A theoretical and experimental kinetic study of phenyl radical addition to butadiene

The reactions of phenyl radical (C₆H₅) are of growing technological and scientific interest. A better understanding of phenyl radical addition to unsaturated hydrocarbons is of great practical interest because it is believed to be an essential component in both soot and fullerene formation. In this study, the rate of phenyl radical addition to butadiene was measured, and the potential surface of the reaction C₆H₅ + C₄H₆ was explored using quantum chemistry with the B3LYP density functional. Vibrational analysis allowed the determination of thermodynamic data and deduction of high-pressure-limit rate constants via transition- state theory. The pressure and temperature dependences of this chemically activated reaction were computed using a weak collision master equation analysis. The comparison of the predictions for the C₆H₅ + C₄H₆ system with experimental data showed good agreement. The rate constant for disappearance of phenyl radical was found to be (3.16 ± 0.29)×10¹² cm³/mol-s exp[-(870 ± 30)/T] over the temperature range 298-450 K. The dominant product at low temperature is the initial adduct, 4-phenylbuten- 3-yl. Around 1000 K, the dominant product is phenyl butadiene formed from the chemically activated adduct, even at 10 atm. Above about 1400 K, bimolecular H-abstraction to form benzene is the most important process. Other products such as 1,4-dihydronaphthalene are much less important.