2025ApJ...982..191R

We present the spectroscopic characterization of cyclopropenethione in the laboratory and detect it in space using the Green Bank Telescope Observations of TMC-1: Hunting Aromatic Molecules survey. The detection of this molecule-the missing link in understanding the C₃H₂S isomeric family in TMC-1-completes the detection of all three low-energy isomers of C₃H₂S, as both CH₂CCS and HCCCHS have been previously detected in this source. The total column density of this molecule (N_T of $5.7{2}_{-1.61}^{+2.65}\times 1{0}^{10}$ cm^–2 at an excitation temperature of $4.{7}_{-1.1}^{+1.3}$ K) is smaller than both CH₂CCS and HCCCHS and follows nicely the relative dipole principle (RDP), a kinetic rule of thumb for predicting isomer abundances that suggests that, all other chemistry among a family of isomers being the same, the member with the smallest dipole (μ) should be the most abundant. The RDP now holds for the astronomical abundance ratios of both the S-bearing and O-bearing counterparts observed in TMC-1; however, CH₂CCO continues to elude detection in any astronomical source.