The sensitivity of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to Brønsted donation and the protonation state of nitrogen in the solid state is investigated through a series of multicomponent bipyridine− acid systems alongside X-ray photoelectron spectroscopy (XPS) data. A large shift to high energy occurs for the 1s → 1π* resonance in the nitrogen K-edge NEXAFS with proton transfer from the acid to the bipyridine base molecule and allows assignment as a salt (CNH + ), with the peak ratio providing the stoichiometry of the types of nitrogen species present. A corresponding binding energy shift for CNH + is observed in the nitrogen XPS, clearly identifying protonation and formation of a salt. The similar magnitude shifts observed with both techniques relative to the unprotonated nitrogen of co-crystals (CN) suggest that the chemical state (initial-state) effects dominate. Results from both techniques reveal the sensitivity to identify proton transfer, hydrogen bond disorder, and even the potential to distinguish variations in hydrogen bond length to nitrogen. ■ INTRODUCTIONProton (hydrogen) transfer can be thought of as one of the simplest chemical reactions, ranging from complete transfer from an acidic to a basic moiety (protonation through Brønsted donation) to varying degrees of sharing through hydrogen bonding. Whether Brønsted proton transfer occurs has a profound effect on the location of protons in crystal structures and influences chemical and physical properties. These interactions can be employed to target properties of solid forms (crystal engineering), with particular relevance to the pharmaceutical industry where acid/base guest molecules can be combined with active ingredients to tailor properties such as solubility and bioavailability through formation of salts and cocrystals. 1−4 Other relevant fields are organic ferroelectrics, 5 energetic materials, 6 and the design of materials with targeted optical properties such as color 7 and luminescence. 8,9 Even among hydrogen bonds, the level of interaction with the donor and acceptor atoms can vary significantly, from relatively weak to strong with quasi-covalent character, 10 and there is also the possibility of disordered hydrogen bonds. 11 While X-ray diffraction (XRD) and solid-state nuclear magnetic resonance spectroscopy (ssNMR) are often techniques of choice for structural characterization, 2,11−14 they are not always unambiguous with regard to proton locations (although further clarity can often be obtained by neutron diffraction). 11,13−16 The importance of accurate characterization of salts vs co-crystals based on this relatively small difference in proton location should not be underestimated, particularly with the wider implications for intellectual property and regulatory control in the pharmaceutical industry. 4,17 X-ray photoelectron spectroscopy (XPS) has recently been shown to unequivocally identify whether intermolecular proton transfer occurs in a range of two-component systems and distinguish protonation (salt...
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).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.