From High-Energy C₇H₂ Isomers with A Planar Tetracoordinate Carbon Atom to An Experimentally Known Carbene

In this work, we use high-level ab initio procedures to show that the high-energy isomers of C₇H₂ with a planar tetracoordinate carbon (ptC) atom serve as reactive intermediate leading to the formation of an experimentally known ring-chain carbene, 1-(buta-1,3-diynyl)cyclopropenylidene (2). Among the experimentally known isomers of C₇H₂, the latter is the only low-lying ring-chain carbene identified by Fourier-transform microwave spectroscopy. Here we investigate the ring-opening pathways of C-C single bonds connected to the ptC atom in three different C₇H₂ isomers using coupled-cluster and density functional theory methods. These three isomers [ptC1 ( C_{2 v}; X̃¹A₁), ptC2 ( C _s; X̃¹A'), and ptC3 ( C _s; X̃¹A')] are found to be local minima on the C₇H₂ potential-energy surface at both CCSD(T)/cc-pVTZ and B3LYP/6-311+G(d,p) levels of theory. The transition states and minimum-energy pathways connecting the reactants (ptC isomers) and the products have been found via intrinsic reaction coordinate calculations at the B3LYP/6-311+G(d,p) level of theory. The high-energy ptC isomers (ptC2 and ptC3) lead to the formation of 2, while the low-energy ptC isomer, ptC1, rearranges to a bicyclic carbene, bicyclo[4.1.0]hepta-4,6-diene-2-yne-7-ylidene (6). In the latter, we note that both the reactant and the product are yet to be identified in the laboratory. Relative energies, activation energies, reaction energies, and nucleus independent chemical shift values have been calculated to access the thermodynamic and kinetic stabilities and the aromatic nature of these peculiar molecules. Rotational and centrifugal distortion constants have also been estimated for all ptC isomers, which may assist the efforts of microwave spectroscopists.