In this work, the structures of the -SO 3 H functionalized acidic ionic liquid 1-(3-sulfonic acid) propyl-3-methylimidazolium hydrogen sulfate ([C 3 SO 3 Hmim]HSO 4 ), including its precursor compound (zwitterion), cation, and cation-anion ion-pairs, were optimized systematically by the DFT theory at B3LYP/6-311??G** level, and their most stable geometries were obtained. The calculation results indicated that a great tendency to form strong intramolecular hydrogen bonds was present in the zwitterion, and this tendency was weakened in the cation that was the protonation product of zwitterion. The intramolecular hydrogen bonds and intermolecular hydrogen bonds coexisted in the ionic liquid, and they played an important role in the stability of the systems. The strongest interaction in the ionic liquid was found between the anion and the functional group. The transition state research and the intrinsic reaction coordinate analysis of the hydrogen transfer reaction showed that, when the cation and the anion interacted near the functional group by double O-HÁÁÁO hydrogen bonds, the ionic liquid was inclined to exist in a form of the zwitterion and H 2 SO 4 .

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Density functional theory has been used to investigate the structure, stability, and aromaticity of a series of nitrogen-containing thiocarbons, which are related to C n S n 2-(n = 3-5). We have identified a large number of minimum-energy species which might be synthesized and calculated their aromaticity using the nucleus-independent chemical shift (NICS) method of Schleyer and magnetic susceptibility. Successive substitution of carbon by nitrogen reduces their stability, as reflected in the calculated bond orders. In general, there is no close correlation between the stability and p-aromaticity of the species studied.

The geometries of the complexes of Li ? , Na ? , K ? , Be 2? , Mg 2? , and Ca 2? metal cations with different possible 2,6-dithiopurine anions (DTP) were studied. The complexes were optimized at the B3LYP level and the 6-311??G(d, p) basis set. The interactions of the metal cations at different nucleophilic sites of various possible 2,6-dithiopurine anions were considered. It was revealed that metal cations would interact with 2,6-dithiopurine anions in a bicoordinate manner. In the gas phase, the most preferred position for the interaction of Li ? , Na ? , and K ? cations is between the N 3 and S 2 sites, while all divalent cations Be 2? , Mg 2? , and Ca 2? prefer binding between the N 7 and S 6 sites of the corresponding 2,6-dithiopurine. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between 2,6-dithiopurine anions and the metal cations. It was revealed that aqueous solution would have significant effect on the relative stability of complexes obtained by the interaction of 2,6-dithiopurine anions with Mg 2? and Ca 2? cations. The effect of metal cations on different NH and CS stretching vibrational modes of 2,6-dithiopurine has also been discussed.