Geometries and vibrational frequencies of oxyacids and carboxylic acids. a study on structural and vibrational effects

Vibrational spectroscopic study of pH dependent solvation at a Ge(100)-water interface during an electrode potential triggered surface termination transition

The charge-dependent structure of interfacial water at the n-Ge(100)-aqueous perchlorate interface was studied by controlling the electrode potential. Specifically, a joint attenuated total reflection infrared spectroscopy and electrochemical experiment was used in 0.1M NaClO₄ at pH ≈ 1-10. The germanium surface transformation to an H-terminated surface followed the thermodynamic Nernstian pH dependence and was observed throughout the entire pH range. A singular value decomposition-based spectra deconvolution technique coupled to a sigmoidal transition model for the potential dependence of the main components in the spectra shows the surface transformation to be a two-stage process. The first stage was observed together with the first appearance of Ge-H stretching modes in the spectra and is attributed to the formation of a mixed surface termination. This transition was reversible. The second stage occurs at potentials ≈0.1-0.3 V negative of the first one, shows a hysteresis in potential, and is attributed to the formation of a surface with maximum Ge-H coverage. During the surface transformation, the surface becomes hydrophobic, and an effective desolvation layer, a "hydrophobic gap," developed with a thickness ≈1-3 Å. The largest thickness was observed near neutral pH. Interfacial water IR spectra show a loss of strongly hydrogen-bound water molecules compared to bulk water after the surface transformation, and the appearance of "free," non-hydrogen bound OH groups, throughout the entire pH range. Near neutral pH at negative electrode potentials, large changes at wavenumbers below 1000 cm^-1 were observed. Librational modes of water contribute to the observed changes, indicating large changes in the water structure.

Electrode potential dependent desolvation and resolvation of germanium(100) in contact with aqueous perchlorate electrolytes

The electrode potential dependence of the hydration layer on an n-Ge(100) surface was studied by a combination of in situ and operando electrochemical attenuated total reflection infrared (ATR-IR) spectroscopy and real space density functional theory (DFT) calculations. Constant-potential DFT calculations were coupled to a modified generalised Poisson-Boltzmann ion distribution model and applied within an ab initio molecular dynamics (AIMD) scheme. As a result, potential-dependent vibrational spectra of surface species and surface water were obtained, both experimentally and by simulations. The experimental spectra show increasing absorbance from the Ge-H stretching modes at negative potentials, which is associated with an increased negative difference absorbance of water-related OH modes. When the termination transition of germanium from OH to H termination occurs, the surface switches from hydrophilic to hydrophobic. This transition is fully reversible. During the switching, the interface water molecules are displaced from the surface forming a "hydrophobic gap". The gap thickness was experimentally estimated by a continuum electrodynamic model to be ≈2 Å. The calculations showed a shift in the centre of mass of the interface water by ≈0.9 Å due to the surface transformation. The resulting IR spectra of the interfacial water in contact with the hydrophobic Ge-H show an increased absorbance of free OH groups, and a decreased absorbance of strongly hydrogen bound water. Consequently, the surface transformation to a Ge-H terminated surface leads to a surface which is weakening the H-bond network of the interfacial water in contact.

Stable solid and aqueous H2CO3 from CO2 and H2O at high pressure and high temperature

Carbonic acid (H₂CO₃) forms in small amounts when CO₂ dissolves in H₂O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H₂CO₃ plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO₂/H₂O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H₂CO₃ inside celestial bodies. We present a novel method to prepare solid H₂CO₃ by heating CO₂/H₂O mixtures at high pressure with a CO₂ laser. Furthermore, we found that, contrary to present understanding, neutral H₂CO₃ is a significant component in aqueous CO₂ solutions above 2.4 GPa and 110 °C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C–O–H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H₂CO₃, a role in subduction related phenomena is inferred.

Overtone spectroscopy of sulfonic acid derivatives

Vapor-phase OH-stretching overtone spectra of methanesulfonic acid and trifluoromethanesulfonic acid were recorded in the Deltav(OH) = 4 and 5 regions using cavity ring-down spectroscopy. We compare these spectra to those of sulfuric acid to consider the effect on vibrational overtone spectra of replacing one of the OH groups with a more or less electronegative group. We complement our experimental work with anharmonic oscillator local mode calculations of the OH-stretching frequencies and intensities. The presence of a weak intramolecular interaction between the hydrogen atom of the OH group and the oxygen atom of the adjacent S=O group in methanesulfonic acid lowers its OH-stretching frequency from what would otherwise be predicted based on the electronegativity of the methyl group.