Core level spectroscopies locate hydrogen in the proton transfer pathway – identifying quasi-symmetrical hydrogen bonds in the solid state

Abstract

Short, strong hydrogen bonds (SSHBs) have been a source of interest and considerable speculation over recent years, culminating with those where hydrogen resides around the midpoint between the donor and acceptor atoms, leading to quasi-covalent nature. We demonstrate that X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy provide deep insight into the electronic structure of the short OHN hydrogen bond of 3,5-pyridinedicarboxylic acid, revealing for the first time distinctive spectroscopic identifiers for these quasi-symmetrical hydrogen bonds. An intermediate nitrogen (core level) chemical shift occurs for the almost centrally located hydrogen compared to protonated (ionic) and non-ionic analogues, and it reveals the absence of twosite disorder. This type of bonding is also evident through broadening of the nitrogen 1s photoemission and 1s -1p* peaks in XPS and NEXAFS, respectively, arising from the femtosecond lifetimes of hydrogen in the potential wells slightly offset to either side of the centre. The line-shape of the core level excitations are thus related to the population occupancies, reflecting the temperature-dependent shape of the hydrogen potential energy well. Both XPS and NEXAFS provide a distinctive identifier for these quasi-symmetrical hydrogen bonds, paving the way for detailed studies into their prevalence and potentially unique physical and chemical properties.

A quantum crystallographic approach to short hydrogen bonds

“…In the absence of a barrier to a second site, the potentials are ‘slide like’ with no clear second minimum. Such a behaviour is also observed from experimental XPS 37 and molecular dynamics studies 83 of 3,5-pyridinedicarboxylic acid and urea phosphoric acid 25 reported in the literature. We also see this for related materials containing the same R 2 2 (8) carboxylic acid : amide hydrogen bonded dimer (Fig.…”

“…30 The experimental EDD analysis is combined with first principle calculations of properties including the reduced density gradient from non-covalent interaction analysis, 31 the electrostatic potential, [32][33][34] PIXEL interaction energies 35,36 and H-atom potentials. [37][38][39] A combined analysis approach is commonly used to assess interaction characteristics in terms of material properties. [40][41][42] Here it allows a complete characterisation of the short hydrogen bonds (Fig.…”

Bulk and Surface Conformations in Solid-State Lovastatin: Spectroscopic and Molecular Dynamics Studies

“…These include a short intermolecular contact between the carbonyl and an acidic proton on adjacent pyranyl rings and a long H-bond between a carbonyl group on the butanoate ester and an alcohol on an adjacent pyranyl ring. Redistribution of electron density at the O atoms from these electrostatic interactions is well known to shift the binding energies of the core-level electrons [69] by a similar magnitude to the shifts observed in the O1s spectrum presented here. The relatively broader peaks in the O1s spectrum also suggest that the 5 O species might be distributed over more than just two defined states.…”

Influences of Absorbed Dose Rate on the Mechanical Properties and Fiber–Matrix Interaction of High-Density Polyethylene-Based Carbon Fiber Reinforced Thermoplastic Irradiated by Electron-Beam

“…However, up-shifts in the binding energies for C=O and O=C-O bonds for C1s and O1s orbital electrons were observed when increasing the absorbed dose rate. These BE shifts originated from the hydrogen bond interactions of C-OH, C=O, and O=C-O generated by irradiation at the HDPE and CF interface [42][43][44]. It is known that as the hydrogen bond interaction is enhanced, the C1s and O1s binding energies of C-OH increase because C-OH (δ + ) acts as a hydrogen bond donor.…”

“…These results were consistent with the FT-IR analysis, which showed that stronger absorption centered at~3350 cm −1 corresponding to intermolecular OH bonding was observed at lower absorbed dose rates. irradiation at the HDPE and CF interface [42][43][44]. It is known that as the hydrogen bond interaction is enhanced, the C1s and O1s binding energies of C-OH increase because C-OH (δ + ) acts as a hydrogen bond donor.…”