Characterizing the properties of the cation and ion pair for the cyclic tetramethylguanidinium nitrate ionic liquid

We have carried out a detailed theoretical study for the geometrical structures and electronic properties of the cation and the ion pair of the cycle tetramethylguanidinium nitrate ionic liquid. It is found that in the cation, the triazolium ring presents almost a plane and the NMe2 group is not coplanar with the ring. The central C2 atom carries high positive charge, which is expected to act as the most active site for the electrostatic attacking of an anion. For the ion pair, three most stable configurations have been located, where the anion lies on either side of the triazolium ring or between C5 and C6 in the cation via multiple hydrogen bonds. However, the calculated results show that the electrostatic interaction between cation and anion plays a crucial role for stabilizing the ion pair. Moreover, the charge-localized character of the cation effectively increases the electrostatic interaction between the cation and anion, which is mainly responsible for the high thermal stabilities of the ionic liquids. There is a close correlation between the charge on C2 and the relative stability of the ion pair: the more positive charge on C2, the more stable the ion pair. Natural bond orbital and frontier molecular orbital analyses reveal that the charge transfer from the anion to cation occurs mainly through the LPO→σC-H∗, LPO→πC2-N3∗ or LPO→πC2-N1∗ interactions.