  The reaction between carbon atoms and vinyl cyanide, CH2CHCN, is a formation
route to interstellar 3-cyano propargyl radical, CH2C3N, a species that has
recently been discovered in space. The 1-cyano propargyl radical (HC3HCN), an
isomer of CH2C3N, is predicted to be produced in the same reaction at least
twice more effciently than CH2C3N. Hence, HC3HCN is a plausible candidate to be
observed in space as well. We aim to generate the HC3HCN radical in the gas
phase in order to investigate its rotational spectrum. The derived
spectroscopic parameters for this species will be used to obtain reliable
frequency predictions to support its detection in space.The HC3HCN radical was
produced by an electric discharge, and its rotational spectrum was
characterized using a Balle-Flygare narrowband-type Fourier-transform microwave
spectrometer operating in the frequency region of 4-40 GHz. The spectral
analysis was supported by high-level ab initio calculations. A total of 193
hyperfine components that originated from 12 rotational transitions, a- and
b-type, were measured for the HC3HCN radical. The analysis allowed us to
accurately determine 22 molecular constants, including rotational and
centrifugal distortion constants as well as the fine and hyperfine constants.
Transition frequency predictions were used to search for the HC3HCN radical in
TMC-1 using the QUIJOTE survey between 30.1-50.4 GHz. We do not detect HC3HCN
in TMC-1 and derive a 3sigma upper limit to its column density of 6.0e11 cm-2.
