The discovery of the isothioforyml radical (HSC) in 2018 by Agúndez and co-workers accompanied the detection of the thioformyl radical (HCS) toward the same source, L483, a dark cloud with an embedded Class 0 protostar. The observations were made with the IRAM 30m telescope. The detection was supported by spectral measurements of Hideta & Yamamoto from 2000 and accurate theoretical results of Puzzarini from 2005.

The isothioformyl (or sulfhydryl carbide) radical is a carbene species with a covalent σ bond between sulfur and carbon strengthened by some dative π bonding involving the out-of-plane 2p² pair on sulfur. The HCS isomer is more stable—by nearly 40 kcal/mol, according to Puzzarini—due in part to better π bonding between sufur and carbon (see figure below). The radical character of HSC is associated with the singly occupied in-plane 2p orbital on carbon.