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Soft X-ray Spectroscopy of the Amine Group: Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid-Base Pairs
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Number of Authors: 82018 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 122, no 31, p. 7737-7746Article in journal (Refereed) Published
Abstract [en]

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 is core electron excitations to elucidate the roles of N-H sigma* and N-C sigma* 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 sigma* to N-C sigma* 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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2018. Vol. 122, no 31, p. 7737-7746
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Chemical Sciences Physical Sciences
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URN: urn:nbn:se:su:diva-160121DOI: 10.1021/acs.jpcb.8b05424ISI: 000441484500011PubMedID: 30024171OAI: oai:DiVA.org:su-160121DiVA, id: diva2:1248872
Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-17Bibliographically approved

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Wernet, PhilippeOdelius, Michael
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