Theoretical Studies of SiC4H2 Isomers Delineate Three Low-Lying Silylidenes Are Missing in the Laboratory

<p>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 (<b>1</b>), diethynylsilylidene (<b>2</b>), 1-sila-1,2,3,4-pentatetraenylidene (<b>4</b>), and 1,3-butadiynylsilylidene (<b>5</b>), 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 (<b>3</b>), 4-sila-2-methylenebicyclo[1.1.0]but-1(3)-en-4-ylidene (<b>6</b>), and 3-ethynyl-1-silapropadienylidene (<b>7</b>)] and three high-lying (>1 eV) silylidenes [2-sila-(didehydrovinylidene)cyclopropene (<b>8</b>), an isomer with a planar tetracoordinate carbon (ptC) atom (<b>10</b>), and 1-ethynyl-1-silapropadienylidene (<b>11</b>)], which remain elusive in the laboratory to date. Isomer <b>9</b> 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 <b>4</b>. Using matrix isolation, three isomers (<b>1</b>, <b>2</b>, and <b>5</b>) 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 (<b>2</b> and <b>5</b>) 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 <b>3</b> and <b>6</b>) 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.</p>