  In recent times, large organic molecules of exceptional complexity have been
found in diverse regions of the interstellar medium. In this context, we aim to
provide accurate frequencies of the ground vibrational state of two key
aliphatic aldehydes, n-butanal and its branched-chain isomer, i-butanal, to
enable their eventual detection in the interstellar medium. We employ a
frequency modulation millimeter-wave absorption spectrometer to measure the
rotational features of n- and i-butanal. We use the spectral line survey ReMoCA
performed with the Atacama Large Millimeter/submillimeter Array to search for
n- and i-butanal toward the star-forming region Sgr B2(N). We also search for
both aldehydes toward the molecular cloud G+0.693-0.027 with IRAM 30 m and
Yebes 40 m observations. Several thousand rotational transitions belonging to
the lowest-energy conformers have been assigned in the laboratory spectra up to
325 GHz. A precise set of the relevant rotational spectroscopic constants has
been determined for each structure. We report non-detections of n- and
i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find that n-
and i-butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde
toward Sgr B2(N1S), respectively, and that n-butanal is at least 63 times less
abundant than acetaldehyde toward G+0.693-0.027. Comparison with astrochemical
models indicates good agreement between observed and simulated abundances
(where available). Grain-surface chemistry appears sufficient to reproduce
aldehyde ratios in G+0.693-0.027; gas-phase production may play a more active
role in Sgr B2(N1S). Our astronomical results indicate that the family of
interstellar aldehydes in the Galactic center region is characterized by a drop
of one order of magnitude in abundance at each incrementation in the level of
molecular complexity.
