  We have extended the pure rotational investigation of the two isomers syn and
anti vinyl mercaptan to the millimeter domain using a frequency-multiplication
spectrometer. The species were produced by a radiofrequency discharge in
1,2-ethanedithiol. Additional transitions have been re-measured in the
centimeter band using Fourier-transform microwave spectroscopy to better
determine rest frequencies of transitions with low-$J$ and low-$K_a$ values.
Experimental investigations were supported by quantum chemical calculations on
the energetics of both the [C$_2$,H$_4$,S] and [C$_2$,H$_4$,O] isomeric
families. Interstellar searches for both syn and anti vinyl mercaptan as well
as vinyl alcohol were performed in the EMoCA (Exploring Molecular Complexity
with ALMA) spectral line survey carried out toward Sagittarius (Sgr) B2(N2)
with the Atacama Large Millimeter/submillimeter Array (ALMA). Highly accurate
experimental frequencies (to better than 100 kHz accuracy) for both syn and
anti isomers of vinyl mercaptan have been measured up to 250 GHz; these deviate
considerably from predictions based on extrapolation of previous microwave
measurements. Reliable frequency predictions of the astronomically most
interesting millimeter-wave lines for these two species can now be derived from
the best-fit spectroscopic constants. From the energetic investigations, the
four lowest singlet isomers of the [C$_2$,H$_4$,S] family are calculated to be
nearly isoenergetic, which makes this family a fairly unique test bed for
assessing possible reaction pathways. Upper limits for the column density of
syn and anti vinyl mercaptan are derived toward the extremely molecule-rich
star-forming region Sgr B2(N2) enabling comparison with selected complex
organic molecules.
