Gerhard Schwaab

Aggregation-Induced Dissociation of HCl(H2O)4 Below 1 K: The Smallest Droplet of Acid

Minimally Acidic Acidity is usually construed in the context of a bulk liquid solvent: billions of trillions of molecules such as HCl, added to hundreds of billions of trillions of water molecules. What happens under sparser conditions, for example, in atmospheric or interstellar environments, when a single HCl molecule might interact with just three or four water molecules? Gutberlet et al. (p. 1545; see the Perspective by Zwier) explored this question using theoretical simulations together with vibrational spectroscopy in ultracold helium droplets that effectively isolated small aqueous HCl clusters. HCl remained intact upon solvation by one, two, or three water molecules. Dissociation into an ion pair, as occurs in bulk water, required the approach of a fourth water molecule and was facilitated by the geometry of the existing (H2O)3 cluster. Just four water molecules are sufficient to dissolve the acid HCl into a charged ion pair of proton and chloride. Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl− ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.

Rattling in the Cage: Ions as Probes of Sub-picosecond Water Network Dynamics

We present terahertz (THz) measurements of salt solutions that shed new light on the controversy over whether salts act as kosmotropes (structure makers) or chaotropes (structure breakers), which enhance or reduce the solvent order, respectively. We have carried out precise measurements of the concentration-dependent THz absorption coefficient of 15 solvated alkali halide salts around 85 cm(-1) (2.5 THz). In addition, we recorded overview spectra between 30 and 300 cm(-1) using a THz Fourier transform spectrometer for six alkali halides. For all solutions we found a linear increase of THz absorption compared to pure water (THz excess) with increasing solute concentration. These results suggest that the ions may be treated as simple defects in an H-bond network. They therefore cannot be characterized as either kosmotropes or chaotropes. Below 200 cm(-1), the observed THz excess of all salts can be described by a linear superposition of the water absorption and an additional absorption that is attributed to a rattling motion of the ions within the water network. By providing a comprehensive set of data for different salt solutions, we find that the solutions can all be very well described by a model that includes damped harmonic oscillations of the anions and cations within the water network. We find this model predicts the main features of THz spectra for a variety of salt solutions. The assumption of the existence of these ion rattling motions on sub-picosecond time scales is supported by THz Fourier transform spectroscopy of six alkali halides. Above 200 cm(-1) the excess is interpreted in terms of a change in the wing of the water network librational mode. Accompanying molecular dynamics simulations using the TIP3P water model support our conclusion and show that the fast sub-picosecond motions of the ions and their surroundings are almost decoupled. These findings provide a complete description of the solute-induced changes in the THz solvation dynamics for the investigated salts. Our results show that THz spectroscopy is a powerful experimental tool to establish a new view on the contributions of anions and cations to the structuring of water.

New p-Ge THz laser spectrometer for the study of solutions: THz absorption spectroscopy of water

We present the development of a high power, tunable far-infrared p-germanium laser spectrometer for the study of dissolved biomolecules in the THz range. As a first application we report on the measurement of the absorption coefficient α for liquid water in the frequency range from 81to96cm−1. Using the p-Ge laser spectrometer we were able to penetrate through layers of up to 100μm thickness. We discuss the advantages and the limitations of this THz spectrometer. We present an analysis of the experimental data based on a χ2 test to provide an objective procedure to minimize the influence of systematic effects, for example of interference due to multiple reflections within the sample chamber. The measured absorption coefficient α lies between (410±6) and (490±6)cm−1 at 81 and 96cm−1, respectively.

Understanding THz Spectra of Aqueous Solutions: Glycine in Light and Heavy Water

THz spectroscopy of aqueous solutions has been established as of recently to be a valuable and complementary experimental tool to provide direct insights into the solute-solvent coupling due to hydrogen-bond dynamics involving interfacial water. Despite much experimental progress, understanding THz spectra in terms of molecular motions, akin to mid-infrared spectra, still remains elusive. Here, using the osmoprotectant glycine as a showcase, we demonstrate how this can be achieved by combining THz absorption spectroscopy and ab initio molecular dynamics. The experimental THz spectrum is characterized by broad yet clearly discernible peaks. Based on substantial extensions of available mode-specific decomposition schemes, the experimental spectrum can be reproduced by theory and assigned on an essentially quantitative level. This joint effort reveals an unexpectedly clear picture of the individual contributions of molecular motion to the THz absorption spectrum in terms of distinct modes stemming from intramolecular vibrations, rigid-body-like hindered rotational and translational motion, and specific couplings to interfacial water molecules. The assignment is confirmed by the peak shifts observed in the THz spectrum of deuterated glycine in heavy water, which allow us to separate the distinct modes experimentally.

Urea, a structure breaker? Answers from THz absorption spectroscopy.

