Profound insight on hydrogen bonding of water in its different phases (liquid, surface, solid phase) has been acquired using soft X-ray absorption spectroscopy, probing the oxygen 1s core excitations to unoccupied molecular orbitals of the water molecule, that directly reflect the hydrogen bonding donating and accepting interactions of bulk water. To obtain information on hydrogen bonding of solvation shell water, one instead can probe solute molecules in aqueous solution, by inspection of element specific transitions associated with the functional groups directly involved in hydrogen bonding with the solvent shell water. For the latter approach we have initiated a study of the amine group using N K-edge spectroscopy of solution phase amine compounds under different charge and solvation conditions. For this we have used a liquid flatjet system with which solution phase soft-x-ray spectroscopy can conveniently be performed. Continuing our studies on aqueous ammonia (NH3) and ammonium (NH4+), we have investigated the N K-edge spectra of alkylamines RxNH3-x (x = 0..3) and respective alkylammonium RyNH4-y+ (y = 0..4) conjugate acids. Consecutive exchange of N-H with alkyl-groups leads to a diminishing of those transitions accentuated by hydrogen bond interactions, i.e. pre-edge and post-edge transitions. Interestingly additional transitions to unoccupied orbitals reflecting N-C σ* character are only pronounced in the neutral alkylamine compounds, whereas for alkylammonium ions these are less apparent. Going to aromatic amines, we learn that for anilinium C6H5NH3+ the spectral features are similar to those of the protonated alkylamines, reflecting the local nature of the LUMOs of the protonated amino-group. Instead aniline C6H5NH2 shows a rich detailed N K-edge XAS pattern, indicative of delocalized LUMOs accessed by the pre- and main-edge transitions. Recent progress in flatjet technology has enabled us to probe the O K-edge of water as a solute in acetonitrile solution. This spectacular advance paves the way for probing O K-edge spectra of water selectively probing its interactions as hydration species in water-poor conditions. With this we expect new detailed insight into the nature of hydrogen bonding and its effects on the electronic structure of water.

Soft X-ray spectroscopy of hydrogen-bonded molecular systems in protic and aprotic solvents

2019 Fall National Meeting

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