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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices
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Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices

机译：γ辐照的低温基体中甲基取代苯的三聚和二聚自由基阳离子的形成

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摘要

Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O-2) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M-3(+)) produced via the corresponding dimer radical cation (M-2(+)) is clearly seen in the solution containing O-2 as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M-3(+) CR absorption is observed; this is due to the large M-3(+)/M-2(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at similar to 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M-3(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M-3(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.
机译：苯，甲苯和二甲苯的二聚体和三聚体阳离子在具有电子清除剂：氧气（O-2）和仲丁基氯化物（sec-BuCl）的低温2-甲基戊烷基质中进行γ射线辐照后有选择地产生。随着温度的升高，在含O-2的溶液中可以清楚地看到通过相应的二聚自由基阳离子（M-2（+））产生的三聚自由基阳离子（M-3（+））的电荷共振（CR）带。在邻二甲苯溶液中，观察到相当强和独特的M-3（+）CR吸收;范围从80到90K。这是由于M-3（+）/ M-2（+）的相对消光系数大。所有的苯衍生物在二聚体和三聚体阳离子之间显示出接近90 K的平衡。然而，甲苯和二甲苯的平衡常数明显低于苯。在通常用作生产阳离子物质的低温光学基质的sec-BuCl中，三聚自由基阳离子的形成受到抑制。从头开始使用DFT方法预测M-3（+）的几何形状，给出“滑动三明治”（对于苯，间二甲苯和对二甲苯）和“滑动扇形”（甲苯和邻二甲苯） ）结构作为最合理的几何形状。实验观察到的光谱参数很好地反映了基于几何的TD-DFT计算所预测的光谱参数，表明M-3（+）的几何形状对取代模式的强烈依赖性。这是第一份报告，不仅直接在缩合相中对芳香族三聚自由基阳离子进行了光谱观察，而且还对它们的平衡进行了定量分析。

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