Quantitative insights into energy contributions of intermolecular interactions in fluorine and trifluoromethyl substituted isomeric N-phenylacetamides and N-methylbenzamides

Abstract

The presence of the C–F bond in organic molecules, particularly in the context of generating different intermolecular interactions of the type C–F⋯F–C, C–H⋯F and C–F⋯π is of extreme significance in the realm of structural chemistry. These interactions generate different packing motifs in the formation of the crystal. It is of interest to evaluate the energetic contributions of such weak interactions to evaluate their important role in crystal packing. In this respect, a library of twelve compounds containing a strong donor and acceptor, along with the presence of a C–F bond in different electronic environments (fluorine atom connected to C(sp²) and C(sp³) carbon atom) have been synthesized and characterized using single crystal X-ray diffraction studies at low temperature. In addition, the non-fluorinated counterpart has also been synthesized. These crystal structures have been analyzed to understand the contribution of weak interactions involving organic fluorine in the crystal packing. Furthermore, the stabilizing–destabilizing roles of such interactions in terms of favourable energetics have been quantified with inputs from calculations performed using PIXEL. It is observed that most of the interactions involving fluorine are of a dispersive character, and in some cases the interaction is also coulombic in origin. These results have been compared with ab initio quantum-chemical calculations (DFT-D3/B-97D level) performed using TURBOMOLE. In addition, the lattice energies of all the compounds have been evaluated, and the total contribution partitioned into the corresponding coulombic, polarization, dispersion and exchange contributions using the CLP module. The results correlate well with thermochemical data experimentally determined for these compounds.