Zeng Zhe, Altarawneh Mohammednoor, Oluwoye Ibukun, Glarborg Peter, Dlugogorski Bogdan Z
School of Engineering and Information Technology, Murdoch University , 90 South Street, Murdoch, WA 6150, Australia.
Department of Chemical Engineering, Technical University of Denmark , DK-2800 Kgs. Lyngby, Denmark.
J Phys Chem A. 2016 Nov 17;120(45):8941-8948. doi: 10.1021/acs.jpca.6b09357. Epub 2016 Nov 8.
This study resolves the interaction of sulfanyl radical (SH) with aliphatic (C-C) hydrocarbons, using CBS-QB3 based calculations. We obtained the C-H dissociation enthalpies and located the weakest link in each hydrocarbon. Subsequent computations revealed that, H abstraction by SH from the weakest C-H sites in alkenes and alkynes, except for ethylene, appears noticeably exothermic. Furthermore, abstraction of H from propene, 1-butene, and iso-butene displays pronounced spontaneity (i.e., ΔG° < -20 kJ mol between 300-1200 K) due to the relatively weak allylic hydrogen bond. However, an alkyl radical readily abstracts H atom from HS, with HS acting as a potent scavenger for alkyl radicals in combustion processes. That is, these reactions proceed in the opposite direction than those involving SH and alkene or alkyne species, exhibiting shallow barriers and strong spontaneity. Our findings demonstrate that the documented inhibition effect of hydrogen sulfide (HS) on pyrolysis of alkanes does not apply to alkenes and alkynes. During interaction with hydrocarbons, the inhibitive effect of HS and promoting interaction of SH radical depend on the reversibility of the H abstraction processes. For the three groups of hydrocarbon, Evans-Polanyi plots display linear correlations between the bond dissociation enthalpies of the abstracted hydrogens and the relevant activation energies. In the case of methane, we demonstrated that the reactivity of SH radicals toward abstracting H atoms exceeds that of HO but falls below those of OH and NH radicals.
本研究使用基于CBS-QB3的计算方法解析了硫氢基（SH）与脂肪族（C-C）碳氢化合物的相互作用。我们获得了C-H键离解焓，并确定了每种碳氢化合物中最薄弱的环节。随后的计算表明，除乙烯外，SH从烯烃和炔烃中最薄弱的C-H位点夺取氢的反应明显放热。此外，由于烯丙基氢键相对较弱，从丙烯、1-丁烯和异丁烯中夺取氢的反应表现出明显的自发性（即300-1200K之间ΔG° < -20 kJ/mol）。然而，烷基自由基很容易从HS中夺取氢原子，在燃烧过程中HS充当烷基自由基的有效清除剂。也就是说，这些反应的进行方向与涉及SH和烯烃或炔烃物种的反应相反，表现出较低的势垒和较强的自发性。我们的研究结果表明，文献中记载的硫化氢（HS）对烷烃热解的抑制作用不适用于烯烃和炔烃。在与碳氢化合物相互作用时，HS的抑制作用和SH自由基的促进相互作用取决于氢夺取过程的可逆性。对于这三类碳氢化合物，埃文斯-波拉尼图显示了夺取氢的键离解焓与相关活化能之间的线性相关性。对于甲烷，我们证明了SH自由基夺取氢原子的反应活性超过HO，但低于OH和NH自由基。
