In this paper we analyse the electronic properties of gas-phase 1-butyl-3-methylimidazolium Cl ion pairs, [C4C1im]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 [C4C1im]+ cation is best described by a C4C5 double bond at the rear, and a delocalised three-centre 4 e− component across the front (N1-C2-N3) 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 C2 and the build up of π-electron density above and below the ring, particularly that associated with the double bond between C4 and C5. The NBO partial charges have been computed and compared with those used in a number of classical simulations.
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