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Description
The saturated n-propyl cyanide has recently been detected in Sagittarius B2(N). The next larger unbranched alkyl cyanide is n-butyl cyanide. We want to 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. The laboratory rotational spectrum of n-butyl cyanide has been investigated between 75 and 348GHz. We have searched for emission lines caused by the molecule in our sensitive IRAM 30m molecular line survey of Sagittarius B2(N). More than one thousand rotational transitions have been identified 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 has been greatly extended to J~120 or more and Ka>35, resulting in accurate spectroscopic parameters and accurate rest frequency calculations up to about 500GHz for strong to moderately weak transitions of the two lower energy conformers. Upper limits of the column densities of N<=3x10^15^cm^-2^ and 8x10^15^cm^-2^ were derived towards Sagittarius B2(N) for the two lower energy conformers, anti-anti and gauche-anti, respectively. The present data will be helpful to identify 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.
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