Characterising the electronic structure of ionic liquids: An examination of the 1-butyl-3-methylimidazolium chloride ion pair

In this paper we analyse the electronic properties of gas-phase 1-butyl-3-methylimidazolium Cl ion pairs, [C(4)C(1)iM]Cl, in order to deepen our understanding of ionic liquids in general. Examination of charge densities, natural bond orbitals (NBO), and delocalised molecular orbitals computed at the B3LYP and MP2/6-31++G-(d,p) levels have enabled us to explain a number of experimental phenomena: the relative acidity of different sites on the imidazolium ring, variations in hydrogen-bond donor and acceptor abilities, the apparent contradiction of the hydrogen-bond-donor parameters for different types of solute, the low probability of finding a Cl- anion at the rear of the imidazolium ring and the expansion of the imidazolium ring in the presence of a strong hydrogen-bond acceptor. The unreactive but coordinating environment and large electrochemical window have also been accounted for, as has the strong electron-donating character of the carbon atoms to the rear of the ring in associated imidazolylidenes. The electronic structure of the [C(4)C(1)im](+) cation is best described by a C-4=C-5 double bond at the rear, and a delocalised three-centre 4e(-) component across the front (N-1-C-2-N-3) of the imidazolium ring; delocalisation between these regions is also significant. Hydrogen-bond formation is driven by Coulombic stabilisation, which compensates for an associated destabilisation of the electronic part of the system. Interactions are dominated by a large positive charge at C-2 and the build up of pi-electron density above and below the ring, particularly that associated with the double bond between C-4 and C-5. The NBO partial charges have been computed and compared with those used in a number of classical simulations.