Johannes Kiefer

Experimental Vibrational Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide and 1-Ethyl-3-methylimidazolium Ethylsulfate

The vibrational structure of two room-temperature ionic liquids with the cation 1-ethyl-3-methylimidazolium [EMIM] and the respective anions bis(trifluoromethylsulfonyl)imide [TFSI] and ethylsulfate [EtOSO3] is investigated. In particular, attenuated total reflection (ATR) infrared (IR) as well as Raman spectra in the spectral range from 500 to 3500 cm−1 have been recorded and analyzed. Moreover, the depolarization ratios of the Raman lines are determined. The individual peaks are assigned to the corresponding vibrational modes of the molecules. While the CH stretching region around 3000 cm−1 is dominating in Raman spectra, it is remarkably weak in IR spectra. Finally, the results for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared to previous studies of single ions available in the literature. This comparison shows good agreement.

Design and characterization of a Raman-scattering-based sensor system for temporally resolved gas analysis and its application in a gas turbine power plant

A sensor system for fast gas composition analysis is presented. Using linear Raman scattering the simultaneous detection of virtually all components of fuel gas mixtures such as natural gas and biogas can be achieved. The system consists of commercially available hardware components, in detail a frequency doubled continuous wave laser at 532 nm and a compact spectrometer with an embedded charge coupled device chip. For the evaluation of the Raman spectra a fast software module based on a contour fit algorithm is developed. Moreover, modules for controlling the hardware components are implemented in the sensor software ensuring simple operability of the entire system. In this paper the sensor is characterized in terms of, e.g., accuracy, reproducibility, detection limits and temporal performance. Finally its application for natural gas analysis in a gas turbine power plant is demonstrated, and the results obtained are compared to gas chromatography results.

Electronic Structure and Normal Vibrations of the 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ion Pair

Electronic and structural properties of the ion pair 1-ethyl-3-methylimidazolium ethyl sulfate are studied using density functional methods. Three locally stable conformers of the ion pair complex are considered to analyze molecular interactions between its cation and anion. Manifestations of these interactions in the vibrational spectra are discussed and compared with experimental IR and Raman spectroscopy data. NBO analysis and difference electron density coupled with molecular electron density topography are used to interpret the frequency shifts of the normal vibrations of the ion pair, compared to the free anion and cation. Excitation energies of low-lying singlet excited states of the conformers are also studied. The density functional theory results are found to be in a reasonable agreement with experimental UV/vis absorption spectra.

Molecular Interactions in 1-Ethyl-3-methylimidazolium Acetate Ion Pair: A Density Functional Study

The density functional method is used to obtain the molecular structure, electron density topography, and vibrational frequencies of the ion pair 1-ethyl-3-methylimidazolium acetate. Different conformers are simulated on the basis of molecular interactions between the 1-ethyl-3-methylimidazolium cation and acetate anion. The lowest energy conformers exhibit strong C-H...O interionic interactions compared with other conformers. Characteristic vibrational frequencies of the ion pair and their shifts with respect to free ions are analyzed via the natural bond orbitals and difference electron density maps coupled with molecular electron density topology. Theoretically scaled vibrational frequencies are also compared with the spontaneous Raman scattering and attenuated total reflection infrared absorption measurements.

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.

Fluorescence spectroscopy of Rhodamine 6G: Concentration and solvent effects

Rhodamine 6G (R6G), also known as Rhodamine 590, is one of the most frequently used dyes for application in dye lasers and as a fluorescence tracer, e.g., in the area of environmental hydraulics. Knowing the spectroscopic characteristics of the optical emission is key to obtaining high conversion efficiency and measurement accuracy, respectively. In this work, solvent and concentration effects are studied. A series of eight different organic solvents (methanol, ethanol, n-propanol, iso-propanol, n-butanol, n-pentanol, acetone, and dimethyl sulfoxide (DMSO)) are investigated at constant dye concentration. Relatively small changes of the fluorescence spectrum are observed for the different solvents; the highest fluorescence intensity is observed for methanol and lowest for DMSO. The shortest peak wavelength is found in methanol (568 nm) and the longest in DMSO (579 nm). Concentration effects in aqueous R6G solutions are studied over the full concentration range from the solubility limit to highly dilute states. Changing the dye concentration provides tunability between ∼550 nm in the dilute case and ∼620 nm at high concentration, at which point the fluorescence spectrum indicates the formation of R6G aggregates.

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.

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N 2 thermometry

Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laser pulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces background caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimination of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotational spectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing of Raman coherences. The sensitivity decreases, however, with increasing temperature, and accurate temperature measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.

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.

Combined coherent anti-Stokes Raman spectroscopy and linear Raman spectroscopy for simultaneous temperature and multiple species measurements

The simultaneous application of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) and vibrational linear Raman spectroscopy (LRS) for the measurement of temperature and species concentrations in combustion systems is demonstrated. In addition to the standard rotational CARS experimental setup, only one detection system (spectrometer and intensified CCD camera) for the collection of the LRS signals was applied. The emission of the broadband dye laser used for CARS was shifted to the deep red to avoid interferences with the LRS signals located in the visible region. First experimental results from a vaporizing propane spray using an engine injection system are shown.