Experimental validation of calculated atomic charges in ionic liquids.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Ionic liquids (ILs) are versatile solvents with unique properties.
• Accurate atomic charge determination is crucial for understanding IL behavior.
• Experimental validation of computational methods for ILs is needed.

Purpose of the Study:

• To experimentally measure nitrogen atomic charges in nine ionic liquids.
• To validate computational methods for calculating atomic charges in ILs.
• To provide physical insights into nitrogen atomic charge distribution in ILs.

Main Methods:

• Combined X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy.
• Experimental charge measurements were compared against three computational methods: ChelpG, Natural Bond Orbital (NBO) analysis, and Atoms in Molecules (AIM).
• Results were integrated with prior studies on sulfur atomic charges.

Main Results:

• Experimental nitrogen atomic charges were successfully measured for nine ILs.
• Charges from electrostatic potentials using a grid-based method (ChelpG) showed the best agreement with experimental data.
• ChelpG charges exhibited significant conformational dependence, necessitating cautious interpretation of small charge differences (<0.3 e).

Conclusions:

• ChelpG is the preferred method for calculating charge distribution in ILs among the tested methods.
• The conformational dependence of ChelpG charges must be considered for accurate analysis.
• Validated atomic charges offer valuable physical insights into the behavior of ionic liquids.