Hydrogen Bonding And Halogen Bonding Interactions In Some Solvent Systems, An Excess Infrared Spectroscopy Study

Ionic liquids(ILs) are green solvents which have been used in diverse fields and developed drastically in recent years. Hydrogen-bonding interactions greatly influence the structure and physical properties of ILs and the mixtures of ILs and cosolvents. Similar to hydrogen-bonds, halogen-bond interactions have been focus of many studies to understand liquid structures recently. In this work, the hydrogen-bond interactions between a number of IL–solvent systems and the halogen-bond and hydrogen-bond interactions between fluoro-benzene derivatives and dimethyl sulfoxide(DMSO) were investigated by infrared spectroscopy, combined with nuclear magnetic resonance and quantum chemical calculations. Excess infrared spectroscopy and two-dimensional(2D) correlation infrared spectroscopy were used to analyze IR spectra in detail.In this work, the hydrogen bond interactions in three IL–solvent systems were investigated. The three IL–solvent systems are 1-butyl-3-methylimidazolium tetrafluoroborate([BMIM][BF4])–acetonitrile(CH3CN), [BMIM][BF4]–DMSO and 1-propylnitrile-3-methylimidazolium tetrafluoroborate([PCNMIM][BF4])–DMSO systems. The conclusions are as follows:(1) In all the three systems, the cation and anion can interact with the organic molecules simultaneously. When interacting with the cation, the orgainic molecules prefer to interact with the aromatic C–H. C2–H is the most preferred interaction site.(2) The methyl group of orgainic molecules locate in the shielding field of imidazolium ring.(3) Excess infrared spetra can enhance the spectra’s resolution.(4) In [BMIM][BF4]–CH3CN system, the cations and anions exist in ion cluster, ion cluster–CH3CN, ion pair and ion pair–CH3CN complexes. In the concentration range(the mole fraction of CH3 CN is between 0.1 and 0.9), CH3 CN cannot break apart the strong Columbic interactions between cation and anion. So there exist no individual cations and anions.(5) In [BMIM][BF4]/[PCNMIM][BF4]–DMSO system, when the mole fraction of DMSO is lower than 0.6, only larger ion clusters can be broken into smaller ion clusters; when the mole fraction of DMSO is between 0.6 to 0.9, ion clusters can be broken into ion pairs; when the mole fraction of DMSO is larger than 0.9, ion pairs begin to be broken into individual cations and anions. Introducing a nitrile group into the alkyl chain did not influence the dilution process of IL in [PCNMIM][BF4]-DMSO system.(6) The [PCNMIM]+ cation can work as hydrogen-bond donor using its electron-poor group such as aromatic hydrogen atoms, and also can be used as hydrogen-bond acceptor using electron-rich nitrile group.Besides the hydrogen bond interactions between IL and solvent, five fluoro-benzene derivatives were also selected to a comparative study of halogen-bond(HB) and hydrogen-bond(XB). Several conclusions can be drawn.(1) XBs are more directional than HBs. The directional order is iodine-based halogen-bond > bromine-based halogen-bond > chlorine-based halogen-bond > hydrogen-bond.(2) The interaction energy of the HB is between the Br-XB and I-XB.(3) The separation distance-dependence of the interaction energies were described in the form of Lennard-Jones Equation. The attractive energies were found to depend on 1/r3.3 for HB, and 1/r4.0, 1/r3.2 and 1/r3.1 for I-XB, Br-XB and Cl-XB, respectively; all the repulsive energies of XBs and HB decay with 1/r8.5.(4) Upon the formation of HB or XBs, electron transfers from DMSO to benzene derivatives.