The region 1350-1500 cm -1 of the spectrum in Fig. is populated by four main bands, associated to the four modes shown in Fig. . As expected from the fragment analysis reported above, this region of the spectrum sees the main participation from the bending and umbrella modes in the two fragments CH 3 and NH 3 . Some distorted CO 2 bending motions contribute as well to the ω1=1082 cm -1 ω2=1043 cm -1 ω4=1297 cm -1 ω3=1154 cm -1 FIG. . Effective modes associated to the spectral region 1000-1350 cm -1 of the IR spectrum of zwitterionic alanine in the gas phase.