Experimental Validation of Calculated Atomic Charges in Ionic Liquids Richard

A combination of X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy have been used to provide an experimental measure of nitrogen atomic charge in nine ionic liquids (ILs). These experimental results are used to validate charges calculated with three computational methods: charges from electrostatic potentials using a grid-based method (ChelpG), natural bond orbital (NBO) population analysis and the atoms in molecules (AIM) approach. By combining these results with those from a previous study on sulfur, we find that ChelpG charges provide the best description of charge distribution in ILs. However, we find that ChelpG charges can lead to significant conformational dependence and therefore advise that small differences in ChelpG charges (<0.3 e) should be interpreted with care. We use these validated charges to provide physical insight into nitrogen atomic charges for the ILs probed.