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The role of halogen C-X<inf>1</inf>⋯X<inf>2</inf>-C contact on the preferred conformation of 2-perhalomethylchromones in solid state

Abstract:

The solid state of 2-chlorodifluoromethyl-3-methylchromone (1) and 3-bromomethyl-2-chlorodifluoromethylchromone (2) was studied by vibrational spectroscopy (IR, Raman) and X-ray diffraction methods. The analysis was extended to solution phase using UV-Vis, NMR (1H, 13C and 19F) and fluorescence spectroscopy with the assistance of theoretical calculations. The crystal structure of 2 shows chlorine and bromine atoms of the -CF2Cl and -CH2Br groups on the same side of the molecular plane, while in the most stable conformer pbkp_redicted by theoretical calculations, these halogen atoms are opposite. These differences were interpreted in terms of intra- and intermolecular interactions in the crystal packing. Particularly, the weak non covalent halogen-halogen contact plays a decisive role in the preferred conformation adopted in solid phase. In addition, results of fluorescence spectroscopy are in accordance with the X-ray spectroscopy, since the proposed association due to intermolecular interactions was also found in solution when the concentration of 2 was increased.