  We report the first laboratory and interstellar detection of the alpha-cyano
vinyl radical (H2CCCN). This species was produced in the laboratory by an
electric discharge of a gas mixture of vinyl cyanide, CH2CHCN, and Ne, and its
rotational spectrum was characterized using a Balle-Flygare narrowband-type
Fourier-transform microwave spectrometer operating in the frequency region of
8-40 GHz. The observed spectrum shows a complex structure due to tunneling
splittings between two torsional sublevels of the ground vibronic state, 0+ and
0-, derived from a large-amplitude inversion motion. In addition, the presence
of two equivalent hydrogen nuclei makes necessary to discern between ortho- and
para-H2CCCN. A least squares analysis reproduces the observed transition
frequencies with a standard deviation of ca. 3 kHz. Using the laboratory
predictions, this radical is detected in the cold dark cloud TMC-1 using the
Yebes 40m telescope and the QUIJOTE line survey. The 404-303 and 505-404
rotational transitions, composed of several hyperfine components, were observed
in the 31.0-50.4 GHz range. Adopting a rotational temperature of 6K we derive a
column density of (1.4+/-0.2)e11 cm-2 and (1.1+/-0.2)e11 cm-2 for ortho-H2CCCN
and para-H2CCCN, respectively. The reactions C + CH3CN, and perhaps also N +
CH2CCH, emerge as the most likely routes to H2CCCN in TMC-1.
