Theoretical study on 1-butyl-3-methyl-4,5-dibromoimidazolium dis(trifluoromethane sulfonyl)imide and 1-(3,4-dibromobutyl)-3-methylimidazolium dis(trifluoromethane sulfonyl) imide ionic liquids

► Two brominated imidazolium ionic liquids were studied. Intra-pair interactions are hydrogen bonds. ► Inter-pair interactions in 2-pair [BMIM-Br]+[Tf2N]− are halogen bonds. ► Inter-pair interactions in 2-pair [Br-BMIM]+[Tf2N]− are hydrogen bonds.