Johannes Hunger

Unraveling the fine details of hydrogen-bond structure and conformation within catalyst-substrate complexes is key to advancing organocatalysis. By employing two-dimensional infrared spectroscopy, this work provides evidence for a conformational transition of diphenylthiourea, induced by hydrogen-bond formation to 1,3-diphenyl-2-propenone in solution. Spectral shifts and coupling patterns indicate only one N–H (probed here via N–D) donor participating in hydrogen bonding. The vibrational spectra reveal a striking inhomogeneity in hydrogen-bond strength, and cross-peak analysis is consistent with single-site donor–acceptor geometry, rendering bifurcated interactions unlikely. Comparison of experimental spectral shifts upon H-bonding to harmonic frequencies from density functional theory calculations suggests a transition of diphenylthiourea from the cis–trans to the trans–trans conformation upon H-bond formation. These results highlight the interplay between molecular geometry and vibrational coupling, thereby helping to resolve the molecular structure of reactive intermediates in thiourea catalysis.

Ion association in aqueous electrolyte solutions depends critically on the interplay between ion hydration and electrostatic attraction such that different ion-pair species have been detected in aqueous 2:2 electrolytes with sulfate as the prototypical divalent anion. Dielectric relaxation (DR) spectroscopy has the potential to resolve and quantify the various ion-pair species, yet literature studies are too scarce to systematically address the effect of the nature of the cation on ion association. Here, we report a DR study of aqueous solutions of MnSO4 and FeSO4 at 25 °C, at frequencies 0.2 ≤ ν/GHz ≤ 115 and concentrations up to near-saturation (∼2.7 and ∼0.8 mol·L–1 respectively). The observed spectra were consistent with the presence of five Debye modes: two at higher frequencies (ν > ca. 10 GHz) that correspond to bulk-water processes, and three solute-related modes at lower frequencies. The slowest mode, centered at ν ≈ 0.3 GHz, was consistent with overlapping contributions from the rotation of double-solvent-separated ion pairs (2SIPs) and hydrated triple ions (M2SO42+, T+) as has been found for some other divalent metal sulfate solutions. The other lower frequency modes, centered at ν ≈ 1.5 GHz and ∼8 GHz, were assigned (respectively) to the rotation of solvent-shared (SIPs) and contact (CIPs) ion pairs, with the latter mode possibly also incorporating an unresolved contribution from “slow” bulk-water molecules. Effective total cation solvation numbers, Zt(M2+, aq), derived from the water-related modes, indicated that both Mn2+ and Fe2+ immobilize approximately 11 water molecules on the DR time scale, independent of solute concentration. Combination of the present results with literature data revealed a strong correlation between the types of ion aggregates detected by DR spectroscopy and the hydration enthalpy of the divalent metal cations (excepting Zn2+). Concentrations of the various species derived from the solute-related modes were used to calculate formation constants for each species that were, where comparisons were possible, in reasonable agreement with literature values obtained by other techniques.

Water and aqueous electrolytes at surfaces, interfaces, and in confinement behave differently from the bulk, yet linear spectroscopy rarely combines the chemical specificity, surface selectivity, and temporal resolution needed to resolve electric double-layer formation, specific-ion adsorption, and hydration at interfaces and under nanoconfinement. This review assesses how ultrafast nonlinear vibrational spectroscopy, comprising two-dimensional infrared spectroscopy, vibrational sum-frequency generation (SFG), and time-resolved SFG, probes the hydrogen-bond network and the dynamics of interfacial water and electrolytes on femtosecond-to-picosecond timescales. We first establish vibrational benchmarks for bulk water and simple electrolytes, then treat soft aqueous boundaries, including air–water, hydrophobic and hydrophilic, lipid, surfactant, and nucleic acid interfaces. We next examine solid–liquid and electrochemical systems, spanning metal oxides, minerals, and field-induced effects, before turning to water and proton dynamics in nanoconfined geometries and complex architectures. Throughout, experimental results are set against theoretical and computational frameworks, from polarizable force fields to machine-learning potentials. Our aim is to identify which molecular-level features of interfacial water are now firmly established, where the prevailing mean-field description of the electric double layer breaks down, and which experiments would resolve the outstanding discrepancies.

