Structural determination and dynamics on floppy molecular systems.

We discuss on the central role of very high resolution spectroscopy for the study of molecular systems weakly bonded or flexible. It will appear evident how the lack of high resolution results can lead to wrong conclusions. The paper will focalize the attention in particular on two different cases: one involving the hydrogen bonded complex anisole-water, the other involving the very floppy 1,3-benzodioxole (BDO) molecule. In the first case the issue is the determination of the structure of the complex that cannot be correctly inferred from resonance enhanced multi photon ionization (REMPI) data and ab initio calculations. In the second case the non-rigidity of the molecule and the possibility of the interaction of two low frequency modes (ring-puckering and ring-butterfly) have lead to a wrong assignment of the laser induced fluorescence (LIF) vibronic spectrum.