Halogen bonding between pyridine and heptafluoro-2-iodopropane (iso-C3F7I)/heptafluoro-1-iodopropane (1-C3F7I) was studied using a combination of FTIR and 19F NMR. The ring breathing vibration of pyridine underwent a blue shift upon the formation of halogen bonds with both iso-C3F7I and 1-C3F7I. The magnitudes of the shifts and the equilibrium constants for the halogen-bonded complex formation were found to depend not only on the structure of the halocarbon, but also on the solvent. The halogen bond also affected the Cα-F (C-F bond on the center carbon) bending and stretching vibrations in iso-C3F7I. These spectroscopic effects show some solvent dependence, but more importantly, they suggest the possibility of intermolecular halogen bonding among iso-C3F7I molecules. The systems were also examined by 19F NMR in various solvents (cyclohexane, hexane, chloroform, acetone, and acetonitrile). NMR dilution experiments support the existence of the intermolecular self-halogen bonding in both iso-C3F7I and 1-C3F7I. The binding constants for the pyridine/perfluoroalkyl iodide halogen bonding complexes formed in various solvents were obtained through NMR titration experiments. Quantum chemical calculations were used to support the FTIR and 19F NMR observations. 1. Introduction Halogen bonding, a noncovalent interaction between a halogen atom acting as an electron acceptor and an electron-rich Lewis base, has been known for nearly 150 years [1–6]. However, only recently has halogen bond started to attract wide-spread attention among scientists in a diversity of fields. Metrangolo and coworkers have reviewed the basic concepts as well as the major applications of the halogen bond [7, 8]. Examples include a wide range of applications in separation science, synthesis of liquid crystals and electronic materials, and the assembly of functional super molecules [9–18]. Recently, Meyer and Dubois [19] highlighted the application of halogen bonding to the synthesis of functional materials such as liquid crystal, nonlinear optical, magnetic conducting material, and halogen bonding based surface modification. A review by Beale et al. [20] provides an informative summary of solution thermodynamics and applications of halogen bonding. Of particular importance are their presentations of detailed thermodynamic parameters measured for the halogen bonding systems made of organic donors and halogen anions in the solution phase and which emphasized their importance in the anion recognition. Erdélyi [21] reviewed the nature of halogen bonding in solution phase and summarized the
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