Specific ion effects are often unpredictable in their manifestation within collective behavior, as the underlying principles of how they mediate water–solute interactions remain unclear. In this study, we investigate the influence of simple ions on amino acids in electrolyte solutions, focusing on arginine, lysine, and serine in potassium iodide, potassium bromide, potassium chloride, sodium chloride, and lithium chloride solutions. We combine experimental techniques and molecular dynamics simulations to examine how monovalent ions affect the dielectric spectra of these complex solutions. Our results reveal three key effects of salt addition: (I) A decrease in the permittivity of the solution, a phenomenon known as the dielectric decrement, which is phenomenologically described by the Langevin function. The extent of the dielectric decrement is dependent on the specific ion. (II) An increase in the relaxation time of amino acids. Furthermore, the relaxation time of water is either increased or decreased depending on the specific salt–amino acid combination. The extent of the change in relaxation time compared to binary mixtures is dependent on the charge density of the added ions. (III) Long-range restructuring of amino acids and water leads to changes in their relative orientation over large distances. These findings provide new insights into how ions affect water–amino acid interactions and their impact on the dielectric properties of electrolyte solutions.

Collagen’s structural integrity and biological function derive from its characteristic triple helix, governed by a unique amino acid sequence containing the rare residue (2S,4R)-hydroxyproline (4R-Hyp). Substituting 4R-Hyp with its diastereomer 4S-Hyp markedly reduces triple-helix thermal stability and impairs collagen function. The origins of this destabilization have mostly been ascribed to stereoelectronic effects such as the gauche effect and n → π * interactions. Here, we dissect the role of different stabilizing interactions on molecular conformation using linear infrared and two-dimensional infrared spectroscopy, aided by density functional theory calculations. We investigate the structure of N -Boc-(2S,4S)-4-hydroxyproline-methyl ester (Boc-4S-Hyp-OMe) and N -Boc-(2S,4R)-4-hydroxyproline-methyl ester (Boc-4R-Hyp-OMe) in chloroform solution, a simplified model for the water-deficient environment in collagen tissues where triple helices assemble into fibrils. We find that, while stereoelectronic effects indeed influence molecular conformation, n → π * interactions only moderately alter conformational equilibria. Conversely, our results show that hydrogen bonding is pivotal: the conformation of Boc-4S-Hyp-OMe is stabilized by an intramolecular hydrogen bond, whereas Boc-4R-Hyp-OMe primarily forms intermolecular hydrogen bonds, leading to a greater intermolecular affinity. These findings highlight hydrogen bonding as a key determinant of hydroxyproline conformation at the single-residue level.

The modulation of acid-base equilibria by salts is fundamental to solution chemistry, yet its molecular origins in multicomponent electrolyte mixtures remain poorly understood. Building on evidence that individual salts induce ion-specific shifts in weak acid pK_a values, we combine experiment and constant pH molecular dynamics to investigate how ion pairing and specific ion effects govern the pK_a of acetic acid in single and mixed sodium salt solutions. Across anions of varying charge density (sulfate, chloride, iodide), we show that cation-anion association (not electrostatic screening alone) controls the observed pK_a shifts. Notably, in mixed sodium chloride-sulfate solutions, the effects on acetic acid pK_a deviate from additivity due to complex ion-ion interactions. We introduce an ion-pairing-augmented Debye-Hückel model that quantitatively captures these behaviors, providing a predictive framework for pH control in multisalt environments. This work establishes a mechanistic basis for interpreting and predicting pH-dependent phenomena in complex, real-world systems relevant to biochemistry and soft matter.

