Ab initio molecular orbital/Rice–Ramsperger–Kassel–Marcus theory study of multichannel rate constants for the unimolecular decomposition of benzene and the H+C6H5 reaction over the ground electronic state

Abstract

The potential energy surface for the unimolecular decomposition of benzene and H+C6H5 recombination has been studied by the ab initio G2M(cc, MP2) method. The results show that besides direct emission of a hydrogen atom occurring without an exit channel barrier, the benzene molecule can undergo sequential 1,2-hydrogen shifts to o-, m-, and p-C6H6 and then lose a H atom with exit barriers of about 6 kcal/mol. o-C6H6 can eliminate a hydrogen molecule with a barrier of 121.4 kcal/mol relative to benzene. o- and m-C6H6 can also isomerize to acyclic isomers, ac-C6H6, with barriers of 110.7 and 100.6 kcal/mol, respectively, but in order to form m-C6H6 from benzene the system has to overcome a barrier of 108.6 kcal/mol for the 1,2-H migration from o-C6H6 to m-C6H6. The bimolecular H+C6H5 reaction is shown to be more complicated than the unimolecular fragmentation reaction due to the presence of various metathetical processes, such as H-atom disproportionation or addition to different sites of the ring. The addition to the radical site is barrierless, the additions to the o-, m-, and p-positions have entrance barriers of about 6 kcal/mol and the disproportionation channel leading to o-benzyne+H2 has a barrier of 7.6 kcal/mol. The Rice-Ramsperger-Kassel-Marcus and transition-state theory methods were used to compute the total and individual rate constants for various channels of the two title reactions under different temperature/pressure conditions. A fit of the calculated total rates for unimolecular benzene decomposition gives the expression 2.26×1014 exp(−53 300/T)s−1 for T=1000–3000 K and atmospheric pressure. This finding is significantly different from the recommended rate constant, 9.0×1015 exp(−54 060/T) s−1, obtained by kinetic modeling assuming only the H+C6H5 product channel. At T=1000 K, the branching ratios for the formation of H+C6H5 and ac-C6H6 are 29% and 71%, respectively. H+C6H5 becomes the major channel at T⩾1200 K. The total rate for the bimolecular H+C6H5 reaction is predicted to be between 4.5×10−11 and 2.9×10−10cm3 molecule−1 s−1 for the broad range of temperatures (300–3000 K) and pressures (100 Torr–10 atm). The values in the T=1400–1700 K interval, ∼8×10−11 cm3 molecule−1 s−1, are ∼40% lower than the recommended value of 1.3×10−10 cm3 molecule−1 s−1. The recombination reaction leading to direct formation of benzene through H addition to the radical site is more important than H disproportionations at T<2000 K. At higher temperatures the recombination channel leading to o-C6H4+H2 and the hydrogen disproportionation channel become more significant, so o-benzyne+H2 should be the major reaction channel at T>2500 K.