Eleven isomers of SiC4H2 lying within 50 kcal mol-1 have been theoretically investigated using density functional theory and high-level coupled-cluster methods. Among them, four isomers, 1-ethynyl-3-silacycloprop-1(2)-en-3-ylidene (1), diethynylsilylidene (2), 1-sila-1,2,3,4-pentatetraenylidene (4), and 1,3-butadiynylsilylidene (5), have already been identified in the laboratory. The current investigation reports three low-lying (<1 eV) silylidenes [2-methylenesilabicyclo[1.1.0]but-1(3)-en-4-ylidene (3), 4-sila-2-methylenebicyclo[1.1.0]but-1(3)-en-4-ylidene (6), and 3-ethynyl-1-silapropadienylidene (7)] and three high-lying (>1 eV) silylidenes [2-sila-(didehydrovinylidene)cyclopropene (8), an isomer with a planar tetracoordinate carbon (ptC) atom (10), and 1-ethynyl-1-silapropadienylidene (11)], which remain elusive in the laboratory to date. Isomer 9 also contains a ptC atom, which turned out to be a transition state at all levels. Though all isomers are polar (μ ≠ 0), rotational spectrum is available only for 4. Using matrix isolation, three isomers (1, 2, and 5) have been trapped in the laboratory at 10 K. Considering the astrochemical relevance of silicon-carbide clusters in the interstellar medium, the current theoretical data demand new molecular spectroscopic studies on SiC4H2. Surprisingly, unlike the isovalent C5H2 isomers, where the bent carbenes are yet to be identified in the laboratory, the bent silylidenes (2 and 5) have been trapped in the case of SiC4H2. In both the cases, molecules with transannular C-C and/or Si-C bonds remain elusive, though they lie in the low-lying region. Using suitable precursors, whether these peculiar geometries (especially 3 and 6) would be identified or not in the laboratory needs to be addressed by molecular spectroscopists. The present investigation documents structural and spectroscopic information of SiC4H2 isomers, which may compliment future molecular spectroscopic observations including radioastronomical searches.

中文翻译：

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SiC4H2异构体的理论研究描述了实验室中缺少的三种低粘硅烷基。

使用密度泛函理论和高级耦合簇方法从理论上研究了50 kcal mol-1内的SiC4H2的11种异构体。其中有四个异构体：1-乙炔基-3-silacycloprop-1（2）-en-3-亚基（1），二乙炔基亚硅烷基（2），1-sila-1,2,3,4-戊二烯基（4），和1,3-丁二炔基亚甲叉基（5）已在实验室中鉴定出来。当前调查报告了三个低洼（<1 eV）的亚甲硅烷基[2-亚甲基硅双环[1.1.0] but-1（3）-en-4-亚基（3），4-sila-2-亚甲基双环[1.1.0] ] but-1（3）-en-4-亚烷基（6）和3-乙炔基-1-硅丙二烯叉二烯（7）]和三个高位（> 1 eV）的亚甲硅烷基[2-sila-（didehydrovinylidene）cyclopropene（ 8），具有平面四配位碳（ptC）原子（10）和1-乙炔基-1-硅丙二烯叉二烯（11）的异构体]至今在实验室中仍难以捉摸。异构体9还包含一个ptC原子，该原子在所有水平上都为过渡态。尽管所有异构体都是极性的（μ≠0），但旋转光谱仅适用于4种。使用基质隔离技术，在实验室中10 K下捕获了三种异构体（1、2和5）。考虑到硅的化学性质，星际介质中的碳化物簇，目前的理论数据需要对SiC4H2进行新的分子光谱研究。出人意料的是，与等价的C5H2异构体不同，在实验室中尚未确定弯曲的碳烯，而在SiC4H2的情况下，弯曲的亚甲硅烷基（2和5）被捕获。在这两种情况下，具有跨环CC和/或Si-C键的分子虽然位于低洼区域，但仍然难以捉摸。使用合适的前体 这些特殊的几何形状（尤其是3和6）是否会在实验室中被识别出，需要由分子光谱学家解决。本研究记录了SiC4H2异构体的结构和光谱信息，这可能会补充包括放射天文搜索在内的未来分子光谱观察。