Through X-ray absorption and emission spectroscopies, the chemical, electronic
and structural properties of organic species in solution can be observed.
Near-edge X-ray absorption fine structure (NEXAFS) and resonant inelastic
X-ray scattering (RIXS) measurements at the nitrogen K-edge of para-
aminobenzoic acid reveal both pH- and solvent-dependent variations in the
ionisation potential (IP), 1s→π* resonances and HOMO–LUMO gap. These changes
unequivocally identify the chemical species (neutral, cationic or anionic)
present in solution. It is shown how this incisive chemical state sensitivity
is further enhanced by the possibility of quantitative bond length
determination, based on the analysis of chemical shifts in IPs and σ* shape
resonances in the NEXAFS spectra. This provides experimental access to
detecting even minor variations in the molecular structure of solutes in
solution, thereby providing an avenue to examining computational predictions
of solute properties and solute–solvent interactions
