The Electronic Structure of Ionic Liquids

The electronic structure of the imidazolium cations is of interest because it impacts on the hydrogen bond acceptor and donor properties of ionic liquids. This in turn relates to the penchant of the solvent to coordinate to, or react with the solvated species. The imidazolium cation is isoelectronic with the carbene -like imidazole-2-ylidene. Theoretical calculations on deprotonation of the unsubstituted imidazolium cations determine pfCaS of24.90 and 32.97 for the proton at the and C positions 

Until recently the primary interest in the electronic stmcture calculations has been to derive partial charges on the imidazohum cation for use in classical simulations [11,13,14,16,62,83]. The charge distribution however depends strongly on the method of analysis. The Natural Atomic Orbital (NAO) [84-88] and MuUiken population analysis methods have been apphed to the imidazolium cations [16,64]. A number of methods have been used to determine partial charges from the electrostatic potential these also show significant variation. There is an especially large 

The Cl anion is thus primarily attracted to the positive C -H unit, which sits exposed in an area depleted of electron density. The association of the anion with 

Solvatochromic experiments have shown [BMIM]+-based ionic liquids to be good hydrogen-bond donors [95], however, complementary GC-based experiments have shown these same ionic liquids to be poor hydrogen-bond donors [96]. This discrepancy has been resolved, and traced back to the nature of the probe molecules. 

The dye used in the solvatochromic experiments is zwitterionic while the probe molecules used in the GC experiment are neutral. Less effective hydrogen bonds are formed with the neutral as opposed to the highly polarized probe molecules. 

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Hunt PA, Kirchner B, Welton T (2006) Characterising the electronic structure of ionic liquids an examination of the l-butyl-3-methylimidazolium chloride ion pair. Chem Eur J 12 ... 

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