  Aminoacetonitrile, a molecule detected in the interstellar medium only
towards the star-forming region Sagittarius B2 (Sgr B2) thus far, is considered
an important prebiotic species. To date, observations were limited to ground
state emission lines, whereas transitions from within vibrationally excited
states remained undetected. We wanted to accurately determine the energies of
the low-lying vibrational states of aminoacetonitrile, which are expected to be
populated in Sgr B2(N1), the main hot core of Sgr B2(N). This step is
fundamental in order to properly evaluate the vibration-rotation partition
function of aminoacetonitrile as well as the line strengths of the rotational
transitions of its vibrationally excited states. This is necessary to derive
accurate column densities and secure the identification of these transitions in
astronomical spectra. The far-infrared ro-vibrational spectrum of
aminoacetonitrile has been recorded in absorption against a synchrotron source
of continuum emission. Three bands, corresponding to the lowest vibrational
modes of aminoacetonitrile, were observed in the frequency region below 500
cm$^{-1}$. The combined analysis of ro-vibrational and pure rotational data
allowed us to prepare new spectral line catalogs for all the states under
investigation. We used the imaging spectral line survey ReMoCA performed with
ALMA to search for vibrationally excited aminoacetonitrile toward Sgr B2(N1).
On the basis of these spectroscopic predictions, we report the interstellar
detection of aminoacetonitrile in its $v_{11}=1$ and $v_{18}=1$ vibrational
states toward Sgr B2(N1) in addition to emission in its vibrational ground
state. The intensities of the identified $v_{11}=1$ and $v_{18}=1$ lines are
consistent with the detected $v=0$ lines under LTE at a temperature of 200 K
for an aminoacetonitrile column density of $1.1 \times 10^{17}$ cm$^{-2}$.
