Molecules with a planar tetracoordinate carbon atom serve as reactive intermediates for carbenes. While isomer 2 has been identified in the laboratory, isomer 5 remains elusive to date.

Here, four stationary points with a planar tetracoordinate carbon (ptC) atom that are considered as anti-van't Hoff Le Bel molecules have been computationally identified for the first time within C₆NH elemental composition. In total, 99 stationary points of C₆NH ranging from 0.00 to 186.95 kcal mol⁻¹ are theoretically identified using density functional theory at the B3LYP-D3BJ/cc-pVTZ level of theory. Low-lying isomers and ptC isomers that are local minima are further investigated with coupled-cluster calculations at the CCSD(T)/cc-pVTZ level of theory. These include the linear triplet isomer, HC₆N, 1-cyanopenta-1,2-dien-4-yn-1,3-diyl (1) and the ring-chain carbene, 2-(2-cyanoeth-1-ynyl)cycloprop-2-en-1-ylidene (2). Both 1 and 2 are polar with dipole moment values of 4.86 and 3.56 Debye, respectively, and are experimentally known molecules through Fourier transform microwave spectroscopy. The current theoretical study reveals that two of these ptC isomers (ptC25 and ptC29) upon ring-opening pathways of CC single bonds connected to the ptC atom lead to the formation of experimentally known cyano ring-chain carbene, 2 and another polar (μ = 4.18 Debye) low-lying ring-chain carbene, 2-cyano-3-ethynylcycloprop-2-en-1-ylidene (5), which is yet to be identified in the laboratory. Formation of both 2 and 5 from the ptC isomers are found to be highly exothermic in nature.