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Acta Cryst. (2014). A70, C531
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Physicochemical properties of molecular crystals are significantly influenced by non-covalent interactions and proton transfer. A well known application is the tuning of solubility, bioavailability and stability of pharmaceutical actives through co-crystal (hydrogen-bonding) or salt (ionic, Brønsted acceptors) formation. X-ray Photoelectron Spectroscopy (XPS) is an intrinsically local structural probe, providing information on the chemical state and chemical environment of atoms in molecules and crystals through the photoemission of core level electrons. We have recently studied a wide range of acid-base complexes in molecular crystals and found that analyzing the chemical shifts of N1s core level binding energies provides a facile route for characterizing the chemical and structural changes at functional groups involved in hydrogen bonding and proton transfer [1]. Very importantly, XPS unequivocally distinguishes protonated (salt) from hydrogen-bonded (co-crystal) nitrogen moieties. We have complemented our results for nitrogen species with 15N Solid-State Nuclear Magnetic Resonance (ssNMR) chemical shifts, which reveal low frequency shifts with protonation, but the magnitude of these shifts is additionally influenced by the wider chemical environment [2]. When crystallographic structure information is available, ssNMR shifts can be computationally predicted and thereby related to H-bond lengths, giving a measure of H-bond strength (NMR crystallography). The wide variety of donor/acceptor systems we have investigated has covered a large range of pKa values and demonstrates the generic nature of taking an XPS/ssNMR/XRD approach to organic molecule crystallography (Fig 1). The excellent agreement between the conclusions drawn by XPS and combined ssNMR/CASTEP investigations opens up a reliable avenue for local structure characterization in molecular systems even in the absence of crystal structure information, for example with non-crystalline or amorphous matter.

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Acta Cryst. (2014). A70, C560
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Determining the location of hydrogen is not always straightforward, despite its potential for wide-reaching effects, such as altering physicochemical properties and biological/chemical processes. Proton transfer can be considered a simple chemical reaction, with a continuum from neutral to protonated states, and short, strong H-bonds (SSHB) and disordered systems between the two extremes. X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) intrinsically probe the local environment, with sensitivity to the chemical state of the atom and, importantly, nature of the local chemical and bonding environment. Organic molecular crystals have been studied by nitrogen XPS and NEXAFS, offering an alternative to X-ray and neutron diffraction. Strong chemical shifts occur with proton transfer to nitrogen (+N-H---O vs. N---H-O), unambiguously characterizing protonated and H-bonded systems,[1] leading to direct observation of an unusual solid-state colour change for 4,4'-bipyridine/squaric acid with heating[2] involving proton transfer to nitrogen with temperature-dependent measurements. Correlation between H-bond lengths and chemical shifts indicates potential for predicting H-bond lengths. SSHBs provide an interesting case, as hydrogen can reside midway between donor and acceptor, having a 3-centre, 4-electron bond with quasi-covalent character and atypical properties. Intermediate chemical shifts are found with hydrogen midway between donor and acceptor in 3,5-pyridinedicarboxylic acid, with increased peak width representative of hydrogen's broadened single minimum potential well.[3] This contrasts with conventional 2-site hydrogen disorder, in which signals from both donor and acceptor environments result in 2 peaks reflecting the % occupancy. Valuable electronic and structural information is obtained from the variety of organic systems investigated, with XPS clearly distinguishing different types of crystallographic materials (Fig 1).

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Acta Cryst. (2014). A70, C1022
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The designed creation of crystalline materials with desired physical properties is a key objective of crystal engineering. Currently the development of multi-component crystals such as salts and co-crystals as a route for the modification of physicochemical properties has been a major focus within the field. However, while the creation of such materials has been repeatedly demonstrated, understanding the structure-property relationships between the component molecules and the final crystal form is great challenge, so limiting the ability to design new materials. Controlling the proton transfer process is vital for the designed creation of protonic conductive materials but also important in other fields as the proton location alters the physical properties of other systems such as pharmaceutical or photochromic materials. The interaction between chemical structure and local crystallographic environment has been shown to alter the energy landscape of the proton transfer process [1,2] This presentation will report on work investigating the relationships between changes in molecular and crystal structure on proton transfer processes in multi-component materials. Both experimental crystal growth and computational modelling have been used to study proton transfer in binary and ternary systems based on the carboxylic acid...pyridine hydrogen bond. Understanding the interplay between packing forces and proton transfer in controlling the observed photochromism in bipy/rac-mandelic acid system and how this can be used to design new material based on these concepts.
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