Gas-phase reactions of aryl radicals with 2-butyne: an experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation
Gas-phase reactions of aryl radicals with 2-butyne: an experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation
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DOI：
10.1039/c2cp22970f
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发表时间：
2012-01-01
影响因子：
3.3
通讯作者：
da Silva, Gabriel
中科院分区：
文献类型：
--
作者：
Lam, Adrian K. Y.;Li, Cong;da Silva, Gabriel
芳香族自由基在各种反应气相体系中形成，它们与不饱和烃的分子量增长反应是相当重要的。采用离子阱质谱法研究了芳香双离子n-甲基吡啶-4-基（NMP）自由基阳离子与2-丁炔（CH3C CCH3）的离子-分子反应。对NMP反应和中性苯基自由基反应的高能级能面进行了从头算比较。NMP自由基阳离子在室温下与2-丁炔反应迅速，因为明显没有任何屏障。活化的乙烯基自由基加合物主要通过H原子的损失离解，CH3的损失较少。高分辨率傅里叶变换离子回旋共振（FT-ICR）质谱法使我们能够识别少量的碰撞失活反应加合物。NMP + 2-丁炔体系的统计反应速率理论计算（主方程/RRKM理论）支持了我们的实验结果，并指出其机制主要是通过H原子在芳香环和C-4侧链之间穿梭形成烯丙基共振稳定自由基，然后进行环化和/或低能H原子的β -裂解反应。中性苯基自由基（Ph中心点）+2-丁炔反应也有类似的反应机理，生成的产物包括3-亚甲基乙烯。预测碰撞失活反应加合物将以共振稳定的甲基苯丙基自由基的形式被淬灭。使用2,5-二氯取代甲基吡啶基自由基阳离子进行的实验表明，在这种情况下，2-丁炔加合物的CH3损失比H原子损失更大，验证了邻位H原子在芳香自由基与炔烃反应中的关键作用及其穿梭机制。作为有用的苯基自由基类似物，吡啶基自由基阳离子可能在土卫六的电离层中形成，在那里它们可以进行快速的分子量增长反应，生成多环芳烃（PANHs）。
Aromatic radicals form in a variety of reacting gas-phase systems, where their molecular weight growth reactions with unsaturated hydrocarbons are of considerable importance. We have investigated the ion-molecule reaction of the aromatic distonic N-methyl-pyridinium-4-yl (NMP) radical cation with 2-butyne (CH3C CCH3) using ion trap mass spectrometry. Comparison is made to high-level ab initio energy surfaces for the reaction of NMP and for the neutral phenyl radical system. The NMP radical cation reacts rapidly with 2-butyne at ambient temperature, due to the apparent absence of any barrier. The activated vinyl radical adduct predominantly dissociates via loss of a H atom, with lesser amounts of CH3 loss. High-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry allows us to identify small quantities of the collisionally deactivated reaction adduct. Statistical reaction rate theory calculations (master equation/RRKM theory) on the NMP + 2-butyne system support our experimental findings, and indicate a mechanism that predominantly involves an allylic resonance-stabilized radical formed via H atom shuttling between the aromatic ring and the C-4 side-chain, followed by cyclization and/or low-energy H atom beta-scission reactions. A similar mechanism is demonstrated for the neutral phenyl radical (Ph center dot)+2-butyne reaction, forming products that include 3-methylindene. The collisionally deactivated reaction adduct is predicted to be quenched in the form of a resonance-stabilized methylphenylallyl radical. Experiments using a 2,5-dichloro substituted methyl-pyridiniumyl radical cation revealed that in this case CH3 loss from the 2-butyne adduct is favoured over H atom loss, verifying the key role of ortho H atoms, and the shuttling mechanism, in the reactions of aromatic radicals with alkynes. As well as being useful phenyl radical analogues, pyridiniumyl radical cations may form in the ionosphere of Titan, where they could undergo rapid molecular weight growth reactions to yield polycyclic aromatic nitrogen hydrocarbons (PANHs).