C K-edge x-ray absorption of imidazole motifs and ionic liquids in aqueous solutions: Explicit solvent simulations and ensemble effects

We report transmission-mode C K-edge x-ray absorption spectra of aqueous solutions containing methanol, acetonitrile, imidazole, 2-methylimidazole, and the chloride salts [BMIM]Cl, [BMMIM]Cl, and [BMPyrr]Cl. The measured spectra exhibit three reproducible pre-edge patterns: (i) no distinct low-energy feature for methanol and [BMPyrr]Cl, (ii) a single broadened band near 287 eV for acetonitrile, imidazole, and [BMIM]Cl, and (iii) a split pre-edge structure around 287.5 eV for 2-methylimidazole and [BMMIM]Cl. To interpret the spectra on a common E−EF energy scale, molecular dynamics sampling was combined with full-potential finite-difference x-ray absorption calculations using FDMNES, including screened core-hole effects. Cluster-size analysis indicates that an intermediate explicit-solvent radius provides stable reproduction of the π*-derived spectral region. Ensemble averaging over 100 MD snapshots reveals that hydration primarily broadens and disperses the π* peak energies, whereas the σ* region remains comparatively insensitive to solvent fluctuations. Site-resolved spectral analysis further demonstrates that imidazole and [BMIM]Cl exhibit unsplit pre-edge features because the low-lying π* contributions from the ring carbon atoms substantially overlap. In contrast, 2-methylimidazole and [BMMIM]Cl develop a distinct split structure when the contribution from the carbon atom located between the two nitrogen atoms becomes energetically separated from those of the other ring carbons. These results clarify how local electronic structure and hydration jointly govern the pre-edge features of aqueous heterocyclic and ionic liquid systems.