Kinetics and Products of Vinyl + 1,3-Butadiene, a Potential Route to Benzene

Abstract

The reaction between vinyl radical, C[subscript 2]H[subscript 3], and 1,3-butadiene, 1,3-C[subscript 4]H[subscript 6], has long been recognized as a potential route to benzene, particularly in 1,3-butadiene flames, but the lack of reliable rate coefficients has hindered assessments of its true contribution. Using laser flash photolysis and visible laser absorbance (λ = 423.2 nm), we measured the overall rate coefficient for C[subscript 2]H[subscript 3] + 1,3-C[subscript 4]H[subscript 6], k[subscript 1], at 297 K ≤ T ≤ 494 K and 4 ≤ P ≤ 100 Torr. k[subscript 1] was in the high-pressure limit in this range and could be fit by the simple Arrhenius expression k[subscript 1] = (1.1 ± 0.2) × 10[superscript –12] cm[superscript 3] molecule[superscript –1] s[superscript –1] exp(−9.9 ± 0.6 kJ mol[superscript –1]/RT). Using photoionization time-of-flight mass spectrometry, we also investigated the products formed. At T ≤ 494 K and P = 25 Torr, we found only C[subscript 6]H[subscript 9] adduct species, while at 494 K ≤ T ≤ 700 K and P = 4 Torr, we observed ≤∼10% branching to cyclohexadiene in addition to C[subscript 6]H[subscript 9]. Quantum chemistry master-equation calculations using the modified strong collision model indicate that n-C[subscript 6]H[subscript 9] is the dominant product at low temperature, consistent with our experimental results, and predict the rate coefficient and branching ratios at higher T where chemically activated channels become important. Predictions of k[subscript 1] are in close agreement with our experimental results, allowing us to recommend the following modified Arrhenius expression in the high-pressure limit from 300 to 2000 K: k[subscript 1] = 6.5 × 10[superscript –20] cm[superscript 3] molecule[superscript –1] s[superscript –1] T[superscript 2.40] exp(−1.76 kJ mol[superscript –1]/RT).