Ion-pair Models and Possible Corrections

Typically, the ionic liquid is best considered as an ion-pair (or ion, on an individual basis). The main forces of attraction between the ions are the electrostatic forces and dispersion (van der Waals) forces. At intermediate distances, there is a slight 

The basis set is 6-31G(d,p), and electron correlation at the MP2 level is included. A similar structure is obtained with the AMI and PM3 semi-empirical methods. Density functional theory at the B3LYP/6-31G(dp,p) level also produced the same structure for this ion-pair. The only observed differences between the semi-empiri-cal and the ab initio structures were slightly shorter hydrogen bonds (PM3 and AMI) between El, E2, and F5 and the C2-H (H18) on the imidazolium ring. 

Note that DFT structures are as reliable as or more reliable than HF structures obtained with similar or less complex basis sets. 

Robert Carper, Zhizhortg Mertg, Andreas Dblle 

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