A Potential New Dimension of Astrophysical Aromatic Functionalization: Phosphaethynyl (CP) Derivatives

Quantum chemical computations outline a thermodynamically viable, low-temperature, gas-phase formation pathway for phosphaethynyl cyclopropenylidene ($c$-C$_3$HCP), resulting from cyclopropenylidene ($c$-C$_3$H$_2$) and the phosphaethynyl (CP) radical. Given the detections of both $c$-C$_3$H$_2$ and the CP radical toward circumstellar shells along with the provided reaction pathway, $c$-C$_3$HCP is a potentially present astro-molecule which could be searched for in phosphorus rich environments. The reaction of CP and benzene (C$_6$H$_6$) to produce phosphaethynyl benzene (C$_6$H$_5$CP) is also explored. A marginally emerged, but tentative, transition state is identified for this reaction using certain quantum chemical approaches but not others. Consequently, the formation of CP-substituted benzoid polycyclic aromatic hydrocarbons (PAHs) in astrophysical environments from CP + PAH reactions is possible but currently uncertain. Additionally, explicitly correlated coupled cluster quartic force field and second-order vibrational perturbation theory rovibrational analysis of the vibrational frequencies and rotational constants of $c$-C$_3$HCP are provided, as well as density functional theory predictions of the same for CP-benzene (C$_6$H$_5$CP) and both isomers of CP-naphthalene (C$_1$$_0$H$_7$CP). Our results suggest that IR searches for these molecules would likely be unsuccessful due to the low intensities and coincidence with PAH features, but their large dipole moments could make them feasible to detect with radio astronomy. The barrierless formation of $c$-C$_3$HCP and possibly barrierless synthesis of C$_6$H$_5$CP stand as examples for future investigations of other CP-substituted aromatic molecules, marking the potential beginning of a new frontier of astronomical aromatic functionalization.