The quest for complex molecules in space: laboratory spectroscopy of n-butyl cyanide, n-C4H9CN, in the millimeter wave region and its astronomical search in Sagittarius?B2(N)

摘要

Context. The saturated n-propyl cyanide was recently detected in Sagittarius?B2(N). The next largest unbranched alkyl cyanide is n-butyl cyanide. Aims. We provide accurate rest frequency predictions beyond the millimeter wave range to search for this molecule in the Galactic center source Sagittarius?B2(N) and facilitate its detection in space. Methods. We investigated the laboratory rotational spectrum of n-butyl cyanide between 75?GHz and 348?GHz. We searched for emission lines produced by the molecule in our sensitive IRAM?30?m molecular line survey of Sagittarius?B2(N). Results. We identified more than one thousand rotational transitions in the laboratory for each of the three conformers for which limited data had been obtained previously in a molecular beam microwave study. The quantum number range was greatly extended to J?≈?120 or more and Ka?>?35, resulting in accurate spectroscopic parameters and accurate rest frequency calculations up to about 500?GHz for strong to moderately weak transitions of the two lower energy conformers. Upper limits to the column densities of N?≤?3?×?1015?cm-2 and 8?×?1015?cm-2 were derived towards Sagittarius?B2(N) for the two lower energy conformers, anti-anti and gauche-anti, respectively. Conclusions. Our present data will be helpful for identifying n-butyl cyanide at millimeter or longer wavelengths with radio telescope arrays such as ALMA, NOEMA, or EVLA. In particular, its detection in Sagittarius?B2(N) with ALMA seems feasible.