  The rotational spectra of thioisocyanic acid (HNCS), and its three energetic
isomers (HSCN, HCNS, and HSNC) have been observed at high spectral resolution
by a combination of chirped-pulse and Fabry-P\'{e}rot Fourier-transform
microwave spectroscopy between 6 and 40~GHz in a pulsed-jet discharge
expansion. Two isomers, thiofulminic acid (HCNS) and isothiofulminic acid
(HSNC), calculated here to be 35-37~kcal/mol less stable than the ground state
isomer HNCS, have been detected for the first time. Precise rotational,
centrifugal distortion, and nitrogen hyperfine coupling constants have been
determined for the normal and rare isotopic species of both molecules; all are
in good agreement with theoretical predictions obtained at the coupled cluster
level of theory. On the basis of isotopic spectroscopy, precise molecular
structures have been derived for all four isomers by correcting experimental
rotational constants for the effects of rotation-vibration calculated
theoretically. Formation and isomerization pathways have also been
investigated; the high abundance of HSCN relative to ground state HNCS, and the
detection of strong lines of SH using CH$_3$CN and H$_2$S, suggest that HSCN is
preferentially produced by the radical-radical reaction HS + CN. A radio
astronomical search for HSCN and its isomers has been undertaken toward the
high-mass star-forming region Sgr B2(N) in the Galactic Center with the 100 m
Green Bank Telescope. While we find clear evidence for HSCN, only a tentative
detection of HNCS is proposed, and there is no indication of HCNS or HSNC at
the same rms noise level. HSCN, and tentatively HNCS, displays clear deviations
from a single-excitation temperature model, suggesting weak masing may be
occurring in some transitions in this source.
