Intramolecular Halogen Bonding in Solution: 15N, 13C, and 19F NMR Studies of Temperature and Solvent Effects

A model system for the investigation of intramolecular halogen bonds is introduced. Two molecules capable of intramolecular halogen bonding have been studied in comparison with eight control compounds by 15N, 13C, and 19F NMR spectroscopy. Iodine- and bromine-centered halogen bonds are indicated by decreases in the 15N NMR chemical shifts of the halogen bond acceptor atom of approximately 6 and 1 ppm, respectively. 13C NMR chemical shifts of the alkynyl carbons in 2-ethynylpyridine systems are good indicators of halogen bonding, with differences of up to 2.4 ppm between halogen-bonded and related control compounds. Halogen bond strengths in different solvents, as indicated by 19F NMR chemical shifts, decrease in the following order: Cyclohexane \u3e toluene \u3e benzene \u3e dichloromethane \u3e acetone \u3e pyridine. Chemical shift effects associated with the structural and electronic properties of intramolecular halogen-bonded systems are modeled well by calculations at the B3LYP/6-311+G(2d,p) level of theory