Brønsted interactions in organic crystal structures: towards crystallographic structure refinement using core level spectroscopies

Hydrogen Bonds are vital to the crystal structure of organic crystals, and the nature of the hydrogen bonding interaction has an effect on the properties of these crystals. Being able to accurately define the position of a hydrogen atom in a molecular structure is difficult with X-ray diffraction based techniques, and more accurate techniques often require onerous sample preparation or long measurement times. As an alternative, core level X-ray spectroscopy is investigated as a technique for hydrogen atom position refinement, with a particular focus on hydrogen bonding in organic crystals. Through the use of XPS and NEXAFS spectroscopy, an analysis procedure has been developed which consistently achieves the absolute core level binding energies and excitation energies. From these, a direct dependence on the hydrogen atom position in relation to a proton acceptor has been determined through a shift in the core level binding energy of +2 eV. These shifts are also reproduced in ab initio DFT calculations and allow hydrogen bonding interactions to be easily classified as salt or cocrystal. An investigation to identify the cause of the +2 eV shift was successful in demonstrating this shift using electrostatics and can be entirely attributed to the electrostatic potential difference between the two hydrogen atom positions as measured at the core level of the proton acceptor. Furthermore, by utilising the different dependencies of XPS and NEXAFS, the molecular structure of a number of crystals has been refined through comparison to theoretical spectra, demonstrating that highly accurate structures can be obtained due to this direct sensitivity to hydrogen atom position.