Importance of the hydrogen bond in tautomeric equilibria and in molecular complex formation. Rotational spectroscopy in supersonic expansions

The importance of weak interactions, especially inter and intramolecular hydrogen bonds, is recognized in all areas of chemistry and biology and the understanding of their role is essential to explain and model many phenomena. In our studies the detailed nature of such interactions is revealed by high resolution spectral data on isolated molecules and molecular complexes observed in free jet expansions. The interpretation of these data is assisted by high level quantum chemical calculations and other models to help justify and rationalize the results. We report microwave spectroscopy studies on molecules isolated in supersonic expansions: 4-hydroxypyrimidine,1 methylsalycilate and erythritol, and show the importance of hydrogen bond formation on the shape of the tautomeric and conformational potential energy surface and on the geometry of the stable structures. The study of small molecular hydrogen bonded complexes such as dimers or trimers is also very interesting and can be considered a starting point towards the understanding of bulk properties. Weak and strong intermolecular hydrogen bonds (C-H...F, C-H...N, O-H...N) can be observed in the structures of the trimer of difluoromethane,2 pyridine- CHF3 or pyridine-water. These molecular complexes are stabilized by more than one hydrogen bond but they are also highly dynamical. 1. Sanchez, R.; Giuliano, B.M.; Favero ,L.B.; Caminati, W., J. Am. Chem. Soc., 2007, 129, 6287-6290 2..Blanco, S.; Melandri, S.; Ottaviani, P.; Caminati, W.; J. Am. Chem. Soc.; 2007; 129(9), 2700 – 2703