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

Context. In recent times, large organic molecules of exceptional complexity have been found in diverse regions the interstellar medium. Aims. this context, we aim to provide accurate frequencies ground vibrational state two key aliphatic aldehydes, n -butanal and its branched-chain isomer, i-butanal, enable their eventual detection We also want test level that chemistry can reach star formation. Methods. employ a frequency modulation millimeter-wave absorption spectrometer measure rotational features - i -butanal. analyze assigned transitions each rotamer separately using A -reduced semirigid-rotor Hamiltonian. use spectral line survey ReMoCA performed with Atacama Large Millimeter/submillimeter Array search for toward star-forming region Sgr B2(N). both aldehydes molecular cloud G+0.693?0.027 IRAM 30 m Yebes 40 observations. The observational results are compared computational from gas-grain astrochemical model. Results. Several thousand belonging lowest-energy conformers distinct linear branched isomers laboratory spectra up 325 GHz. precise set relevant spectroscopic constants has determined structure as first step identifying report non-detections -and i-butanal sources, B2(N1S) G+0.693-0.027. find at least 2-6 6-18 times less abundant than acetaldehyde B2(N1S), respectively, is 63 G+0.693?0.027. While propanal not detected either, an abundance 5–11 lower acetaldehyde, be 7 Comparison models indicates good agreement between observed simulated abundances (where available). Grain-surface appears sufficient reproduce aldehyde ratios G+0.693?0.027; gas-phase production may play more active role B2(N1S). Model estimates larger indicate upper limits close underlying values. Conclusions. Our astronomical family Galactic center characterized by drop one order magnitude incrementation complexity.