2025ApJ...980L..13B

We report an extensive rotational spectroscopic analysis of singly deuterated methyl mercaptan (CH₂DSH) using both millimeter and far-infrared synchrotron spectra to achieve a global torsional analysis of the three lowest torsional substates (e₀, e₁, and o₁) of this nonrigid species. A fit including 3419 millimeter-wave transitions along with 43 infrared torsional subband centers was performed with rms deviations of 0.233 MHz and 0.270 cm^–1, respectively, resulting in 68 fit parameters. A spectroscopic catalog built from this analysis for a temperature of 125 K has led to the first interstellar detection of CH₂DSH toward the solar-like protostar IRAS 16293-2422 B. We report the identification of 46 transitions, including eight relatively unblended lines, resulting in a derived column density of (3.0 ± 0.3) × 10¹⁴ cm^–2. The column density ratio for HDCS/CH₂DSH compared to HDCO/CH₂DOH suggests a difference in the interstellar chemistry between the sulfur and oxygen complex organics, in particular a different link between H₂CO and CH₃OH and between H₂CS and CH₃SH. This is the first interstellar detection of a deuterated sulfur-bearing complex organic molecule and therefore an important step toward understanding the chemical origin of sulfur-based prebiotics.