Ring fluorination effects on molecular water clusters: the cases of 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine. A rotational spectroscopy study.

The importance of organic fluorinated compounds in science and in everyday life is growing year after year.1 The effects of fluorination are related to the high electronegativity of this atom and its polarizability. In general the introduction of a fluorine atom is used in order to modulate physicochemical properties, like hydrophilicity and lipophilicity, of bio-organic molecules and functional materials.1,2 Additionally, structural changes can also be related to the electronic effect of the fluorine atom or to the molecule capability of creating new hydrogen bonds or non-covalent bonds, having fluorine as acceptor. For this reason we investigate clusters where a molecule of water is used as a probe to reveal the changes on the electrostatic potential on the fluorinated compounds. The experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy technique (MBFTMW).3 The high resolution and sensitivity of rotational spectroscopy give direct access to the structural arrangement of the systems, allowing the measurement of bond lengths and angles. Moreover this gas phase technique allows unveiling subtle structural and dynamical effects usually related to changes in non-covalent interactions. The series of clusters studied between different fluorinated pyridines and a molecule of water are: 2-fluoropyridine-water, 3-fluoropyridine-water and penta-fluoropyridine-water (see Figure 1) complexes. The results clearly show that the introduction of a single fluorine atom into a molecule already induces significant effects, but as the number of fluorine atoms increases, such as in the case of penta-fluoropyridine, the system starts to behave as a completely novel species.4 The perfluorination effect is clearly observable in the penta-fluoropyridine-water adduct where the water oxygen lone pairs point towards the aromatic ring. Figure 1: the observed adducts of 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine with a molecule of water.