The IR absorption bands of the fragment NH 3 are slightly blue shifted with respect to those of CH 3 , and are mainly distributed in the region 1450-1750 cm -1 , as shown by the colored bands in the lower panel of Fig. . Despite the blue shift, the vibrations within the NH3 fragments are expected to be coupled to those of CH 3 since the absorption bands partially overlap. Therefore, similarly to CH 3 , NH 3 contributes to the large absorption region of zwitterionic alanine in the gas phase in the region 1300-1600 cm -1 (black line in the upper panel of Fig. ) as well as to the isolated band at 1700 cm -1 . In contrast, we find that the bands are not well localized in frequency as it was the case for the CH 3 fragment, which might be a manifestation of strong anharmonicity and coupling of NH 3 with other modes that have not been considered in this fragment analysis. The effective modes identified by our analysis are very similar to those in CH 3 , namely they are a HNH bending mode (mode 1, depicted as arrows in the inset of Fig. ), the umbrella mode (mode 2 in Fig. ), and a combination of a HNH bending mode with an out-of-plane vibration of the remaining NH bond (mode 3 in Fig. ). Similar to the CH 3 fragment, these modes will be denoted as δ as NH 3 and δ s NH 3 in Section III A.