邻苯炔的单分子热裂解

Zhang Xu, Maccarone Alan T, Nimlos Mark R, Kato Shuji, Bierbaum Veronica M, Ellison G Barney, Ruscic Branko, Simmonett Andrew C, Allen Wesley D, Schaefer Henry F

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA.

J Chem Phys. 2007 Jan 28;126(4):044312. doi: 10.1063/1.2409927.

The ortho-benzyne diradical, o-C(6)H(4) has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C(6)H(4)+Delta--> products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C(6)H(4)+Delta-->HC triple bond CH+HC triple bond C-C triple bond CH. The experimental Delta(rxn)H(298)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH) is found to be 57+/-3 kcal mol(-1). Further experiments with the substituted benzyne, 3,6-(CH(3))(2)-o-C(6)H(2), are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o-benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C(7s6p5d4f3g2h1i)H(6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C(2v)-symmetric decomposition of o-benzyne, E(b)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH)=88.0+/-0.5 kcal mol(-1). A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C(6)H(6)-->H+[C(6)H(5)]-->H+[o-C(6)H(4)]-->HC triple bond CH+HC triple bond C-C triple bond CH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500 K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene.

邻苯炔双自由基（o-C₆H₄）已通过超音速喷嘴产生，并对其随后的热分解进行了研究。随着喷嘴温度升高，苯炔分子发生裂解：o-C₆H₄ + Δ → 产物。热解离产物通过三种实验方法进行鉴定：（i）飞行时间光电离质谱法，（ii）基质隔离傅里叶变换红外吸收光谱法，以及（iii）化学电离质谱法。在阈值解离温度下，邻苯炔通过明显的逆狄尔斯-阿尔德反应干净利落地分解为乙炔和丁二炔：o-C₆H₄ + Δ → HC≡CH + HC≡C-C≡CH。实验测得的Δ(rxn)H(298)(o-C₆H₄ → HC≡CH + HC≡C-C≡CH)为57±3 kcal mol⁻¹。对取代苯炔3,6-(CH₃)₂-o-C₆H₂的进一步实验与逆狄尔斯-阿尔德裂解相符。但在较高的喷嘴温度下，裂解模式变得更加复杂。为了解释这些实验，通过严格的从头算电子结构计算研究了邻苯炔的逆狄尔斯-阿尔德裂解。这些计算使用了高达[C(7s6p5d4f3g2h1i)H(6s5p4d3f2g1h)] (cc-pV6Z)的基组，并采用了尽可能广泛的电子相关处理，如通过三重激发的完全耦合簇方法（CCSDT），在某些情况下还包含连接四重激发的微扰项[CCSDT(Q)]。计算数据的焦点外推得出邻苯炔协同的、具有C₂v对称性分解的0 K势垒，E(b)(o-C₆H₄ → HC≡CH + HC≡C-C≡CH)=88.0±0.5 kcal mol⁻¹。如此大小的势垒与实验结果一致。对苯高温裂解的热化学进行了仔细评估：C₆H₆ → H + [C₆H₅] → H + [o-C₆H₄] → HC≡CH + HC≡C-C≡CH。苯炔可能是许多烷基苯（芳烃）热分解过程中的重要中间体。高于1500 K的高发动机温度可能会使这些烷基苯裂解为烷基自由基和苯基自由基的混合物。然后苯基自由基首先会解离为苯炔，接着再分解为乙炔和丁二炔。