (1) and C4H4 + OH → products (2) in a quasi-static reactor with helium as bath gas. The hydroxyl radicals were produced by laser flash-photolysis of nitric acid at a wavelength of 248 nm and detected by laser-induced fluorescence with excitation at 282 nm. The rate coefficients were obtained from the intensity-time profiles under pseudo-first order conditions with respect to OH. We found a virtually temperature-independent rate coefficient for reaction (1): k1 = (1.0 ± 0.3) × 10−11 cm3 s–1 (T = 290–670 K, P = 2.7–30.5 bar) and a weakly negative temperature-dependent rate coefficient for reaction (2): k2(T) = (6.4 ± 1.9) × 10−12 exp (486 K/T) cm3 s–1 (T = 295–740 K, P = 1.7–19.2 bar). For neither of the two reactions pressure dependence was observed. From comparisons with analogous reaction systems, we conclude that the dominating reaction pathway is OH addition, where in the case of C4H4 the double bond is preferred over the triple bond.