Soft X-ray Spectroscopy of the Amine Group: Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid–Base Pairs

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 Et NH ( x = 0...3; Et- = CH-) and the conjugate positively charged ethylammonium cation acids Et NH ( y = 0...4; Et- = CH-) 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.

Inner-shell X-ray absorption spectra of the cationic series NHy+ (y = 0–3)

“…2 a clear shift of the resonant structures to lower photon energies with increasing number y of H atoms is found for the series of free NH y + ions. This is contrary to shifts found for the alkylamines, 37 where the N 1s resonances are shifting to higher photon energy with increasing number of H atoms.…”

“…35 Yet, there is no detailed information available on the inner-shell XAS of the cationic series NH y + apart from the atomic cation, N + , which has been studied previously by Gharaibeh et al 36 Furthermore, the role of NH y with different numbers y of hydrogen atoms has recently been studied in the liquid phase by Ekimova et al for alkylamines in different solutions. 37,38 They show a strong dependence of the N 1s X-ray spectra on the number y of H atoms and the solution itself.…”

Sensitivity of Nitrogen K-Edge X-ray Absorption to Halide Substitution and Thermal Fluctuations in Methylammonium Lead-Halide Perovskites

“…This can be improved with a larger broadening, but consequently, the spectral detail (in particular the pre-edge shoulder) is reduced. The performance of the transition potential approximation has been evaluated before in N 1s XA spectra of ammonium species 12 , 13 and in which similar artifacts related to the transition potential approximation have been observed. Thus, we probe the sensitivity in the calculated spectra to geometric variation only around the pre- and main-edge features.…”

“…This is also in line with the previous results of Ekimova et al, who argued that the appearance of an ammonia pre-edge was due to differences in the symmetry of the lowest unoccupied molecular orbital (LUMO) as compared to the symmetric ammonium which had no pre-edge and that the slight pre-edge for aqueous ammonium is related to instantaneous distortions of symmetry. 13 …”

“… 12 Analogous studies of a series of ethylammonium with Et y NH 4– y + ( y = 0...4; Et = C 2 H 5 ) in solution is of direct relevance for comparison to our present data. 13 Additionally, recently, Kot et al showed that nitrogen does cause a noticeable change in the electronic structure of the MAPI system using X-ray photoelectron spectroscopy (XPS) and XAS on the N 1s edge. 3 …”