HALOGEN SUBSTITUTION EFFECTS ON THE TAUTOMERIC AND CONFORMATIONAL EQUILIBRIA OF MOLECULES AND MOLECULAR COMPLEXES: A ROTATIONAL STUDY

Halogen substitution in molecules is a common practice to modulate physicochemical properties of bio-organic molecules and functional materials [1,2]. The effects of halogenation are related to the high electronegativity of the atoms and their different polarizabilities. The presence of a halogen atom greatly alters lipophilicity, dipole moment, reactivity, chemical stability and interactions with other functional groups present in the molecule. Above all, the introduction of a fluorine atom becomes particularly interesting because it can activate or increase the molecule capability of creating hydrogen bonds or non- covalent bonds. For this reason we also investigate clusters where a molecule of water is used as a probe to reveal the changes on the electrostatic potential on the halogenated compounds. The experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy (MBFTMW) [3] or Free Jet Absorption Microwave Spectroscopy (FJAMW) [4] techniques. The high resolution and sensitivity of rotational spectroscopy give direct access to the structural arrangement of the compounds, allowing the measurement of bond length and angles. Moreoverthis gas phase technique can help to unveil subtle structural and dynamical effects usually related to changesb in non-covalent interactions. Regarding chlorination we present results on model compounds such as 5-and 6- clorohydroxypyridine in which we investigate the effects of halogenations on the tautomeric equilibrium. Regarding fluorination we first present a study on the changes in the conformational potential energy surface of 2-fluorobenzylamine with respect to the un-substituted molecule. The conformational space of such molecules is shaped by non bonding interactions which can be changed drastically through substitution of even a single atom. We have also studied a series of clusters between different fluorinated pyridines and a molecule of water: 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine. The results clearly show that the introduction ofa 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 material starts to behave as a completely novel species [5]. The perfluorination effect is clearly observable: and in the penta-fluoropyridine-water adduct the water oxygen lone pairs point towards the aromatic ring. [1] K. Müller, C. Faeh, F. Diederich, Science 317, 1881 (2007). [2] H. Matter, M. Nazar, S. Gussregen, D. Will, H. Schreuder, A.,Angew. Chem. 121, 2955(2009). [3] Caminati W.; Millemaggi A.; Alonso J.L.; Lesarri A.; Lopez J.C.; Mata S., Chem. Phys. Lett. 392, 1 (2004). [4] C. Calabrese, A. Maris, L. Evangelisti, L.B. Favero, S. Melandri and W. Caminati, J. Phys. Chem. A, 117, 13712 (2013). [5] Cametti M.,Crousse B., Metrangolo P., Milanicd R., Resnati G., Chem. Soc. Rev. 41, 31 (2012).