Research Topics
- The Incremental scheme for local correlation calculations
- Ab initio modelling of chemical reaction mechanisms
Theoretical Chemistry
Research
Research
It is well-known that a quantitative description of the electronic structure in molecules is usually not possible with the Hartree-Fock (HF) method. One way to achieve higher accuracy in electronic structure calculations is to improve the wavefunction, which can be routinely done by many-body perturbation theory (MBPT), configuration interaction theory (CI), or coupled-cluster theory (CC). The major drawback of these approaches is their strong dependence of the computational effort on the size of the one-particle basis set. This means that these approaches depend heavily on the system size too, if atom-centered basis functions are used. Since for large systems the canonical HF orbitals are not necessarily the best choice for a PT-, CI-, or CC-expansion of the wavefunction, many groups use a local orbital basis instead, to include electron correlation. This allows to screen out insignificant contributions to the energy and therefore the computational cost is reduced. Conceptually different approaches divide the total system into parts and then perform a perturbation expansion to obtain the total correlation energy. An approach designed in this way is the incremental scheme of Stoll. It is based on the Bethe-Goldstone expansion which was introduced to quantum chemistry by Nesbet more than 40 years ago.
The incremental scheme was successfully applied during the last 15 years to various periodic systems and molecules. Recently we proposed a fully automated implementation of the incremental scheme for CCSD(T), CCSD and MP2 energies, implemented an automatic distance screening, extended the approach for the usage of symmetry and to the RCCSD method for open-shell calculations. In order to account for the local character of the core electrons, we introduced an efficient scheme to treat the core and core-valence correlation.
Density functional theory is a useful tool to obtain qualitative and quantitative insights into chemical reaction mechanisms. For instance one can study the potential energy hyper surface (PES) of chemical reactions. It enables a theoretician to study highly reactive compounds and transition states of chemical reactions which are hardly or not accessible experimentally. One can directly study substitution effects on chemical reactions. The parent cyclopropylcarbinyl radical is not stable and the ring opening is observed. If a CO2Me-group is introduced, the 3-exo cyclization becomes thermodynamically favorable because the cyclopropylcarbinyl radical is stabilized due to the CO2Me-group. This explains very nicely why the 3-exo cyclization can take place in the ester substituted system and why the reverse reaction is observed in the parent system.
In this subproject we investigate how bases catalyse the twin polymerization of the monomers of TP 5, 6, 7 and 10 using ab initio quantum chemistry methods. The aim of the project is to develop a detailed molecular model of all reaction steps, to understand the reaction mechanism on the basis of the potential energy surface (PES). This model together with the PES itself will be used in TP 8 to fit coarse grained models, which are capable to model the structure formation process. Therefore the results obtained in this project are an important ingredient to model the structure formation during the twin polymerization.
A main point of the A. Gansäuer group is the experimental investigation of the radical reaction of metallocenes. The utilisation of Titanocene for catalysis, materials science and medical chemistry is of high interest.
The influence of the solvent of the stability of the formyl azide as well as the Curtius rearrangement of the formyl azide are investigated with the K. Banert group. Moreover the reactions of the formyl azide with different substances like cyclooctine are investigated and the resulting intramolecular rearrangements are quantum chemically related. The structure of the energetically lowest lying transition state of this rearrangement is shown in this figure below (left), as well as the relative energy diagram of the Curtius rearrangement of the formyl azide (right).