There has been a controversial debate of whether urea can be regarded as structure breaker or a structure maker. Here, we present concentration and temperature dependent absorption coefficients of urea-water mixtures in the THz range (1.5-10 THz, 50-350 cm(-1)). Our results are in agreement with the hypothesis that urea adapts ideally into the water network. Using a semi-ideal chemical association model and accompanying MD simulations, the observed spectra could be decomposed in three contributions: one is attributed to bulk water, a second one to rattling modes of weakly solvated urea in the surrounding water cage, and the third part accounts for THz modes describing a doubly hydrogen bonded strong solvent-solute interaction. The bands attributed to the rattling motion of the solute scale linearly with concentration. The intensity of this contribution is temperature independent in contrast to the water and strongly solvated solute absorption. We find that even at high urea concentrations the majority of water retains a bulklike absorption spectrum, whereas only a small number (about 0.5-1.1 per urea on average) are strongly bound in the temperature range between 9 and 36 °C. The THz absorption data provide no evidence for urea aggregation in the concentration range investigated (1-10 M).

Far-infrared multilayer mirrors

We present a concept for highly reflecting broadband mirrors in the far-infrared. We report on the fabrication of dielectric multilayer mirrors consisting of 1–6 layers of silicon wafers with vacuum gaps in between. By comparison to gold mirrors, we can demonstrate the high reflectivity (>99%) over the frequency range between 40 and 130 cm−1. The measurements are in very good agreement with theoretical predictions.

High resolution spectroscopy of HCl–water clusters: IR bands of undissociated and dissociated clusters revisited

We report a detailed study on the IR spectroscopy of HCl-water complexes in superfluid helium nanodroplets in the frequency range from 2660 to 2675 cm(-1). We have recorded spectra of HCl-H2(16)O as well as of HCl-H2(18)O complexes and compared these results with theoretical predictions. In addition, we have carried out mass-selective intensity measurements as a function of partial pressure of HCl as well as of H2(18)O (pick-up curves). The results support a scenario where the IR-absorption in this part of the spectrum contains contributions from undissociated as well as from dissociated clusters with Cl(-)(H2O)3(H3O)(+) being the smallest dissociated complex. These findings are corroborated by additional electric field measurements yielding the orientation of the vibrational transition moment with respect to the permanent dipole moment. As a result we are able to assign a broad absorption band starting at 2675 cm(-1) to dissociated HCl-water clusters (HCl)1(H2O)n with n ≥ 4. The two narrow absorption lines at 2667.9 cm(-1) and 2670 cm(-1) are assigned to an undissociated cluster, in agreement with previous studies.

Solvation Dynamics of Trimethylamine N-Oxide in Aqueous Solution Probed by Terahertz Spectroscopy

We have studied the hydration dynamics of trimethylamine N-oxide (TMAO) in aqueous solution using a combination of concentration-dependent terahertz/far-infrared (THz/FIR) and Raman spectroscopic techniques. Terahertz/FIR absorption was measured using narrowband (76-93 cm(-1)) p-Ge laser and broad band (30-400 cm(-1)) Fourier transform spectroscopy. We used principal component analysis in combination with a semi-ideal chemical equilibrium model to dissect the spectra into linear and nonlinear contributions of the solvated solute extinction. We attribute the linear part to the average extinction and Raman scattering of TMAO-water aggregates with approximately 3-4 water strongly hydrogen bonded to TMAO. An additional nonlinear concentration dependence indicates a decrease of the number of attached water molecules with increasing TMAO concentrations due to a shift in association equilibria. The Raman spectra reveal a frequency shift of the (narrowband) intramolecular vibrations with decreasing dilution. Based on the results of a detailed analysis and isotopic substitution, the experimentally observed absorption bands at 0, 176, and 388 cm(-1) could be assigned to water relaxation modes, an intermolecular TMAO-H2O stretch, and the C-N-C bending mode, respectively. Our results provide evidence for a local modification of the water structure.

Terahertz time-domain spectroscopy as a new tool for the characterization of dust forming plasmas

We report the application of terahertz time-domain spectroscopy as a new tool for plasma diagnostics. The short broadband THz pulses were radiated from a low temperature grown GaAs emitter by free charge carriers which were generated by focusing a 20 femtosecond TiSa-laser pulse onto the emitter. For sensitive signal recording a coherent detection scheme was applied. This allowed the measurement of the amplitude and sign of the electromagnetic field of the THz pulse after passing the plasma chamber. Fourier transformation allowed us to obtain the full spectrum in the frequency domain.We compared the transmitted THz intensities of a pure argon (Ar) and an acetylene (C2H2)/argon plasma. The presence of the ethynyl–radical (CCH) and cyclopropenylidene (c-C3H2) in the (C2H2)/argon plasma could be confirmed by the observations of rotational transitions in the region from 8 to 16 cm−1 corresponding to 0.3–0.5 THz.

Terahertz cavity-enhanced attenuated total reflection spectroscopy

We realized cavity-enhanced absorption spectroscopy in the terahertz spectral region by combining multilayer mirrors with an attenuated total reflection technique. Using this technique, we were able to observe an absorbance of biological samples with a minimum detectable concentration as low as 8×10−10mol∕mm2. The absorbance between 75cm−1 and 115cm−1 was measured using a monolithic resonator of high-purity silicon. We demonstrate that the sensitivity of the attenuated total reflection design is significantly improved by adding multilayer mirrors for the THz region.