Structure and Bonding Patterns in C₅H₄ Isomers: Pyramidane, Planar Tetracoordinate Carbon, and Spiro Molecules

Simple Summary Tetra-coordinated carbon-containing molecules generally form tetrahedral carbon geometries. However, there are continuous searches for other possible geometries of tetra-coordinated carbon which are non-classical in nature. However, planar tetra-coordinated carbon (ptC), where all four bonds of the carbon atom are in the same plane or the pyramidane structure in which tetra-coordinated carbon with all four bonds are on the same side of a plane, are good examples of non-classical structures. However, synthesis and experimental realization of these molecules are very challenging in the laboratory. In this paper, we have theoretically investigated nine unusual isomers of the molecular formula C5H4 and studied their stability and novel bonding patterns. One of the ptC compounds has been found to be thermodynamically more stable compared to its tetrahedral counterpart with a high value of dipole moment (& mu; = 4.64 D), which may be helpful for experimental detection in the laboratory or in the low-temperature regions of the interstellar medium. We have theoretically investigated nine unusual isomers of the molecular formula C5H4 using coupled cluster (CC) and density functional theory (DFT) methods. These molecules possess non-classical structures consisting of two pyramidanes, three planar tetracoordinate carbon (ptC), and four spiro types of isomers. Both the pyramidanes (tetracyclo-[2.1.0.0(1,3).0(2,5)]pentane; py-1 and tricyclo-[2.1.0.0(2,5)]pentan-3-ylidene; py-2) are minima on the potential energy surface (PES) of C5H4. Among the three isomers containing ptC, (SP4)-spiro [2.2]pent-1-yne (ptC-2) is a minimum, whereas isomer, (SP4)-spiro [2.2]pent-1,4-diene (ptC-1) is a fourth-order saddle point, and (SP4)-sprio[2.2]pent-1,4-diylidene (ptC-3) is a transition state. The corresponding spiro isomers spiro[2.2]pent-1,4-diene (spiro-1), sprio[2.2]pent-1,4-diylidene (spiro-3) and spiro[2.2]pent-4-en-1-ylidene (spiro-4) are local minima, except spiro[2.2]pent-1-yne (spiro-2), which is a second-order saddle point. All relative energies are calculated with respect to the global minimum (pent-1,3-diyne; 1) at the CCSD(T)/cc-pVTZ level of theory. Quantum chemical calculations have been performed to analyze the bonding and topological configurations for all these nine isomers at the B3LYP/6-311+G(d,p) level of theory for a better understanding of their corresponding electronic structures. ptC-2 was found to be thermodynamically more stable than its corresponding spiro counterpart (spiro-2) and possesses a high dipole moment (& mu; = 4.64 D). The stability of the ptC structures with their higher spin states has been discussed.