Aims. The detection of c-C3HC2H and possible future detection of c-C3HCN provide new molecules for reaction chemistry in the dense ISM where R-C2 and R-CN species are prevalent. Determination of chemically viable c-C3HC2H and c-C3HCN derivatives and their prominent spectral features can accelerate potential astrophysical detection for this chemical family. This work will characterize three such derivatives: c-C3(C2H)2, c-C3(CN)2, and c-C3(C2H)(CN).
Methods. Interstellar reaction pathways of small carbonaceous species are well-replicated through quantum chemical means. Highly-accurate cc-pVX Z-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-C3(C2H)2 is as thermodynamically and, likely, stepwise favorable as the formation of c-C3HC2H, rendering its detectability to be mostly dependent on the concentrations of the reactants. c-C3(C2H)2 and c-C3(C2H)(CN) will be detectable through radioastronomical observation with large dipole moments of 2.84 D and 4.26 D, respectively, while c-C3(CN)2 has an exceedingly small and likely unobservable dipole moment of 0.08 D. The most intense frequency for c-C3(C2H)2 is ν2 at 3316.9 cm−1 (3.01 µm) with an intensity of 140 km mol−1. c-C3(C2H)(CN) has one frequency with a large intensity, ν1, at 3321.0 cm−1 (3.01 µm) with an intensity of 82 km mol−1. c-C3(CN)2 lacks intense vibrational frequencies within the range that current instrumentation can readily observe.
Conclusions. c-C3(C2H)2 and c-C3(C2H)(CN) are viable candidates for astrophysical observation with favorable reaction profiles and spectral data produced herein, but c-C3(CN)2 will not be directly observable through any currently-available remote sensing means even if it forms in large abundances
