The role of halogen C–X1⋯X2–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<br />(2) was studied by vibrational spectroscopy (IR, Raman) and X-ray diffraction methods.<br />The analysis was extended to solution phase using UV?Vis, NMR (1H, 13C and 19F) and fluorescence<br />spectroscopy with the assistance of theoretical calculations. The crystal structure of 2 shows chlorine and<br />bromine atoms of the ?CF2Cl and ?CH2Br groups on the same side of the molecular plane, while in the<br />most stable conformer predicted by theoretical calculations, these halogen atoms are opposite. These differences<br />were interpreted in terms of intra- and intermolecular interactions in the crystal packing.<br />Particularly, the weak non covalent halogen?halogen contact plays a decisive role in the preferred conformation<br />adopted in solid phase. In addition, results of fluorescence spectroscopy are in accordance with<br />the X-ray spectroscopy, since the proposed association due to intermolecular interactions was also found<br />in solution when the concentration of 2 was increased.