Ab initio molecular orbital/Rice-Ramsperger-Kassel-Marcus theory study of multichannel rate constants for the unimolecular decomposition of benzene and the H + C_6H_5 reaction over the ground electronic state

摘要

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 sequentiall,2-hydrogen shifts to 0-, 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 9f 121.4 kcaVmol relative to benzene. 0- and m-C6H6 can also isomerize to acyclic isomers, ac-C6~, 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 kca1/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 preserice 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 0-, 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 2.26XI014exp(-53300/T)s-I for T=1000-3000K and atmospheric pressure. This ~nding ~s signific~tl~ differe?t from th~ recommended rate constant, 19.0X 1015 exp( -54 060/T) s I, obtaIned by kinetic modelIng assulDlng only the H +C6H5 product channel. At T= 1000 K, the branching ratios for the formation of H +C6H5 and ac-C~6 are 29% i a~d 71 %, resp~ctively. H +C6.H5 b~comes ~he major channel at T~ 1200 K. -The total rate for ~e IbImolecular H+C6H5 reaction IS predIcted to be between 4.5X10 II and 2.9X10 10 cm3 molecule-1 S-I for the broad range of temperatures (300-3000 K) and pressures (100 Torr-10 jatm). The values in the T=1400-1700K interval, -8XI0-IIcm3molecule-Is-I, are -40% lower than the recommended value of 1.3X 10~IOcm3molecule-I S-I. The recombination reaction I 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 I 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.