In this work, the geometrical and electronic properties of the mono cationic ionic liquid 1-hexyl-3-methylimidazolium halides ([C6(mim)]+ _X , X=Cl, Br and I) and dicationic ionic liquid 1,3-bis[3-methylimidazolium-1-yl]hexane halides ([C6(mim)2X2], X=Cl, Br and I) were studied using the density functional theory (DFT). The most stable conformer of these two types ionic liquids (IL) are determined and compared with each other. Results show that in the most stable conformers, in both monocationic ILs and dicationic ILs, the Cl and Br anions prefer to locate almost in the plane of the imidazolium ring whereas the I anion prefers nearly vertical location respect to the imidazolium ring plan. Comparison of hydrogen bonding and ionic interactions in these two types of ionic liquids reveals that these ionic liquids can be formed hydrogen bond by Cl and Br anion. The calculated thermodynamic functions show that the interaction of cation – anion pair in the dicationic ionic liquids are more than monocationic ionic liquids and these interactions decrease with increasing the halide anion atomic weight.
