Discriminating Halogen-Bonding from Other Noncovalent Interactions by a Combined NOE NMR/DFT Approach

Herein a combined NOE NMR/DFT methodology to discriminate between adducts held together by halogen bonding (XB) and other noncovalent interactions (non-XB, such as lone pair/p), based on the determination of the XB donors' and acceptors' relative orientation, is proposed. In particular, F-19, H-1 HOESY NMR spectroscopy experiments and DFT calculations on different XB donors, such as perfluorohexyl iodide (I1), iodopentafluorobenzene (I2) and bromopentafluorobenzene (Br), combined with different Lewis bases, such as 1,4-diazabicyclo[2.2.2] octane (DABCO) and 2,4,6-trimethylpyridine (Me3Py), were performed. The results clearly show that in the case DABCO/I1 the XB adduct is practically the only one present in solution, whereas for the other pairs a certain amount of non-XB adduct is present. Combining DFT and HOESY results, the amount of non-XB adducts can be roughly quantified under our experimental conditions as 4% for DABCO/I2, between 10 and 20% for Me3Py/I1 and Me3Py/I2, and 44% for DABCO/Br.