We report the gas-phase detection and spectroscopic characterisation of ethynethiol (?), a metastable isomer of thioketene (?) using a combination of Fourier-transform microwave and submillimetre-wave spectroscopies. Several a-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, b-type lines using double resonance provided accurate predictions in the submillimetre region. With these, searches using a millimetre-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280-660 GHz, ultimately yielding nearly 100 transitions up to ? and ?. From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of HCCSH and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction ?, which is calculated to be highly exothermic (-477 kJ/mol) using the HEAT345(Q) thermochemical scheme.
