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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation
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Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

机译：漫游是乙醛光解离中分子产物的主要机制

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摘要

Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed "roaming," has been described in the photodissociation of H_2CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH_3CHO to CH_4 + CO. The CH_4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at ≈95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH_4 + CO products is via a roaming pathway where a CH_3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH_4 + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH_3 roaming mechanism is attributed to the fact that the CH_3 + HCO radical asymptote and the TS saddle-point barrier to CH_4 + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H_2CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.
机译：绕过常规鞍点过渡态（TS）的反应途径引起了极大的兴趣和重要性。 H_2CO的光解离中已经描述了这种途径的一个例子，称为“漫游”。在实验和理论相结合的研究中，我们表明，漫游路径在CH_3CHO与CH_4 + CO的308 nm光解离中很重要。发现CH_4产物具有极强的振动激发，其振动分布达到了峰值的约95％。总可用能量。在全尺寸势能面上的准经典轨迹计算重现了这些结果，并用于推断通向CH_4 + CO产品的主要途径是通过漫游途径，其中CH_3片段从HCO提取H。 CH_4 + CO形成的常规鞍点TS途径仅扮演次要角色。还观察到H原子漫游，但这也是一个次要途径。 CH_3漫游机制的优势归因于以下事实：CH_3 + HCO自由基渐近线和对CH_4 + CO的TS鞍点势垒几乎是等能量的。因此，漫游动力学不限于诸如H_2CO的小分子，也不限于H原子为漫游片段。观察到的漫游机制相对于传统TS机制的主导地位对当前的反应速率理论提出了重大挑战。

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