The ubiquitous role of amine compounds in biological systems, as functional groups in proteins, or as messenger molecules (neurotransmitters), as nucleophilic agents in (SN1, SN2) reactions, as reducing agent in electron transfer and as proton accepting site in acid-base chemistry cannot be overemphasized. In all these cases the interactions between amines and the solvent play a decisive role in the reaction dynamics. We explore the solute-solvent interactions of alkylamines and conjugate protonated alkylammonium ions using both infrared and soft-x-ray spectroscopies as local probes of the specific interactions of the amine groups. We have developed a novel liquid flatjet system with which solution phase soft-x-ray spectroscopy can conveniently be performed. This liquid water flatjet with thicknesses in the micrometer range, stable for tens to hundreds of minutes, fully operational under vacuum conditions (&lt;10-3 mbar), opens up new frontiers in steady-state and time-resolved soft-x-ray spectroscopy of solution phase systems. We have quantitatively assessed the hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH3) and ammonium ion (NH4+) in terms of electronic structure, solvation structure, and dynamics. Aqueous NH4+ ion interacts via comparatively strong N–H donating interactions, whereas aqueous NH3 has only one strongly accepting hydrogen bond with the water molecule at the nitrogen lone pair but only weakly N–H donating hydrogen bonds. Energy mismatch between occupied orbitals of both the solutes NH3 and NH4+ and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding, and –thus – inhibits substantial charge transfer between solute and solvent. Using our findings on aqueous ammonia and ammonium ion, 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. Our results benchmark the N K-edge spectroscopy of amine compunds with which we can design time-resolved soft-x-ray spectroscopy experiments using the flatjet technology.

Soft-X-ray spectroscopy of the amine group: Hydrogen bond motifs in alkylamine/alkylammonium acid-base pairs

256th National Meeting (2018)

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