The modulation of acid-base equilibria by salts is a cornerstone of solution chemistry, yet the underlying mechanisms in complex, multi-component electrolyte mixtures remain poorly understood. While prior studies have established that individual salts can induce ion-specific shifts in the pKa of weak acids, the molecular origins of these effects-especially in mixed salt environments-have not been systematically elucidated. Here, we present a comprehensive investigation combining experiment and constant pH molecular dynamics simulations to unravel how ion pairing and specific ion effects govern the pKa of acetic acid in both single and mixed sodium salt solutions. By spanning a range of anion charge densities (sulfate, chloride, iodide), we demonstrate that the extent of cation-anion association, rather than electrostatic screening alone, dictates the observed pKa shifts. Notably, we reveal that in mixtures of sodium chloride and sodium sulfate, the effects on acetic acid pKa deviate from additivity owing to the underlying complexity of ion–ion interactions in solution. Our work introduces an ion-pairing-augmented Debye-Hückel framework that quantitatively predicts these behaviors, offering a predictive tool for pH control in complex ionic environments. This study establishes a mechanistic foundation for interpreting and forecasting pH-dependent processes in real-world, multi-salt systems—an advance with broad implications for biochemistry, buffer design, and soft matter science.

Aluminium-based aqueous batteries hold promises for next-generation sustainable and large-scale energy storage due to the favorable metrics of Al and water-based electrolytes. However, the performance of current aluminium-based aqueous batteries falls significantly below theoretical expectations, with a critical bottleneck of realizing cathodes with high areal capacities. Herein, we present a hydrate-melt electrolyte design utilizing cost-effective AlCl₃ and organic halide salts, which enables the demonstration of aqueous Al-Br batteries with enhanced energy-power characteristics. The optimal electrolyte features suppressed water activity and loosely bound halogen anions, attributed to its unique electrolyte structure, where the majority of water molecules engage in robust ion solvation (>98% as suggested by simulations) and halogen anions reside in the outer solvation sheath of cations. These distinctive features ensure good compatibility of the electrolyte with the reversible Br⁻/Br⁰/Br⁺ conversion, enabling cathodes with a high areal capacity of 5 mAh cm⁻². Besides, the electrolyte allows for Zn-Al alloying/de-alloying with minimal polarization (around 100 mV at 5 mA cm⁻²) and a smooth alloy surface. The assembled Al-Br cell delivers an energy density (267 Wh L⁻¹, based on the volume of anode, cathode and separator) comparable to commercial Li-ion batteries and a substantial power density (1069 W L⁻¹) approaching electrochemical capacitors.

The electric double layer (EDL) is critical in electrochemical capacitors and transistors, on-water chemistry, and bioelectric technologies. Ion dynamics within the EDL define the limits for charging and discharging processes. Classical EDL models struggle at high electrolyte concentrations, and observing EDL dynamics has been challenging. In this study, an all-optical technique allowed real-time monitoring of EDL dynamics at arbitrary concentration by quasi-instantaneously changing the surface propensity of protons (H ₃ O ⁺ ) adsorbed at the air-aqueous electrolyte solution interface and by subsequently tracking EDL relaxation with femtosecond time-resolved spectroscopy. EDL reorganization occurred on picosecond timescales and was strongly concentration dependent. Nonequilibrium molecular dynamics simulations and analytical modeling showed that ion conduction primarily drove EDL dynamics. This research quantified EDL dynamics and identified its primary driver, providing insights for optimization of electrochemical applications.

Nuclear quantum effects (NQEs) significantly influence material properties upon isotopic substitution, particularly with light atoms such as hydrogen. While water is rich in hydrogen, its hydrogen-bonded structure exhibits only moderate NQEs. Simulations ascribe this to competing zero-point energies (ZPEs): Intermolecular ZPEs stabilize hydrogen-bonds, while intramolecular ZPEs destabilize them. However, experimental validation has been lacking due to the difficulty in quantifying NQEs. The air/water interface provides an ideal platform to quantify NQEs in liquid water using surface-specific vibrational spectroscopy. By analyzing the excess/depletion of interfacial HOD, H ₂ O, and D ₂ O molecules with one free OH/OD group and the other H-bonded OH/OD group, we found that the intermolecular ZPE destabilizes the hydrogen-bonds by 0.74 ± 0.20 kilojoule per mole upon isotope substitution from H to D, while the intramolecular ZPE stabilizes them by 0.78 ± 0.33 kilojoule per mole. This near-complete cancellation explains the overall moderate NQE in liquid water. The interface thus allows for quantifying NQEs in water.