Ekimova, M.; Kubin, M.; Ochmann, M.; Ludwig, J.; Huse, N.; Wernet, P.; Odelius, M.; Nibbering, E.: Soft X-ray Spectroscopy of the Amine Group: Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid-Base Pairs. The Journal of Physical Chemistry B 122 (2018), p. 7737-7746
10.1021/acs.jpcb.8b05424
Open Access Version
Abstract:
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 postedge 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.