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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2015-07, Vol.119 (28), p.7325-7338
• Main Authors: Buras, Zachary J, Dames, Enoch E, Merchant, Shamel S, Liu, Guozhu, Elsamra, Rehab M. I, Green, William H
• Format: Article
• Language: English
• Subjects: Absorption; Aromatic compounds; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Lasers; Photodissociation; Vinyl
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp512705r

The reaction between vinyl radical, C2H3, and 1,3-butadiene, 1,3-C4H6, 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 C2H3 + 1,3-C4H6, k 1, at 297 K ≤ T ≤ 494 K and 4 ≤ P ≤ 100 Torr. k 1 was in the high-pressure limit in this range and could be fit by the simple Arrhenius expression k 1 = (1.1 ± 0.2) × 10–12 cm3 molecule–1 s–1 exp­(−9.9 ± 0.6 kJ mol–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 C6H9 adduct species, while at 494 K ≤ T ≤ 700 K and P = 4 Torr, we observed ≤∼10% branching to cyclohexadiene in addition to C6H9. Quantum chemistry master-equation calculations using the modified strong collision model indicate that n-C6H9 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 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 1 = 6.5 × 10–20 cm3 molecule–1 s–1 T 2.40 exp­(−1.76 kJ mol–1/RT).