| In the search for alternatives to conventional solvents partly driven by the needfor ‘green’ chemistry and sustainable technology, but primarily because of thepotential for novel synthetic routes and process designs, a number ofenvironmentally benign media have been explored recently. Room-temperature ionicliquids （RTILs） are one such class of solvents. To expand the utility of ionic liquids,IL-based mixed solvents have come into focus. In particular the solubility of water isan important factor for the industrial use of ILs in synthesis, electrochemistry,catalysis, industrial cleaning, solvent extraction and separations. It has beendemonstrated that the addition of water can strongly affect the physical and chemicalproperties of ILs, such as viscosity, electrical conductivity, reactivity, polarity,surface characters, as well as solvation and solubility properties. Hence, theinformation on the structures of ILs and their interactions with water are importantnot only in the fundamental researches but also in the practical applications. In thispaper, new insights into the structure, interaction energy, and frequency of[Tf2N]-（W）nand [EMIM][Tf2N]（W）n（n=16） were revealed from Gaussian basedDFT/B3LYP methods, and the water states and its effect on the interaction andarrangement of [EMIM][Tf2N] were explored. This provides reliable parameter forfurther dynamics simulations.Firstly, the intereaction between water moleculers and [Tf2N]-of corresponding[Tf2N]-（H2O）n（n=16） were systematically studied at the B3LYP/6-311+G（d,p）levels by optimized strucrures, interaction energy, and atoms in the moleculecalculation （AIM）, and so on.[Tf2N]-anion has two geometries:cis-[Tf2N]-（C1-[Tf2N]-） and trans-[Tf2N]-（C2-[Tf2N]-）. It has been indicated thatC1-[Tf2N]-anion prefer to make a H-bonded network with the water molecules, dueto its relatively large dipole moment. It has been shown that for [Tf2N]-（H2O）n, thewater molecules toward to form clusters. Additionally, the molecular state of waterin ILs has been fully discussed through the calculated Ï… shifts, defined as thedifferences of the stretching vibration Ï…3（asymmetric） and Ï…1（symmetric）. The interaction between [EMIM]+and [Tf2N]-were studied atB3LYP/6-311+G（d,p） levels. According to the results, three most stable isomerswere selected to interact with water moleculers. According to the electrostaticpotential energy surface of [EMIM][Tf2N], the negative charges are more locatedaround the atoms of [Tf2N]-which are easily attacked by protons, meanwhile theimidazole ring of cation contains positive charges which are more easily attacked byO atom of water. At the low water content, the water molecules preferly formedH-bond between the anion and cation, and the water molecules are “brige-likeâ€.With increasing water content in the mixtures, the environment of both ILs andwater are changing all the time, and the state of water molecules changes frommonomer and dimer to water clusters which interact with the anion and cation. Theelectrostatic interactions between [EMIM]+and [Tf2N]-are partially screened by theintruding water molecules, and as the water content increases the continuousattenuation of the cation-anion interactions is observed. Simultaneously, the dipolemoment gradually decreases as the water aggregation between the cation and anion,thus if the cation involves the moderate or long alkyl tail, the nonpolar groups willprefer to aggregate, and the most ordered micelle structure should be formed.The investigation on interaction between [EMIM][Tf2N] ionic liquids andwater molecules can preliminary explore the micromechanism of interaction, thestate of water molecules, provide some initial hints as to the nanostructural evolutionand the presence of turnover point in ILs/water mixtures. This dissertationestablishes theoretical base for the application of [EMIM][Tf2N] ionic liquid in thepreparation of nanostructured, enzyme catalysis, and so on. |
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