The high temperature pyrolysis of 1,3‐butadiene: heat of formation and rate of dissociation of vinyl radical

The high temperature pyrolysi of 1,3‐butadiene has been investigated in the shock tube with two time‐resolved diagnostic techniques: laser schlieren measurements of density gradient with 1, 2, 4, and 5% C 4 H 6 in Ar or Kr, 0.26 < P 2 < 0.66 atm, over 1550–2200 K, and time‐of‐flight mass spectra for 3% C 4 H 6 –Ne, P 5 ∼ 0.4 atm, 1400–2000 K. When combined with a recent single‐pulse shock tube product analysis covering 1050–2050 K, these measurements permit a complete modeling of major species in C 4 H 6 pyrolysis. Extrapolated density gradients and product analyses show initiation is dominated by C 4 H 6 → 2C 2 H 3 ., significant falloff and Arrhenius curvature being seen in the derived rates. A restricted rotor, Gorin model RRKM fit to these rates with reasonable parameters generates The derived barrier, Δ H = 99 ± 4 kcal/mol, translates to Δ H ,298 = 63.4 ± 2 kcal/mol for the heat of formation of vinyl radical. A mechanism for the formation of all products detected in the above experiments is given, together with a successful but semiquantitative kinetic model for major products. The measurements require the rate of vinyl radical dissociation, C 2 H 3 + M → C 2 H 2 + H + M, to be extremely low, k < 10 9 cm 3 /mol s for 1600 K, so that the dominant chain carrier in C 4 H 6 pyrolysis is vinyl radical.