  The saturated n-propyl cyanide was recently detected in Sagittarius B2(N).
The next larger unbranched alkyl cyanide is n-butyl cyanide. 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. 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). 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 x 10E15 cm-2 and 8 x 10E15 cm-2 were derived towards
Sagittarius B2(N) for the two lower energy conformers, anti-anti and
gauche-anti, respectively. 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.
