  The interstellar detections of isocyanic acid, methyl isocyanate, and very
recently also ethyl isocyanate, open the question of the possible detection of
vinyl isocyanate in the interstellar medium. The aim of this study is to extend
the laboratory rotational spectrum of vinyl isocyanate into the millimeter wave
region and to undertake a check for its presence in the high-mass star forming
region Sgr B2. The rotational spectrum of vinyl isocyanate was recorded in the
frequency regions 127.5-218 and 285-330 GHz using the Prague millimeter wave
spectrometer. The spectral analysis was supported by high-level
quantum-chemical calculations. We assumed local thermodynamic equilibrium to
compute synthetic spectra of vinyl isocyanate and to search for it in the
ReMoCA survey performed with ALMA toward Sgr B2(N). We also searched for ethyl
isocyanate in the same source. Accurate values for the rotational and
centrifugal distortion constants are reported for the ground vibrational states
of trans and cis vinyl isocyanate. We report nondetections of vinyl and ethyl
isocyanate toward the main hot core of Sgr B2(N). We find that vinyl and ethyl
isocyanate are at least 11 and 3 times less abundant than methyl isocyanate in
this source, respectively. Although the precise formation mechanism of
interstellar methyl isocyanate itself remains uncertain, we infer from existing
astrochemical models that our observational upper limit for the CH3NCO:C2H5NCO
ratio in Sgr B2(N) is consistent with ethyl isocyanate being formed on dust
grains via the abstraction or photodissociation of an H atom from methyl
isocyanate, followed by the addition of a methyl radical. The dominance of such
a process for ethyl isocyanate production, combined with the absence of an
analogous mechanism for vinyl isocyanate, would indicate that the ratio
C2H3NCO:C2H5NCO should be rather less than unity.
