Solid phases of water, such as ice (Ih) and clathrate hydrates, form characteristic hydrogen bond network motifs, such as hexagonal ice, pentagons, and dodecahedrons. The same motifs might be present in supercooled water and in the hydration structure around hydrophobes. Here, we present the characteristic low frequency fingerprints of ice (Ih), tetrahydrofuran (THF) clathrate hydrates, and tetrabutyl-ammonium bromide (TBAB) semiclathrate close to their melting point, as well as supercooled water at 266.6 K and aqueous alcohol solutions. Interestingly, we find in all these cases two characteristic resonances in the THz frequency range: at least, one intensive band in the frequency range between 190 cm−1 and 220 cm−1 which is a characteristic of a tetrahedral hydrogen bond network configuration and a second band in the frequency range between 140 cm−1 and 170 cm−1, indicating a component with weaker hydrogen bonds. For solvated alcohols, we find spectroscopic fingerprints of a clathratelike structure at 164 cm−1 as well as a tetrahedral network structure at 194 cm−1, which is close to one of ice (Ih) at 192 cm−1. We propose that in the hydration shell of hydrophobes, both structural motifs are present. In the case of supercooled water—unlike ice—only one peak was found in the frequency range between 190 cm−1 and 220 cm−1. Interestingly, the latter peak center-frequency (204 cm−1) corresponds to the average of those of the two peaks observed for ice Ih (191 cm−1 and 215 cm−1). This indicates a homogeneous intermediate hydrogen bonding, providing no evidence for any heterogeneity in two high-density and low-density phases.

Based upon precise terahertz (THz) measurements of the solvated amino acid glycine and accompanying ab‐initio molecular‐dynamics simulations, we show that the N‐C‐C‐O open/close mode at 315 cm−1 serves as a sensitive, label‐free probe for the local protonation of the amide group. Experimentally, we can show that this holds not only for glycine but also for diglycine and valine. The approach is more general, since the changes due to protonation result in intensity changes which can be probed by THz time domain (0–50 cm−1) as well as by precise THz‐FT spectroscopy (50–400 cm−1). A detailed analysis allows us to directly correlate the titration spectra with pKa values. This demonstrates the potential of THz spectroscopy to probe the charge state of a natural amino acid in water in a label‐free manner.

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl− hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

Based upon precise terahertz (THz) measurements of the solvated amino acid glycine and accompanying ab‐initio molecular‐dynamics simulations, we show that the N‐C‐C‐O open/close mode at 315 cm−1 serves as a sensitive, label‐free probe for the local protonation of the amide group. Experimentally, we can show that this holds not only for glycine but also for diglycine and valine. The approach is more general, since the changes due to protonation result in intensity changes which can be probed by THz time domain (0–50 cm−1) as well as by precise THz‐FT spectroscopy (50–400 cm−1). A detailed analysis allows us to directly correlate the titration spectra with pKa values. This demonstrates the potential of THz spectroscopy to probe the charge state of a natural amino acid in water in a label‐free manner.

Transport mechanisms of solvated protons of 1 M HCl acid pools, confined within reverse micelles (RMs) containing the negatively charged surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field simulations to interpret dynamical signatures from TeraHertz absorption and dielectric relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs as due to headgroup and counterion charges that expel hydronium and chloride ions from the interface and into the bulk interior, thereby increasing the pH of the acid pools relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of hydronium near a counterion or conjugate base reduces the Eigen and Zundel configurations that enable forward hopping. Thus, localized oscillatory hopping dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even oscillatory hopping is slow.