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

摘要

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 (lambda = 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) x 10(12) cm(3) 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 <=similar to 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 x 10(20) cm(3) molecule(-1) s(-1) T-2.40 exp(-1.76 kJ mol(-1)/RT).