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Theα-form of crystalline para-aminobenzoicacid (PABA) has been examined as a model system for demonstratinghow the core level spectroscopies X-ray photoelectron spectroscopy(XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can becombined with CASTEP density functional theory (DFT) to provide reliablemodeling of intermolecular bonding in organic molecular crystals.Through its dependence on unoccupied valence states NEXAFS is an extremelysensitive probe of variations in intermolecular bonding. Predictionof NEXAFS spectra by CASTEP, in combination with core level shiftspredicted by WIEN2K, reproduced experimentally observed data verywell when all significant intermolecular interactions were correctlytaken into account. CASTEP-predicted NEXAFS spectra for the crystallinestate were compared with those for an isolated PABA monomer to examinethe impact of intermolecular interactions and local environment inthe solid state. The effects of the loss of hydrogen-bonding in carboxylicacid dimers and intermolecular hydrogen bonding between amino andcarboxylic acid moieties are evident, with energy shifts and intensityvariations of NEXAFS features arising from the associated differencesin electronic structure and bonding. [ABSTRACT FROM AUTHOR]

X ‐ ray Absorption Spectroscopy In many chemical and biological processes, proton transfer plays a crucial role by impacting the electronic structure and functional properties of materials. Direct evidence of a proton transfer within the solid state is captured by chemical shifts in X ‐ ray absorption spectroscopy (XAS) in the Communication by J. S. Stevens et al. on page 15600 ff. Heating of the squaric acid/4,4′ ‐ bipyridine system leads to a dramatic, rapid phase and colour transition through a cascading Brønsted acid ‐ base reaction, showcasing XAS as an unambiguous identifier for proton transfer without the need for a priori information and the ability to monitor organic reactions in situ. [ABSTRACT FROM AUTHOR]

The dramatic colour and phase alteration with the solid-state, temperature-dependent reaction between squaric acid and 4,4'-bipyridine has been probed in situ with X-ray absorption spectroscopy. The electronic and chemical sensitivity to the local atomic environment through chemical shifts in the near-edge X-ray absorption fine structure (NEXAFS) revealed proton transfer from the acid to the bipyridine base through the change in nitrogen protonation state in the high-temperature form. Direct detection of proton transfer coupled with structural analysis elucidates the nature of the solid-state process, with intermolecular proton transfer occurring along an acid-base chain followed by a domino effect to the subsequent acid-base chains, leading to the rapid migration along the length of the crystal. NEXAFS thereby conveys the ability to monitor the nature of solid-state chemical reactions in situ, without the need for a priori information or long-range order., (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Ionic bond or hydrogen bridge? Brønsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)