  3-Hydroxypropenal (HOCHCHCHO) is the lower energy tautomer of malonaldehyde
which displays a complex rotation-tunneling spectrum. It was detected
tentatively toward the solar-type protostar IRAS 16293$-$2422 B with ALMA in
the framework of the Protostellar Interferometric Line Survey (PILS). Several
transitions, however, had large residuals, preventing not only their detection,
but also the excitation temperature of the species from being determined
unambiguously. We want to extend the existing rotational line list of
3-hydroxypropenal to shed more light on the recent observational results and to
facilitate additional radio astronomical searches for this molecule. We
analyzed the rotation-tunneling spectrum of 3-hydroxypropenal in the frequency
regions between 150 and 330 GHz and between 400 and 660 GHz. Transitions were
searched for in the PILS observations of IRAS 16293$-$2422. Local thermodynamic
equilibrium (LTE) models were carried out and compared to the observations to
constrain the excitation temperature. Additional transitions were searched for
in other ALMA archival data of the same source to confirm the presence of
3-hydroxypropenal. More than 11500 transitions were assigned in the course of
our investigation with quantum numbers $2 \le J \le 100$, $K_a \le 59$, and
$K_c \le 97$, resulting in a greatly improved set of spectroscopic parameters.
The comparison between the LTE models and the observations yields an excitation
temperature of 125 K with a column density $N = 1.0 \times 10^{15}$ cm$^{-2}$
for this species. We identified seven additional lines of 3-hydroxypropenal
that show a good agreement with the model in the ALMA archive data. The
calculated rotation-tunneling spectrum of 3-hydroxypropenal has sufficient
accuracy for radio astronomical searches. The detection of 3-hydroxypropenal
toward IRAS 16293$-$2422 B is now secure.
