N K-edge XAS measurements for ionic liquids and N-heterocycle molecules in liquid phase analyzed by simulations with explicit inclusion of solvent molecules

Abstract

We present a framework for interpreting nitrogen K-edge x-ray absorption of ionic liquids and imidazole derivatives in water on a single absolute energy scale. Liquid transmission measurements provide reproducible spectra that can be compared across solutes without ad hoc shifting. Experimental line shapes fall into three patterns defined by the availability of low-lying π acceptors. Pyrrolidinium and ammonium-like centers show no discrete pre-edge. Imidazolium cations show a single pre-edge. Imidazole derivatives show a robust doublet. Real-space calculations with explicit inclusion of water molecules test how hydration controls these patterns. A radius scan in simulation identifies a practical cluster size that captures the first hydration shell while avoiding spurious long-range structure. Direct summation of simulated spectra based on 100 molecular dynamics snapshots on a common energy scale converts discrete transitions into a continuous liquid-phase profile. It merges the two ring nitrogen atoms of imidazolium into one pre-edge and preserves the imidazole doublet with improved intensity ratios. A minimal protocol that combines absorber-centered p DOS with targeted COOP maps separates σ and π channels and locates hydration-sensitive π windows without broader projections. Hydration increases low-energy π overlap while leaving the σ onset comparatively stable. The result is a transferable set of rules for assignment and for choosing simulation settings that match spectra for liquid phases.