(Vibrational) Spectroscopy in complex systems and condensed phases
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Molecular spectroscopy provides valuable information, e.g. on the structure of molecules, the composition of a given sample, the concentration of substances therein, and the mechanism and rate of chemical reactions. The theoretical prediction of vibrational spectra does not only help interpreting experimental spectra, but based on molecular simulations it additionally provides deeper insight into the molecular vibrations of flexible molecules and their interplay with the environmental medium. Different techniques, like infrared (IR), Raman, vibrational circular dichroism (VCD) can be routinely applied via our analysis tool TRAVIS. While TRAVIS follows the autocorrelation appraoch, we also provide an alternative tool with the quantum cluster equilibrium (QCE) method (Link to our QCE Code Peacemaker). QCE averages over the spectra of different clusters, which can consist of different oligomers and different compositions. This allows the concentration-dependent determination of spectra. |
Paper examples:
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Glucose in ionic liquids (214)
Ionic liquid/water mixtures show remarkable features in cellulose processing, i.e., their strongly coordinating hydrogen bonding properties enable them to dissolve biomass entirely or particular biomass components selectively, that are not soluble by conventional solvents, keeping the biopolymers fully intact.
In order to elucidate the complex interaction network, we investigated three D-glucose isomers (α, β, open-chain) in 1-ethyl-3-methylimidazolium acetate with and without water, through AIMD simulations. While structural analysis was applied to unravel the hydrogen bonding network, showing evidence for the beginning of a crystallization process upon addition of water, the calculated IR and VCD spectra reveal insight into the intermolecular interactions. The results can help to develop new pulping and corresponding refining technologies for the regeneration of natural and technical fibers.
Keywords: Glucose, ionic liquids, structure, vibrational spectroscopy -
Novel method: Thomas-Kirchner approach to VCD (169)
While nowadays quantum chemical gas phase predictions of vibrational circular dichroism (VCD) spectra are in wide use, the calculation of bulk phase VCD spectra from molecular dynamics simulations is still a field of development. Since measurements are usually performed in solution, and such simulations are able to model the dynamics and explicit solvation effects, further development in this direction is indispensable. A new approach is presented for the calculation of VCD spectra from ab initio molecular dynamics simulations based on a time cross-correlation ansatz of the electric and magnetic dipole moment. While the electric dipole moments are obtained via integration of the electron density, the novelty lies in the calculation of the magnetic dipole moments from the electric current density. Calculated gas and bulk phase spectra match the experiment with excellent agreement and solvent-induced peak shifts can be modeled properly, yielding insight into the intermolecular interplay.
Keywords: Vibrational circular dichroism, ab initio molecular dynamics, magnetic dipole moment -
Vibrational spectra via Voronoi tessellation of the electron density (156)
With the development and application of new spectrocsopic functions within TRAVIS, an excellent tool for comparison between experiment and ab initio molecular dynamics AIMD simulations is provided, which also helps in assigning peaks of the experimental spectrum. One key step in the spectrum calculation is the determination of molecular properties like dipole moments and polarizabilities from the electron density of the systems, which is available in each time step of an AIMD simulation. Compared to the well established Wannier localization scheme, utilizing the Voronoi tessellation of the electron density, infrared and Raman spectra of similar and even superior quality could be achieved, especially in systems with significant charge transfer.
Keywords: Vibrational spectroscopy, ab initio molecular dynamics simulations, Voronoi tessellation
