Vibrationally resolved dynamics of the reaction of Cl atoms with 2,3-dimethylbut-2-ene in chlorinated solvents
Vibrationally resolved dynamics of the reaction of Cl atoms with 2,3-dimethylbut-2-ene in chlorinated solvents
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氯化溶剂中 Cl 原子与 2,3-二甲基丁-2-烯反应的振动解析动力学
DOI：
10.1039/c2sc21267f
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
Abou-Chahine F
中科院分区：
文献类型：
--
作者：
Abou-Chahine F
据报道，宽带瞬态红外吸收测量对比了氯化溶剂中 Cl 原子与 2,3-二甲基丁-2-烯和正戊烷的反应动力学。在这两种情况下，氢原子转移都会产生 HCl 和烃自由基，但前一种反应中释放的能量更大，因为形成了共振稳定的烯丙基自由基。 Cl 原子与 CH2Cl2 溶液中的正戊烷反应仅在其最低振动能级 (v = 0) 下形成 HCl，我们测得的速率系数与 Sheps 等人之前的报告一致，J. Phys。化学。 A, 2006, 110, 3087。HCl 基本吸收带强度增长的时间依赖性显示了两个反应域：迅速上升与光解生成的 Cl 原子与位于第一个溶剂壳内的正戊烷分子的反应相关，而较慢的进一步上升归因于扩散通过溶液后的反应。对于 Cl 原子与 2,3-二甲基丁-2-烯的反应，也观察到这两个反应域，并且拟合包含这些组分的动力学模型给出了在 CDCl3 中形成 HCl 的双分子速率系数为 (1.7 ± 1.4) × 1010 M−1 s−1 ，在 CCl4 中形成 HCl 的双分子速率系数为 (3.4 ± 1.2) × 1010 M−1 s−1 。然而，观察到额外的瞬态吸收，其波数比基本 HCl 吸收带低约 115 cm−1，并被分配给 HCl 的 v = 2 ← v = 1 热带。新生 HCl(v = 1) 的吸收在 ∼20 ps 后达到峰值，随后由于振动弛豫而衰减至基线水平，这表明能量转移到 2,3-二甲基丁-2-烯会增强振动弛豫。在 CDCl3 溶液中，v = 1 时最初形成的 HCl 分数确定为 0.24 ± 0.04，在 CCl4 中则为 0.15 ± 0.02。溶液中这些反应的 HCl(v = 1) 分支明显低于气相中 Cl 原子与丙烯反应报道的 0.48 ± 0.06 分数 [Pilgrim 和 Taatjes, J. Phys.化学。 A, 1997, 101, 5776]。因此，溶液中和气相中类似双分子反应之间的比较可以识别由与溶剂相互作用引起的动力学变化。与过渡态附近势能表面的拓扑结构的溶剂修饰相比，反应分离产物所经历的溶剂摩擦被认为更有可能导致新生HCl的较低振动激发。还讨论了将 Cl 添加到 2,3-二甲基丁-2-烯中的不饱和键上可能产生的后果。
Broadband transient infra-red absorption measurements are reported that contrast the dynamics of reaction of Cl atoms with 2,3-dimethylbut-2-ene and n-pentane in chlorinated solvents. In both cases, H-atom transfer produces HCl and a hydrocarbon radical, but the energy release in the former reaction is greater because of the formation of a resonance-stabilized allylic radical. The reaction of Cl atoms with n-pentane in solution in CH2Cl2 forms HCl exclusively in its lowest vibrational level (v = 0), and our measured rate coefficient agrees with a prior report by Sheps et al., J. Phys. Chem. A, 2006, 110, 3087. The time-dependence of the growth of intensity in the HCl fundamental absorption band shows two domains of reaction: a prompt rise is associated with reaction of the photolytically generated Cl atoms with n-pentane molecules lying within the first solvent shell, whereas a slower further rise is attributed to reaction following diffusion through the solution. For the reaction of Cl atoms with 2,3-dimethylbut-2-ene, these two domains of reaction are also observed, and fitting to a kinetic model incorporating these components gives bimolecular rate coefficients for formation of HCl of (1.7 ± 1.4) × 1010 M−1 s−1 in CDCl3 and (3.4 ± 1.2) × 1010 M−1 s−1 in CCl4. However, an additional transient absorption is observed ∼115 cm−1 lower in wavenumber than the fundamental HCl absorption band and is assigned to the v = 2 ← v = 1 hot band of HCl. The absorption by the nascent HCl(v = 1) peaks after ∼20 ps and subsequently decays to baseline levels because of vibrational relaxation, which is shown to be enhanced by energy transfer to 2,3-dimethylbut-2-ene. In solution in CDCl3, the fraction of HCl formed initially in v = 1 is determined to be 0.24 ± 0.04 and in CCl4 it is 0.15 ± 0.02. The branching to HCl(v = 1) for these reactions in solution is significantly lower than the 0.48 ± 0.06 fraction reported for reaction of Cl atoms with propene in the gas phase [Pilgrim and Taatjes, J. Phys. Chem. A, 1997, 101, 5776]. This comparison between similar bimolecular reactions in solution and in the gas phase therefore identifies changes to the dynamics caused by interaction with the solvent. Solvent friction experienced by the separating products of the reaction is considered more likely to be responsible for the lower vibrational excitation of the nascent HCl than is solvent modification of the topology of the potential energy surface in the vicinity of the transition state. Possible consequences of addition of Cl to the unsaturated bond in 2,3-dimethylbut-2-ene are also discussed.
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