From Molecules with a Planar Tetracoordinate Carbon to an Astronomically Known C₅H₂ Carbene

Abstract

Ethynylcyclopropenylidene ( 2 ), an isomer of C 5 H 2 , is a known molecule in the laboratory and has recently been identified in Taurus Molecular Cloud-1 (TMC-1). Using high-level coupled-cluster methods up to the CCSDT(Q)/CBS level of theory, it is shown that two isomers of C 5 H 2 with a planar tetracoordinate carbon (ptC) atom, (SP-4)-spiro[2.2]pent-1,4-dien-1,4-diyl ( 11 ) and (SP-4)-spiro[2.2]pent-1,4-dien-1,5-diyl ( 13 ), serve as the reactive intermediates for the formation of 2 . Here, a theoretical connection has been established between molecules containing ptC atoms ( 11 and 13 ) and a molecule ( 2 ) that is present nearly 430 light years away, thus providing evidence for the existence of ptC species in the interstellar medium. The reaction pathways connecting the transition states and the reactants and products have been confirmed by intrinsic reaction coordinate calculations at the CCSDT(Q)/CBS//B3LYP-D3BJ/cc-pVTZ level. While isomer 11 is non-polar (μ = 0), isomers 2 and 13 are polar, with dipole moment values of 3.52 and 5.17 Debye at the CCSD(T)/cc-pVTZ level. Therefore, 13 is also a suitable candidate for both laboratory and radioastronomical studies.

Theoretical spectra and energetics for c-C3HC2H, l-C5H2, and bipyramidal D3h C5H2

“…However, some small PAH-like molecules, like isomers of C 5 H 2 , have a lower symmetry which can allow for a nonzero dipole moment and possible rotational activity. Recently, two isomers of C 5 H 2 , l-C 5 H 2 and ethynyl cyclopropenylidene (c-C 3 HC 2 H), have been independently detected in Taurus Molecular Cloud-1 and pathways of formation have been determined experimentally and supported theoretically (He et al, 2020;Cabezas et al, 2021;Cernicharo et al, 2021;Karton and Thimmakondu, 2022). The detection of C 5 H 2 isomers could allude to other rotationally dark PAHs lurking in the interstellar medium.…”

The Formation of Monosubstituted Cyclopropenylidene Derivatives in the Interstellar Medium via Neutral–Neutral Reaction Pathways

“…The plausibility of an ethynyl-functionalized c-C 3 H 2 derivative was first hypothesized by Vrtilek et al (1987) and computationally validated by Cooper & Murphy (1988) before being detected in the laboratory by Travers et al (1997). The structural isomers of c-C 3 HC 2 H have also been studied in detail, showing that this form is the lowest in energy for the singlet spin multiplicity (Seburg et al 1997;Karton & Thimmakondu 2022). Following the astrophysical detection of c-C 3 HC 2 H (Cernicharo et al 2021), a mechanistic study of its formation found that c-C 3 HC 2 H may form through a gas-phase reaction between c-C 3 H 2 and the ethynyl radical (Fortenberry 2021).…”

The structure, stability, thermochemistry, and bonding in SO3-(H2O)n (n = 1–7) clusters: a computational analysis

“…The obtained new structure is optimized at the DFT level without any geometrical constraints. The above intuitive generation is also essential to arrive at the correct PGM geometries [37,38].…”