Gas-phase formation and spectroscopic characterization of the disubstituted cyclopropenylidenes c-C3(C2H)2, c-C3(CN)2, and c-C3(C2H)(CN)

Aims. The detection of c-C₃HC₂H and possible future detection of c-C₃HCN provide new molecules for reaction chemistry in the dense interstellar medium (ISM) where R-C₂H 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 of this chemical family. This work characterizes 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-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-C₃(C₂H)₂ is as thermodynamically and, likely, as stepwise favorable as the formation of c-C₃HC₂H, rendering its detectability to be mostly dependent on the concentrations of the reactants. Both 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 v₂ at 3316.9 cm^–1 (3.01 μm), with an intensity of 140 km mol^–1. The mixed-substituent molecule c-C₃(C₂H)(CN) has one frequency with a large intensity, v₁, at 3321.0 cm^–1 (3.01 μm), with an intensity of 82 km mol^–1. The molecule c-C₃(CN)₂ lacks intense vibrational frequencies within the range that current instrumentation can readily observe.

Conclusions. Both 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.