Si3C2H2 isomers with a planar tetracoordinate carbon or silicon atom(s)

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We report molecules with a planar tetracoordinate carbon (ptC) or silicon (ptSi) atom(s) for the first time within Si3C2H2 elemental composition using density functional theory and coupled-cluster (CC) quantum chemical calculations. In total, nine isomers of Si3C2H2 (1-9) have been theoretically investigated. These include the lowest-energy trigonal bipyramidal geometry (1) and the other eight isomers (2-9) with either a ptC or ptSi atom(s). Among the latter, four isomers are identified to be local minima (2, 5, 6, and 9), three are found to be transition states (3, 4, and 8), and one geometry is found to be a second-order saddle-point (7) on the singlet ground electronic state of Si3C2H2 potential energy surface at various levels of theory employed here. While the isomer with a ptC atom show aromatic characteristics (5), the isomers with ptSi atom(s) show either aromatic (2) or anti-aromatic characteristics (6 and 9). Apart from 1, the kinetic stabilities of the latter four isomers also remain as an open-ended question at the moment. However, unlike 1, the latter four isomers are associated with a non-zero dipole moment (μ≠0). Therefore, energetic, aromatic, and spectroscopic parameters have been documented here, which may trigger the laboratory search for these elusive molecules in the future.