Context : We performed a spectral line survey called Exploring Molecular Complexity with ALMA ( EMoCA ) toward Sagittarius B2 ( N ) between 84.1 and 114.4 GHz with the Atacama Large Millimeter/submillimeter Array ( ALMA ) in its Cycles 0 and 1 .
We determined line intensities of n -propyl cyanide in the ground vibrational states of its gauche and anti conformers toward the hot molecular core Sagittarius B2 ( N2 ) which suggest that we should also be able to detect transitions pertaining to excited vibrational states .

Aims : We wanted to determine spectroscopic parameters of low-lying vibrational states of both conformers of n -propyl cyanide to search for them in our ALMA data .

Methods : We recorded laboratory rotational spectra of n -propyl cyanide in two spectral windows between 36 and 127 GHz .
We searched for emission lines produced by these states in the ALMA spectrum of Sagittarius B2 ( N2 ) .
We modeled their emission and the emission of the ground vibrational states assuming local thermodynamic equilibrium ( LTE ) .

Results : We have made extensive assignments of a - and b -type transitions of the four lowest vibrational states of the gauche conformer which reach J and K _ { a } quantum numbers of 65 and 20 , respectively .
We assigned mostly a -type transitions for the anti conformer with J and K _ { a } quantum numbers up to 48 and 24 , respectively .
Rotational and Fermi perturbations between two anti states allowed us to determine their energy difference .
The resulting spectroscopic parameters enabled us to identify transitions of all four vibrational states of each conformer in our ALMA data .
The emission features of all states , including the ground vibrational state , are well-reproduced with the same LTE modeling parameters , which gives us confidence in the reliability of the identifications , even for the states with only one clearly detected line .

Conclusions : Emission features pertaining to the highest excited vibrational states of n -propyl cyanide reported in this work have been identified just barely in our present ALMA data .
Features of even higher excited vibrational states may become observable in future , more sensitive ALMA spectra to the extent that the confusion limit will not have been reached .
The ^ { 13 } C isotopomers of this molecule are expected to be near the noise floor of our present ALMA data .
We estimate that transitions of vibrationally excited iso -propyl cyanide or aminoacetonitrile , for example , are near the noise floor of our current data as well .