Thermodynamic Property Of Imidazolium-based ILs, Theoretical And Experimental Study On Structure Of ILs

In this work, the density of two binary mixtures formed by 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] with aromatic compound (benzyl alcohol or benzaldehyde) has been determined, and the excess molar volumes (VmE) have been obtained from these experimental results and been fitted by the fourth-order Redlich-Kister equation. It is showed that the minimum in VmE reached with mole fraction of ionic liquid is different due to the difference of the substitution of the aromatic compound. In addition, the VmE values for benzaldehyde mixtures are more negative than those for the benzyl alcohol mixtures, which imply that in the benzaldehyde solutions there are stronger ion-dipole interactions and packing effects than in the benzyl alcohol solutions. As a result, it is indicated that the quasi-clathrates are probably formed in the mixtures of {[bmim][PF6] + aromatic compound (benzyl alcohol or benzaldehyde)}.Furthermore, structures and interaction details of the ionic liquids, which have same cation and different anions, were investigated systematically. It is found that the anion X- (X=OH, F, Cl, Br, I) can dramatically change the structure of the ionic liquids whose cation is [bmim]+ and anion is X-. As a result, [bmim]Cl, [bmim]Br and [bmim]I can exist stably in ionic pairs state, and [bmim]OH, [bmim]F can not exist stably in ionic pairs state. For [bmim]F, the most stable geometry have carbene-like species. For [bmim]OH, the most stable geometry is that hydroxide is added to the carbon two of the imidazole-ring.The transition state between the isomers of [bmim]X (X=OH, F) were performed by QST2 method. It is displayed that the energy between the transition state and the most stable geometry of [bmim]F is 46.38 kJ/mol, and the energy between the transition state and the most stable geometry of [bmim]OH is 123.59 kJ/mol. It is suggested that [bmim]F is more liable to converse its structure than that of [bmim]OH, and it is helpful to explain the difficulty in preparing [bmim]F.Considered with the results obtained by theoretical investigation, the structure of [bmim]OH was studied here. Determined with FT-IR, there was a peak of 1666 cm-1, which indicated that the double bonds of C4=C5 formed and the imidazole-ring did not exist in same plane anymore, agreed with the geometry obtained by theoretical investigation. Furthermore, the results obtained by UV-spectrum and LC-MS also indicated that [bmim]OH did not exist stably in ionic pairs state. All of these results implied that theoretical investigation on the structure of the ionic liquids guided the experiment effectively.