Chemical Speciation and Bond Lengths of Organic Solutes by Core-Level Spectroscopy: pH and Solvent Influence on p-Aminobenzoic Acid

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. Seeing into solution: Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy acts as a “chemical detector” for the species present in solution. It identifies neutral, anionic and cationic forms of p‐aminobenzoic acid depending on the pH and solvent, and provides an indicator of CN bond lengths in situ (see figure).