Kristina Noack

Concentration-dependent hydrogen-bonding effects on the dimethyl sulfoxide vibrational structure in the presence of water, methanol, and ethanol.

The effects of hydrogen bonding between dimethyl sulfoxide (DMSO) and the co-solvents water, methanol, and ethanol on the symmetric and antisymmetric CSC stretching vibrations of DMSO are investigated by means of Raman spectroscopy. The Raman spectra are recorded as a function of co-solvent concentration and reflect changes in structure and polarizability as well as hydrogen-bond donor and acceptor ability. In all cases studied a nonideal mixing behavior is observed. The spectra of the DMSO/water system show blue-shifted CSC stretching modes. The antisymmetric frequencies are always further blue-shifted than the symmetric stretching ones. The DMSO/methanol system also features blue-shifted CSC stretching frequencies but at high mole fractions a pronounced red shifting is observed. In the binary DMSO/ethanol system, the co-solvent also gives rise to blue shifts of the CSC stretching frequencies but restricted to mole fractions between x=0.38 and 0.45. The different magnitudes and occurrences of both blue- and red-shifted spectral lines are comprehensively and critically discussed with respect to the existing literature concerning wavenumbers and Raman intensities in both absolute and normalized values. In particular, the normalized Raman intensities show a higher sensitivity for the nonideal mixing behavior because they are independent of the mole fraction.

Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(Trifluoromethylsulfonyl)imide

Electronic structure theory (density functional and Møller-Plesset perturbation theory) and vibrational spectroscopy (FT-IR and Raman) are employed to study molecular interactions in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Different conformers of a cation-anion pair based on their molecular interactions are simulated in the gas phase and in a dielectric continuum solvent environment. Although the ordering of conformers in energy varies with theoretical methods, their predictions for three lowest energy conformers in the gas phase are similar. Strong C-H---N interactions between the acidic hydrogen atom of the cation imidazole ring and the nitrogen atom of the anion are predicted for either the lowest or second lowest energy conformer. In a continuum solvent, different theoretical methods yield the same ion-pair conformation for the lowest energy state. In both phases, the density functional method predicts that the anion is in a trans conformation in the lowest energy ion pair state. The theoretical results are compared with experimental observations from Raman scattering and IR absorption spectroscopies and manifestations of the molecular interactions in the vibrational spectra are discussed. The directions of the frequency shifts of the characteristic vibrations relative to the free anion and cation are explained by calculating the difference electron density coupled with electron density topography.

The Peculiar Nature of Molecular Interactions between an Imidazolium Ionic Liquid and Acetone

We present novel insights into the molecular interactions between polar solvents and imidazolium ionic liquids using the example of 1-ethyl-3-methylimidazolium ethyl sulfate and acetone. Recently published volumetric property data of this particular system have revealed peculiarities which could not be fully explained by steric effects. In order to shed light on the behavior at a molecular level, we apply IR spectroscopy and analyze solvent-induced line shifts as well as the excess IR spectra. From the spectroscopic results a conclusive picture of the site-specific molecular interactions is developed and our explanation is in concert with the volumetric effects. The data suggest the initial formation of trimers in which acetone interacts with existing ion pairs through interactions of the acetone oxygen atom with the imidazolium ring rather than forming directed hydrogen bonds at the CH moieties. With further addition of acetone, tetramers are formed which significantly weaken the interionic interactions and eventually initiate ion pair dissociation. Once the ions are released, the anion is rapidly saturated with acetone while the cation solvation proceeds more slowly with acetone addition.

