Ab initio/RRKM-ME study on the mechanism and kinetics of the reaction of phenyl radical with 1,2-butadiene.

Ab initio G3(MP2,CC)//B3LYP/6-311G** calculations have been performed to investigate the potential energy surface and mechanism of the reaction of phenyl radical with 1,2-butadiene followed by kinetic RRKM-ME calculations of the reaction rate constants and product branching ratios at various temperatures and pressures. The results show that the reaction can proceed by direct hydrogen abstraction to produce benzene and C(4)H(5) radicals or by addition of phenyl to different carbon atoms in CH(2)CCHCH(3) followed by isomerizations of C(10)H(11) adducts and their dissociation by H or CH(3) losses. The H abstraction channels are found to be kinetically preferable and to contribute 70-90% to the total product yield in the 300-2500 K temperature range, with the products including C(6)H(6) + CH(2)CHCCH(2) (approximately 40%), C(6)H(6) + CH(3)CHCCH (5-31%), and C(6)H(6) + CH(2)CCCH(3) (24-20%). The phenyl addition channels are calculated to be responsible for 10-30% of the total product yield, with their contribution decreasing as the temperature increases. The products of the addition channels include collisionally stabilized C(10)H(11) adducts, 1-phenyl-2-buten-2-yl, 3-phenyl-2-buten-2-yl, and 2-phenyl-2-buten-1-yl/2-phenyl-1-buten-3-yl, which are favored under low temperatures, as well as their dissociation products, 1-phenyl-propyne + CH(3), phenylallene + CH(3), and 2-phenyl-1,3-butadiene + H, preferred at higher temperatures. Indene is predicted to be a very minor reaction product at the temperatures relevant to combustion, with the maximal calculated yield of only 2% at 700 K and 7.6 Torr. Our calculations showed that at typical combustion temperatures product branching ratios are practically independent of pressure, and collisional stabilization of reaction intermediates does not play a significant role. Three-parameter modified Arrhenius expressions have been generated for the total reaction rate constants and rate constants for the most important product channels, which can be utilized in future kinetic modeling of reaction networks related to the growth of hydrocarbons in combustion processes.