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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 predicted 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.