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


School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK and The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK. [Email]


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 two-site disorder. This type of bonding is also evident through broadening of the nitrogen 1s photoemission and 1s → 1π* 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.

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