Soft X-ray Spectroscopy of the Amine Group: Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid–Base Pairs
2018
Ekimova, Maria | Kubin, Markus | Ochmann, Miguel | Ludwig, Jan | Huse, Nils | Wernet, Philippe | Odelius, Michael | Nibbering, Erik T. J.
We use N K-edge absorption spectroscopy to explore the electronic structure of the amine group, one of the most prototypical chemical functionalities playing a key role in acid–base chemistry, electron donor–acceptor interactions, and nucleophilic substitution reactions. In this study, we focus on aliphatic amines and make use of the nitrogen 1s core electron excitations to elucidate the roles of N–H σ* and N–C σ* contributions in the unoccupied orbitals. We have measured N K-edge absorption spectra of the ethylamine bases EtₓNH₃–ₓ (x = 0...3; Et– = C₂H₅−) and the conjugate positively charged ethylammonium cation acids EtyNH₄–y⁺ (y = 0...4; Et– = C₂H₅−) dissolved in the protic solvents ethanol and water. Upon consecutive exchange of N–H for ethyl-groups, we observe a spectral shift, a systematic decrease of the N K-edge pre-edge peak, and a major contribution in the post-edge region for the ethylamine series. Instead, for the ethylammonium ions, the consecutive exchange of N–H for ethyl groups leads to an apparent reduction of pre-edge and post-edge intensities relative to the main-edge band, without significant frequency shifts. Building on findings from our previously reported study on aqueous ammonia and ammonium ions, we can rationalize these observations by comparing calculated N K-edge absorption spectra of free and hydrogen-bonded clusters. Hydrogen bonding interactions lead only to minor spectral effects in the ethylamine series, but have a large impact in the ethylammonium ion series. Visualization of the unoccupied molecular orbitals shows the consecutive change in molecular orbital character from N–H σ* to N–C σ* in these alkylamine/alkylammonium ion series. This can act as a benchmark for future studies on chemically more involved amine compounds.
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