  Hydrogen bonds (HBs) play a crucial role in the physicochemical properties of
ionic liquids (ILs). At present, HBs between cations and anions (Ca-An) or
between cations (Ca-Ca) in ILs have been reported extensively. Here, we
provided DFT evidences for the exists of HBs between anions (An-An) in the IL
1-(2-hydroxyethyl)-3-methylimidazolium 4-(2-hydroxyethyl)imidazolide
[HEMIm][HEIm]. The thermodynamics stabilities of anionic, cationic, and H2O
dimers together with ionic pairs were studied by potential energy scans. The
results show that the cation-anion pair is the most stable one, while the HB in
anionic dimer possesses similar thermodynamics stability to the water dimer.
The further geometric, spectral and electronic structure analyses demonstrate
that the inter-anionic HB meets the general theoretical criteria of traditional
HBs. The strength order of four HBs in complexes is cation-anion pair > H2O
dimer = cationic dimer > anionic dimer. The energy decomposition analysis
indicates that induction and dispersion interactions are the crucial factors to
overcome strong Coulomb repulsions, forming inter-anionic HBs. Lastly, the
presence of inter-anionic HBs in ionic cluster has been confirmed by a global
minimum search for a system containing two ionic pairs. Even though
hydroxyl-functionalized cations are more likely to form HBs with anions, there
still have inter-anionic HBs between hydroxyl groups in the low-lying
structures. Our studies broaden the understanding of HBs in ionic liquids and
support the recently proposed concept of anti-electrostatic HBs.