Hydrogen Bonding in Mixtures of Dimethyl Sulfoxide and Cosolvents

Abstract: Dimethyl sulfoxide (DMSO) represents a dipolar aprotic solvent which incorporates a strongly polar sulfoxide group and two hydrophobic methyl moieties. Owing to its beneficial properties including low toxicity and environmental compatibility, DMSO has been and still is widely used as a solvent in industry as well as research. Applications can be found in many different areas ranging from medicine and biotechnology to electrochemistry and laser physics. In practical systems, DMSO is usually accompanied by other substances with whom it interacts at molecular scale. These interactions include, for instance, hydrogen bonds and van der Waals forces and determine the microscopic dissolution properties as well as the macroscopic solution behavior. Moreover, such interactions exert influence on the molecular structures of the involved molecules. In turn, this means that analyzing the molecular structure by means of theoretical and experimental approaches can shed light on the nature of interactions and help to achieve better understanding of the phenomena observed. In this article we review the literature reporting DMSO hydrogen bonding interactions with cosolvent molecules. In this context, theoretical as well as experimental studies are considered and compared in order to get a clear picture of this important solvent. Special attention is paid to the DMSO/water system which is well known for exhibiting a strongly nonideal mixing behavior. In addition, mixtures of DMSO with alcohols and other organic solvents are discussed.

Raman excess spectroscopy vs. principal component analysis: probing the intermolecular interactions between chiral molecules and imidazolium-based ionic liquids

Raman spectroscopy is a very sensitive and specific measurement tool for probing intermolecular interaction structures. As imidazolium-based ionic liquids can favorably be used for enantioseparation, in this work two approaches for Raman signal analysis, namely by means of excess spectra and principal component analysis (PCA), are compared to extract detailed information about the interaction structure of the enantiomers d- and l-glucose in an aqueous solution of the ionic liquid [EMIM][EtSO4]. In contrast to the excess calculations, the loadings obtained from PCA lead to significant results since the interactions are weighted by their strength and significance. Moreover, the analysis of the weighted vibrations in the loadings indicate that hydrogen bonds are particularly formed between the ethyl sulphate anion of [EMIM][EtSO4] and the hydrogen atom of the OH-group at the C6-atom of glucose.

Design and validation of a multimodal low-budget Raman microscope for liquid and solid phase applications

Confocal Raman microscopy is a powerful tool to measure small sample volumes or solids. Since commercial Raman microscopes are expensive and a change of the laser wavelength or the excitation path is hardly possible after the installation, we constructed a multimodal low-budget Raman microscope. Thus, it was possible to significantly increase the flexibility in terms of excitation wavelengths, paths, and planes. Furthermore, the asset costs were reduced by a factor of 1.7. By using commercial as well as home-built objectives to adapt the working distance and the magnification to the system under investigation, the self-constructed Raman microscope offers the possibility to measure big sample volumes, too. The obtained Raman spectra were validated by Raman spectra from a commercial Raman microscope. With a comparable measurement setting it was possible to increase the signal intensities, but with a slightly lower SNR. However, based on the great flexibility of the set-up, e.g., the laser power or the excitation wavelength can be adapted to increase the SNR. Furthermore, measurement times can be decreased. With this low-budget self-constructed Raman microscope high quality Raman microscopy and micro spectroscopy can be performed with a high flexibility to fast adapt the set-up to the sample under investigation which is not offered by commercial microscopes.

Raman difference spectroscopy approach for monitoring of a bioreactor

We present polarization-resolved shifted excitation Raman difference spectroscopy (pol-SERDS) and its application to monitor a bioreactor in which the microalga Porphyridium purpureum produces antiviral exopolysaccharides and pigments.

Interionic Interactions in Imidazolium‐Based Ionic Liquids: The Role of the C2‐Position Revealed by Raman Scattering and Supported by IR and NMR Spectroscopy

Intermolecular interactions determine the state of aggregation of a substance at given temperature. Based on that, changes in intermolecular interactions can lead to microscopic reordering which may be observed macroscopically in terms of altered physicochemical properties. Especially, when chemicals are employed in technical processes, it is important to control and regulate their properties to guarantee product quality. A special group of chemical substances increasingly gaining interest in the field of chemical and process engineering are room temperature ionic liquids (RTILs). In general, RTILs are organic salts with melting points “below the boiling point of water”. The variety of possible combinations of cations and anions lead to a wide range of chemical and thermo‐physical properties. In fact, it is possible to tune their properties by adjusting the ratio of Coulomb and van der Waals interactions. However, because it is hardly possible to investigate a reasonable fraction of the potential cation‐a...