The density functional method and DNP basis set were employed to study 1-pair, 2-pair, and 3-pair of cation-anion interactions of 1-propyl-4,5-dibromo-3-methylimidazolium bromides. The calculated results reveal that H-2 atoms of 1-propyl-4,5-dibromo-3-methylimidazolium form hydrogen bondings with Br. The natural bond orbital (NBO) and atoms in molecules (AIM) analyzes show that halogen bondings between Br anion and imidazolium ring substituted bromine atoms were formed in the gas phase, in agreement with the crystal phase of 1-propyl-4,5-dibromo-3-methylimidazolium bromide which is governed by both hydrogen bondings and halogen bondings. (C) 2011 Elsevier B.V. All rights reserved.