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

Abstract

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-[2.1.0.01,3.02,5]pentane; py-1 and tricyclo-[2.1.0.02,5]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 (μ = 4.64 D). The stability of the ptC structures with their higher spin states has been discussed.

Energetic and Spectroscopic Properties of Astrophysically Relevant MgC4H Radicals Using High-Level Ab Initio Calculations

“…However, the synthesis, characterization, and eventual identification of such transient species in the laboratory are not a very easy task. Therefore, computational studies on the structure and electronic properties of these astrophysically important molecules may aid the radioastronomers, molecular spectroscopists, and synthetic organic chemists to identify them both in the laboratory as well as in the ISM. ,− Moreover, the spectroscopic detection and identification of these molecules can be challenging due to the presence of the metal atom having many closely spaced electronic states with different spin multiplicities.…”

CCSD(T) Rotational Constants for Highly Challenging C5H2 Isomers—A Comparison between Theory and Experiment

“…It should be noted that the electronic structure of the C H isomers in Figure 1 renders these isomers challenging from the electronic structure point of view. As illustrated in Figure 1 , these isomers are characterized by carbenes (mono, bi, or tri), biradicals, polyynics, molecules with a planar tetracoordinate carbon (ptC) atom [ 15 , 43 , 45 , 46 , 47 , 48 , 49 ] or cumulenic electronic structures [ 41 , 42 , 43 ]. In addition, several isomers are associated with high strain energies.…”