Kinetic Stability of Si₂C₅H₂ Isomer with a Planar Tetracoordinate Carbon Atom

• Main Authors: Krishnan Thirumoorthy, Vijayanand Chandrasekaran, Andrew L. Cooksy, Venkatesan S. Thimmakondu
• Format: Article
• Language: English
• Published: MDPI AG 2021-12-01
• Series: Chemistry
• Subjects: Si₂C₅H₂; planar tetracoordinate carbon; kinetic stability; dissociation pathways; ab initio calculations
• Online Access: https://www.mdpi.com/2624-8549/3/1/2

Dissociation pathways of the global minimum geometry of Si<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula>C<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>5</mn></msub></semantics></math></inline-formula>H<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> with a planar tetracoordinate carbon (ptC) atom, 2,7-disilatricyclo[4.1.0.0<inline-formula><math display="inline"><semantics><msup><mrow></mrow><mrow><mn>1</mn><mo>,</mo><mn>3</mn></mrow></msup></semantics></math></inline-formula>]hept-2,4,6-trien-2,7-diyl (<b>1</b>), have been theoretically investigated using density functional theory and coupled-cluster (CC) methods. Dissociation of Si-C bond connected to the ptC atom leads to the formation of 4,7-disilabicyclo[4.1.0]hept-1(6),4(5)-dien-2-yn-7-ylidene (<b>4</b>) through a single transition state. Dissociation of C-C bond connected to the ptC atom leads to an intermediate with two identical transition states and leads back to <b>1</b> itself. Simultaneous breaking of both Si-C and C-C bonds leads to an acyclic transition state, which forms an acyclic product, cis-1,7-disilahept-1,2,3,5,6-pentaen-1,7-diylidene (<b>19</b>). Overall, two different products, four transition states, and an intermediate have been identified at the B3LYP/6-311++G(2d,2p) level of theory. Intrinsic reaction coordinate calculations have also been done at the latter level to confirm the isomerization pathways. CC calculations have been done at the CCSD(T)/cc-pVTZ level of theory for all minima. Importantly, all reaction profiles for <b>1</b> are found be endothermic in Si<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula>C<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>5</mn></msub></semantics></math></inline-formula>H<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula>. These results are in stark contrast compared to the structurally similar and isovalent lowest-energy isomer of C<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>7</mn></msub></semantics></math></inline-formula>H<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> with a ptC atom as the overall reaction profiles there have been found to be exothermic. The activation energies for Si-C, C-C, and Si-C/C-C breaking are found to be 30.51, 64.05, and 61.85 kcal mol<inline-formula><math display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></semantics></math></inline-formula>, respectively. Thus, it is emphasized here that <b>1</b> is a kinetically stable molecule. However, it remains elusive in the laboratory to date. Therefore, energetic and spectroscopic parameters have been documented here, which may be of relevance to molecular spectroscopists in identifying this key <i>anti-van’t-Hoff-Le Bel</i> molecule.