Halogen bonding in iodo-perfluoroalkane/pyridine mixtures

Mole fraction and temperature studies of halogen bonding between 1-iodo-perfluorobutane, 1-iodoperfluorohexane, or 2-iodo-perfluoropropane and pyridine were performed using noisy light-based coherent anti-Stokes Raman scattering (I⁽²⁾ CARS) spectroscopy. The ring breathing mode of pyridine both is highly sensitive to halogen bonding and provides a strong I⁽²⁾ CARS signal. As the lone pair electrons from the pyridinyl nitrogen interact with the σ-hole on the iodine from the iodo-perfluoroalkane, the ring breathing mode of pyridine blue-shifts proportionately with the strength of the interaction. The measured blue shift for halogen bonding of pyridine and all three iodo-perfluoroalkanes is comparable to that for hydrogen bonding between pyridine and water. 2-Iodo-perfluoropropane displays thermodynamic behavior that is different from that of the 1-iodo-perfluoroalkanes, which suggests a fundamental difference at the molecular level. A potential explanation of this difference is offered and discussed.