previously noted, this slight difference can be ascribed to the different rigidities of the environment where alanine is embedded. The effective mode at higher frequency clearly points at the asymmetric stretching motion of CO 2 in the crystalline form, which becomes instead a wagging motion in the water solvent that is coupled to vibrations in the other fragments of the solute molecule. We can thus conclude that in water, in the region spectral region 1000-2000 cm -1 , the pure antisymmetric stretching motion of CO 2 is suppressed.