In this work, we investigated the detailed reactivity of dehydro‐diazine radicals through unimolecular dissociation pathways. Aromatic six‐membered rings with two nitrogens as heteroatoms are classified as diazines, which include pyrimidine 1, pyridazine 2, and pyrazine 3. The C–H bond scission in pyrimidine, pyridazine, and pyrazine leads to three (1a–1c), two (2a and 2b), and one (3a) radical isomers, respectively. Through quantum chemical calculations, reactivity aspects of all the six radical isomers have been investigated by considering different modes of unimolecular reactions. On the basis of the computations at (U)B3LYP/cc‐pVTZ, (U)M06‐2X/cc‐pVTZ, and (U)CCSD(T)/cc‐pVTZ//(U)B3LYP/cc‐pVTZ levels of theory and analysis, we found out that ring‐opening pathways were kinetically more favorable compared with isomerization (1,2‐H shift) pathways or C–H bond dissociation leading to biradicals. In all the cases, ring‐opening via C–N bond cleavage has a lower energy barrier compared with C–C bond cleavage owing to the stabilizing interaction between radical electron and the nitrogen lone pair. Moreover, the unimolecular decomposition channels can lead to several fragmentation products, which are interesting in the astrochemical context. On the basis of these investigations and also the impact of three centered–five electrons (3c–5e) interactions on the reactivity aspects, we determined the kinetic stability order among the six isomeric dehydrodiazine radicals.

中文翻译：

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脱氢二嗪基中的单分子热反应路径

在这项工作中，我们研究了脱氢二嗪自由基通过单分子解离途径的详细反应性。具有两个氮原子作为杂原子的芳香族六元环被分类为二嗪类，其中包括嘧啶1，哒嗪2和吡嗪3。嘧啶，哒嗪和吡嗪中的C–H键断裂导致三个（1a – 1c），两个（2a和2b）和一个（3a）自由基异构体。通过量子化学计算，已经通过考虑不同的单分子反应模式研究了所有六个自由基异构体的反应性方面。基于（U）B3LYP / cc-pVTZ，（U）M06-2X / cc-pVTZ和（U）CCSD（T）/ cc-pVTZ //（U）B3LYP / cc-pVTZ级别的计算从理论和分析的角度来看，我们发现开环途径在动力学上比异构化（1,2-H移位）途径或C-H键解离导致双自由基更有利。在所有情况下，由于自由基电子和氮孤对之间的稳定相互作用，通过C–N键断裂的开环与C–C键断裂相比具有较低的能垒。此外，单分子分解通道会导致多种碎片产物，在星化学方面很有趣。在这些研究的基础上，以及三个居中的五个电子（3c-5e）相互作用对反应性的影响，我们确定了六个异构体脱氢二嗪基团之间的动力学稳定性顺序。