Gas-phase formation and spectroscopic characterization of the disubstituted cyclopropenylidenes c -C 3 (C 2 H) 2 , c -C 3 (CN) 2 , and c -C 3 (C 2 H)(CN)

• Published in: Astronomy and astrophysics (Berlin) 2023-03, Vol.671, p.A95
• Main Authors: Flint, Athena R., Watrous, Alexandria G., Westbrook, Brent R., Patel, Dev J., Fortenberry, Ryan C.
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202245643

Aims. The detection of c -C 3 HC 2 H and possible future detection of c -C 3 HCN provide new molecules for reaction chemistry in the dense interstellar medium (ISM) where R-C 2 H and R-CN species are prevalent. Determination of chemically viable c -C 3 HC 2 H and c -C 3 HCN derivatives and their prominent spectral features can accelerate potential astrophysical detection of this chemical family. This work characterizes three such derivatives: c -C 3 (C 2 H) 2 , c -C 3 (CN) 2 , and c -C 3 (C 2 H)(CN). Methods. Interstellar reaction pathways of small carbonaceous species are well replicated through quantum chemical means. Highly accurate cc-pVXZ-F12/CCSD(T)-F12 ( X = D,T) calculations generate the energetics of chemical formation pathways as well as the basis for quartic force field and second-order vibrational perturbation theory rovibrational analysis of the vibrational frequencies and rotational constants of the molecules under study. Results. The formation of c -C 3 (C 2 H) 2 is as thermodynamically and, likely, as stepwise favorable as the formation of c -C 3 HC 2 H, rendering its detectability to be mostly dependent on the concentrations of the reactants. Both c-C 3 (C 2 H) 2 and c -C 3 (C 2 H)(CN) will be detectable through radioastronomical observation with large dipole moments of 2.84 D and 4.26 D, respectively, while c -C 3 (CN) 2 has an exceedingly small and likely unobservable dipole moment of 0.08 D. The most intense frequency for c -C 3 (C 2 H) 2 is v 2 at 3316.9 cm –1 (3.01 μm), with an intensity of 140 km mol –1 . The mixed-substituent molecule c -C 3 (C 2 H)(CN) has one frequency with a large intensity, v 1 , at 3321.0 cm –1 (3.01 μm), with an intensity of 82 km mol –1 . The molecule c -C 3 (CN) 2 lacks intense vibrational frequencies within the range that current instrumentation can readily observe. Conclusions. Both c -C 3 (C 2 H) 2 and c -C 3 (C 2 H)(CN) are viable candidates for astrophysical observation, with favorable reaction profiles and spectral data produced herein, but c -C 3 (CN) 2 will not be directly observable through any currently available remote sensing means, even if it forms in large abundances.