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In recent times, large organic molecules of exceptional complexity have beenfound in diverse regions of the interstellar medium. In this context, we aim toprovide accurate frequencies of the ground vibrational state of two keyaliphatic aldehydes, n-butanal and its branched-chain isomer, i-butanal, toenable their eventual detection in the interstellar medium. We employ afrequency modulation millimeter-wave absorption spectrometer to measure therotational features of n- and i-butanal. We use the spectral line survey ReMoCAperformed with the Atacama Large Millimeter/submillimeter Array to search forn- and i-butanal toward the star-forming region Sgr B2(N). We also search forboth aldehydes toward the molecular cloud G+0.693-0.027 with IRAM 30 m andYebes 40 m observations. Several thousand rotational transitions belonging tothe lowest-energy conformers have been assigned in the laboratory spectra up to325 GHz. A precise set of the relevant rotational spectroscopic constants hasbeen determined for each structure. We report non-detections of n- andi-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 acetaldehydetoward Sgr B2(N1S), respectively, and that n-butanal is at least 63 times lessabundant than acetaldehyde toward G+0.693-0.027. Comparison with astrochemicalmodels indicates good agreement between observed and simulated abundances(where available). Grain-surface chemistry appears sufficient to reproducealdehyde ratios in G+0.693-0.027; gas-phase production may play a more activerole in Sgr B2(N1S). Our astronomical results indicate that the family ofinterstellar aldehydes in the Galactic center region is characterized by a dropof one order of magnitude in abundance at each incrementation in the level ofmolecular complexity.

Toward the limits of complexity of interstellar chemistry: Rotational spectroscopy and astronomical search for n- and i-butanal