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Abstract 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. 

While quantum chemical treatment of the prebiotic, astrochemically relevant H2NCO molecule’s rotational constants and fundamental vibrational frequencies produces exceptionally accurate values, the cis-HNCHO isomer is more of a challenge to standard electronic structure methods. The trans-HNCHO conformer is well-behaved like H2NCO due to the nature of the local potential energy surface, but the zero-point vibrational energy of the torsional motion is significantly greater than the barrier, implying that this isomer cannot be observed. The 2.6 kcal mol−1 lower-energy cis-HNCHO conformer showcases a small pseudo-Jahn–Teller distortion about the HNCO torsion coordinate at planarity. This produces a challenging environment for the quartic force field (QFF) treatment of the rotational and vibrational spectroscopic data. Even so, the accuracy of the H2NCO spectroscopic constants from this explicitly correlated coupled cluster theory QFF implies that similar accuracy should be present for the mathematically well-behaved trans-HNCHO, which exists in a shallow but true minimum. The constants computed herein may allow for a more detailed search of experimental rotation spectra for cis-HNCHO, which would provide reference data necessary for potential searches for this molecule in space. Rotational constants are also provided for the four conformers of HNCOH, and the fundamental vibrational frequencies in the range of James Webb Space Telescope observation are provided for all isomers in order to allow for the possibility of multiple wavelength observation of this family of molecules, which may play a role in the molecular origins of life. 

The creation of larger prebiotic molecules in astronomical regions may require aminoketene (NH2CHCO) as an intermediate, and the two conformers of this molecule exhibit infrared vibrational frequencies with intensities larger even than the antisymmetric stretch in CO2. While the present quantum chemically computed frequencies of these fundamentals of ∼4.7 μm are in the same spectroscopic region as features from functionalized polycyclic aromatic hydrocarbons, they provide clear markers for what James Webb Space Telescope IR observations may be able to distinguish. Additionally, the IR and radioastronomical spectral characterization of two additional 2-iminoacetaldehyde, HN=CHC(=O)H, conformers are also computed as are the same data for a new carbene isomer (NH2CC(=O)H). All conformers of aminoketene and 2-iminoacetaldehyde exhibit dipole moments of more than 2.0 D, if not greater than 4.0 D, implying that they would be notable targets for radioastronomical searches. Additionally, the 2-iminoacetaldehyde conformers have a notable mid-IR C=O stretch around 1735 cm−1 slightly below the same fundamental in formaldehyde. This quantum chemical study is providing a more complete set of reference data for the potential observation of these tautomers and conformers of NH2CHCO in the laboratory or even in space.