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Context.In recent times, large organic molecules of exceptional complexity have been found in diverse regions of the interstellar medium.

Aims.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 also want to test the level of complexity that interstellar chemistry can reach in regions of star formation.

Methods.We employ a frequency modulation millimeter-wave absorption spectrometer to measure the rotational features ofn- andi-butanal. We analyze the assigned rotational transitions of each rotamer separately using theA-reduced semirigid-rotor Hamiltonian. We use the spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array to search forn- andi-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. The observational results are compared with computational results from a recent gas-grain astrochemical model.

Results.Several thousand rotational transitions belonging to the lowest-energy conformers of two distinct linear and branched isomers 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 as a first step toward identifying both molecules in the interstellar medium. We report non-detections ofn-and i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find thatn- andi-butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde toward Sgr B2(N1S), respectively, and thatn-butanal is at least 63 times less abundant than acetaldehyde toward G+0.693−0.027. While propanal is not detected toward Sgr B2(N1S) either, with an abundance at least 5–11 lower than that of acetaldehyde, propanal is found to be 7 times less abundant than acetaldehyde in 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). Model estimates for the larger aldehydes indicate that the observed upper limits may be close to the underlying values.

Conclusions.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.