Stability and IR Spectra of Functionalized, Cyclic Hexamers of Cyclopropenylidene, C18Xy (X = C, O, S; y = 0, ..., 6)

The astronomically abundant molecule cyclopropenylidene (c-C3H2) can form stable hexamer rings. Additionally, the related carbene cyclopropenylidene (c-C3H2C) can do the same, and this work explores the continuum of carbon allotropes constructed in this manner ranging from C18 to C24. The stability and IR features of each molecule are quantum chemically computed showcasing a surprising stability in C23 and C24. The latter is actually more stable than even C18 itself. Regardless, each molecule in the class exhibits notable IR features, including a high intensity C═C stretch, which may be observable from the James Webb Space Telescope (JWST). This feature falls in the vicinity of 1600 cm-1 (6.2 μm), a region typically associated with polycyclic aromatic hydrocarbon cations. Additionally, each apical carbon on the carbene cyclopropenylidene hexamer is further replaced by an oxygen and sulfur to create a hexamer ring of each molecule, but the stability patterns for these substitutions are not favorable for higher substitutions, reducing the likelihood that C18 functionalized with O or S could contribute to any observable, naturally produced IR features.