After analyzing the influence of the level of theory and basis sets for the monomeric ions, to further investigate the molecular interactions and conformational states of 1-ethyl-3 cation-anion pairs were optimized at B3LYP level in the gas phase and in a dielectric continuum solvent environment, and found that the anion in the lowest energy ion pair state adopted a trans conformation. Those studies showed that, in the gas phase, the lowest energy conformer exhibited a characteristically strong C 1 -H 1 ---N 1 interaction through the C The study of H-bonds in ILs is multifaceted with a range of experimental and computational methods employed. A traditional H-bond involves complex intermolecular interactions represented by X-H----Y, where a H-atom intercedes between two electronegative species X and Y, such as O or N. Standard H-bonds are primarily electrostatic interactions with covalent and dispersion contributions . The concept of X-H----Y hydrogen bonds may be extended to ionic liquids. Nevertheless, the characteristics and features of H-bonding in ILs cover an extremely wide and diverse range which is still the subject of scientific debate, and are thus not fully understood. H-bonding is not a binary on-off phenomenon but occurs in a graduated scale which makes quantifying and clearly demarking H-bonding difficult. To differentiate the H-bond in IL from other ionic H-bonds, we refer to the H-bond between [EMI] + and [TFSI] -species as the doubly ionic H-bond. Doubly ionic H-bonds are very common and occur within a large range of ILs. There are best reviews available in this topic . The doubly ionic H-bond between ). This suggests that the interionic H-bond interaction of the former is stronger than the latter two. When imidazolium cations are associated with large anionic groups, like [TFSI] -anion , there exist multiple H-bond interaction sites within the liquid environment and exhibit varying levels of Hbond strength and directionality. Additionally, these large anions take preferential on-top distributions above and below imidazolium rings, leading to π-type interactions. The delicate interplay of H-bond and π-type interactions in [EMI][TFSI] ionic species becomes more complicated than this and requires further investigation.