Computational Chemistry Computational Chemistry

Spectroscopic Analysis Science Case

Description

A highly useful quantum chemical Science case is the so-called spectroscopic analysis. After a first geometry optimization of the desired molecule several further simulations are performed which serve for a spectroscopic analysis. Chemists describe this in a rather complex workflow, which comprises a multitude of consecutive and subsequent steps. First of all, the geometry of the desired molecules needs to be energetically optimized. In real-life most inorganic chemists look at molecules which possess at least 50-200 atoms. A quantum chemist wants to explore the spectroscopic properties of such molecules. This can be on the one hand the vibrations of the molecule (IR and Raman frequencies) and on the other hand UV/Vis spectra of such a molecule. The vibrations require a so-called frequency calculation. In case of only positive vibrations, the molecule geometry represents a true minimum. UV/Vis spectra with good accordance to experimental data are obtained by time-dependent density functional theory calculations (TD-DFT).

Scientific Merit

The spectroscopic analysis of selected molecules allows a better interpretation of experimental spectroscopic data and helps to an identification of highly reactive chemical species. The species can then be further developed towards sustainable catalysis.

Steps

When dissecting the single steps of the idea described above, we identified that there are smaller workflows of fundamental quality (such as the optimization workflow) which are embedded in the larger entity. The dissection followed the principle of identifying small tasks which can be reused within other workflows. Hence, one can define several small workflows as part of a larger meta-workflow as depicted in Figure 5.
The workflow dissection provides with the insight that the first workflow is a simple geometry optimization (opt WF). Such a basic workflow can be reused in many more applications. The subsequent workflows are similar to each other: a converter script extracts the output geometry from the optimization output and combines it with blank input files (i.e. just lacking input coordinates) with the corresponding keywords for frequency calculations (leading to a Freq WF), time-dependent DFT (TD-DFT WF), population analyses (pop WF) and subsequent calculations in solvents (Solv WF). All these small workflows in Figure 5 are highly valuable since they can be reused in larger quantum chemical workflows. The whole systems gains flexibility as the small workflows can be freely combined to new meta-workflows.

Example

Understanding of the formation of peroxo (Figure 6) and the isomeric oxo cores as well as their distinct reactivity relies on comprehensive orbital analyses. First the geometry is optimised, then TD-DFT and all other steps are performed. Figure 7 shows as example the optical spectra.
Time dependent-DFT calculated spectra predict the optical spectrum with the four LMCT bands at 340 nm, 366 nm, 381 nm and 547 nm in good accordance to the experimental spectrum.

Related Publications

  1. S. Herres-Pawlis, A. Hoffmann, S. Gesing, J. Krüger, A. Balasko, P. Kacsuk, R. Grunzke, G. Birkenheuer, L. Packschies, User-Friendly Workflows in Quantum Chemistry, CEUR Workshop Proceedings 2013, 993, Paper 14.
  2. S. Herres-Pawlis, A. Hoffmann, A. Balasko, P. Kacsuk, G. Birkenheuer, A. Brinkmann, L. de la Garza, J. Krüger, S. Gesing , R. Grunzke, G. Terstyansky, N. Weingarten, Quantum chemical metaworkflows in MoSGrid, Concurrency Computat.: Pract. Exper. 2014, in print. 
  3. A. Hoffmann, R. Grunzke, S. Herres-Pawlis, Insights into the influence of dispersion correction in the theoretical treatment of guanidine-quinoline copper(I) complexes, J. Comp. Chem. 2014, doi: 10.1002/jcc.23706.

Contacts

          

    This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 312579.