Novel diol functionalized dicationic ionic liquids: synthesis, characterization and DFT calculations on H-bonding influence on thermophysical properties

Abstract

Two new dicationic ionic liquids (DILs) functionalized with diols on their linkages were synthesized. Elemental analysis, FT-IR, ¹H and ¹³CNMR spectroscopies were applied to identify the molecular structure of these compounds. Some thermophysical properties of these DILs, including density, thermal behavior and heat capacity, were investigated. To find the effect of the hydroxyl groups and H-bonding on the thermophysical properties of these DILs, another DIL with the same anion and linkage chain length but without a hydroxyl functionality was synthesized. Density functional theory (DFT) was used to study the adsorption of the anion on the cation and the strength of H-bonding in 2,2-bis(1-(1-methylimidazolium)methylpropane-1,3-diol dicyanamide). For this purpose, the electronic and structural properties including binding energies (E_b), band gap values (E_g) and partial densities of state (PDOS) were calculated. DFT calculations revealed both strong H-bonding and electrostatic interactions between the hydroxyl groups of the cation and dicyanamide anion in this DIL. Among the more interesting findings for diol functionalized DILs, their thermal stability range generally is higher than DILs without a hydroxyl functionality. The decomposition temperature for 1,3-bis(3-methylimidazolium-1-yl)propane dicyanamide is 465.2 K, whereas 2,2-bis(1-(1-methylimidazolium)methylpropane-1,3-diol dicyanamide) and 2,2-bis(1-(1-vinylimidazolium)methylpropane-1,3-diol dicyanamide) have higher T_d values of 524.6 and 525.8 K, respectively. In addition, diol functionalized DILs have higher heat capacities and densities than corresponding DILs without diol functionality.