THE EFFECT OF RING FLUORINATION AND WATER COMPLEXATION ON MOLECULAR FLEXIBILITY AND TUNNELING PATHWAYS: THE ROTATIONAL SPECTRUM OF 2-FLUOROBENZYLAMINE AND BENZYLAMINE-WATER

We present the rotational spectra of different organic molecules, Benzylamine (BA) and 2-Fluorobenzylamine (2FBA), and we report the effects of water complexation for BA. These flexible molecules are characterized by high number of low energy conformations and the presence of large amplitude motions. Non-bonding interactions compete to shape their conformational space and potential energy surfaces and these interactions can be changed drastically with water complexation and halogenation. Halogen substitution is in fact a common practice in chemistry in order to modulate physicochemical properties of bio-organic molecules and functional materials.[1,2] In particular, the role of organic fluorine atoms, has been extensively investigated at the molecular level and its effects are correlated to the high electronegativity of the atoms and their different polarizabilities. We recorded the spectra with Free Jet Absorption Microwave Spectroscopy (FJ-AMMW) or Fourier Transform Microwave Spectroscopy (FTMW). The high resolution and sensitivity of these techniques can help unveil subtle structural and dynamical effects usually related to changes in non-covalent interactions. In the study of the two monomers BA and 2FBA we want to evaluate the changes that substitution of H atoms by fluorine causes to Benzylamine. The rotational spectra of 2-FBA, recorded with FTMW spectrometer, show the presence of two of the four stable conformers predicted with quantum chemical calculations: the global minimum is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group, whereas the other conformer is characterized by a complex tunnelling motion of the aminic hydrogen atom. On the basis of the BA monomer, we predicted a fourth conformer and we found a greater splitting of the same tunneling motion due to the new geometry generated by the presence of the fluorine. The motion was analysed with a one-dimensional flexible model which allowed to estimate the energy barrier for the transition state. As to the rotational spectrum of the complex of BA with one molecule of water, recorded with FJ-AMMW spectrometer, it shows only one conformation with the water molecule’s hydrogen bound to the nitrogen atom of BA. We can note that the introduction of the water molecule does not disrupt the initial conformation of the monomer BA, but inverts the stability order of the conformations and blocks the tunneling motion. REFERENCES [1] K. Müller, C. Faeh, F. Diederich, Science, 2007, 317, 1881; H. Matter, M. Nazar, S. Gussregen, D.W. Will, H. Schreuder, A., Angew. Chem., 2009, 121, 2955. [2] R. Berger, G. Resnati, P. Metrangolo, E. Weberd and J. Hulliger, Chem. Soc. Rev., 2011, 40, 3496.