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We present the first measurements of the rotational spectrum of thiocyanic acid (HSCN) in the microwave and mm-wave regions. HSCN is an isomer of the spectroscopically and astronomically well-studied molecule isothiocyanic acid (HNCS). HSCN has been calculated to lie only 4-14~kcal/mol higher in energy than the most stable isomer HNCS, yet to date HSCN had only been characterised experimentally by matrix-IR spectroscopy. We now succeeded to produce HSCN in a discharge of H$_2$S and either (CN)$_2$ or CH$_3$CN in sufficient amounts for its spectroscopic investigation in the gas phase. Three {\it a}-type rotational transitions in the $K_a=0$ ladder and four in the $K_a=1$ ladder were measured in the frequency range $10-35$~GHz with our Fourier Transform Microwave Spectrometer (FTM) in a molecular beam. The identification was confirmed by the observation of 6 singly and doubly substituted isotopic species at predicted isotopic shifts. Furthermore, we were able to resolve and analyse the distinctive hyperfine structure due to $^{14}$N and D in all species containing these nuclei, providing additional evidence for the identification. An experimental structure derived from the isotopic measurements will be presented. For the main isotopic species additional $\Delta K_a=0$ transitions were observed in selected frequency regions up to 350~GHz with a free-space millimeter-wave absorption spectrometer. Lines of HSCN were surprisingly strong through a dc discharge of H$_2$S and (CN)$_2$, allowing to observe spectra up to $J=30$ and $K_a=7$, as shown in the figure. \\ We also report a tentative detection of HSCN towards SgrB2(M) via four $K_a=0$ $a$-type transitions observed in the 3-mm band with the IRAM 30m telescope. The observations yield an abundance of HSCN only a factor 2-5 lower than that of the lowest energy isomer HNCS.


Author Institution: Harvard Smithsonian Center for Astrophysics and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; Max-Planck-Institut fur Radioastronomie, 53121 Bonn, Germany