Aqueous Solvation of Ammonia and Ammonium: Probing Hydrogen Bond Motifs with FT-IR and Soft X-ray Spectroscopy

Abstract

In a multifaceted investigation combining local soft X-ray and vibrational spectroscopic probes with ab initio molecular dynamics simulations, hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH) and ammonium ion (NH), are quantitatively assessed in terms of electronic structure, solvation structure, and dynamics. From the X-ray measurements and complementary determination of the IR-active hydrogen stretching and bending modes of NH and NH in aqueous solution, the picture emerges of a comparatively strongly hydrogen-bonded NH ion via N-H donating interactions, whereas NH has a strongly accepting hydrogen bond with one water molecule at the nitrogen lone pair but only weakly N-H donating hydrogen bonds. In contrast to the case of hydrogen bonding among solvent water molecules, we find that energy mismatch between occupied orbitals of both the solutes NH and NH and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding and, thus, inhibits substantial charge transfer between solute and solvent. A close inspection of the calculated unoccupied molecular orbitals, in conjunction with experimentally measured N K-edge absorption spectra, reveals the different nature of the electronic structural effects of these two key principal amine compounds imposed by hydrogen bonding to water, where a pH-dependent excitation energy appears to be an intrinsic property. These results provide a benchmark for hydrogen bonding of other nitrogen-containing acids and bases.

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

“…The absorption spectrum of NH 4 + (aq) has been published in our previous study on NH 3 (aq)/NH 4 + (aq). 5 We found that NH 4 + (aq) exhibits a strong main-edge band located at In addition, we measured ammonia NH 3 and diethylamine Et 2 NH both in water and in ethanol to investigate the influence of solute-solvent hydrogen bonding (see Figure 3). Here we note that NH 3 dissolved in ethanol has been prepared using concentrated aqueous NH 3 , which amounts to a molar ratio of shift, or the consequence of somewhat larger line broadening (in conjunction with larger spectral overlap with main-edge transitions) cannot be determined within the current measurement accuracy.…”

“…This suggests a different type of underlying transition than those ascribed to the post-edge of NH 3 (aq). 4,5 Moreover, despite similarities in pre-and main-edge frequency positions for the alkylamines, the step-wise change in the character of the unoccupied orbitals as the consecutive exchange of N-H for ethyl-groups is expected to be manifested in the N K-edge XAS. In Figure 2 (right column) we show the experimental N K-edge absorption spectra measured for the alkylammonium R y NH 4-y + (R= C 2 H 5 -(Et-); y=0..4) series dissolved in water.…”

“…1,2 Determination of the electronic structure of the amine group allows for a better understanding of its chemical reactivity. 3 Reports on the electronic structure of amines and conjugate protonated ammonium compounds deduced from soft-x-ray spectra are scarce, ranging from the benchmark ammonia/ammonium ion system, 4,5 to amino acids, 6,7,8,9,10,11 aromatic amines, 12,13,14 ionic liquids 15 and nucleic acid bases. 16 Among the soft x-ray spectroscopic methods suited for studying amines and ammonium compounds, N K-edge x-ray absorption spectroscopy (XAS)…”

“…This systematic investigation of the electronic sensitivity to ligand substitution and hydrogen bonding builds on and extends from our previously published results on ammonia and the ammonium ion in water. 5 (Merck, for analysis EMSURE). Concentrations used were ranging between 0.2 -0.5 M. All samples were filtered and degassed before each XAS measurement.…”

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

“…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.…”