Before reporting on the full IR spectrum of alanine, let us briefly analyze the spectrum of its gas phase zwitterionic form in terms of fragments contributions. Since the spectral region 1000-2000 cm -1 of interest in this work is densely populated by overlapping and not well-defined absorption bands, this preliminary study aims to clarify the results that will be discussed in the following sections on the alanine molecule. From the trajectory of the alanine molecule, positions and velocities of three fragments, namely CH 3 , NH 3 , and CO 2 were extracted in order to identify their effective modes and the bands of the full IR spectrum arising from their vibrations. Note that, while the analysis is performed on the isolated fragments, their dynamics accounts for the presence of the remaining degrees of freedom of the molecule. The IR absorption bands of the fragment CH 3 are mainly located in the region 1400-1500 cm -1 , as shown by the colored bands in the lower panel of Fig. , and thus contribute 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. ). The effective modes identified by our analysis are, rather unsurprisingly, combinations of bending modes (mode 1 and mode 3, depicted as arrows in the insets of Fig. ) and the umbrella mode (mode 2 in Fig. ). These modes will be referred to as δ as CH 3 (modes 1 and 3) and δ s CH 3 (mode 2) in Section III A.