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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Quantenchemie der Festkörper/ Katalyse

Research in the Quantum Chemistry Group


Overview

Combined use is made of methods of ab initio quantum chemistry and molecular simulation for solving problems of chemical interest. We focus on solids and their surfaces. Contributions are made to elucidate structures and elementary processes. Of current interest are catalysts, in particular zeolites and metal oxids. Our aim is to rationalize the activity and selectivity of these industrial important materials on the basis of first principles of quantum mechanics.

Methods for investigating the structure, dynamics and reactivity of large chemical systems:
  • Quantum chemical ab initio methods (DF, HF, MP2)
  • Combined approach quantum mechanics/interatomic potential functions (QM-Pot; embedded cluster)
  • Car-Parrinello molecular dynamics (plane waves)
Applications to solid materials and molecules:
  • Solid catalysis, in particular zeolites
  • Proton transfer as elementary reaction step
Structure and dynamics of molecular clusters in the gas phase (hydrogen bonds, transition metal compounds)
  • Potential energy surfaces by "conventional" ab initio calculations including electron correlation
  • Solution of multi-dimensional nuclear motion problem quantum mechanically (variational methods) and classically (Born-Oppenheimer molecular dynamics with ab initio forces)


Applications

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Methods

Development of hybrid methods for treating large systems (solids, molecules in solution, biomolecules)
A combined approach has been developed that describes the inner part of a system by a quantum mechanical approach and its environment by interatomic potential functions (127, 128, 131, 138, 139). The success of the method relies on interatomic potential functions that take the polarizability of atoms or ions of the environment into account. We also take advantage of the fact that the interatomic potential functions are parametrized on ab initio data (vide infra). Current extensions (PhD work of Marek Sierka) allow flexible combination of different quantum chemical (e.g. Turbomole, Dmol3) and simulation codes (e.g. Discover). It includes an efficient optimizer for both minima and transition structures (2). To describe the making and breaking of bonds Warshel's EVB method is adapted in a modified form. All this together makes it possible to find transition structures in systems with several hundred degrees of freedom - which is prerequisite for reactivity studies on large chemical systems.
The code is available from the authors and also distributed by MSI to the members of the Catalysis and Sorption Consortium.

Ab intio parametrization of interatomic potential functions
Results of ab initio calculations on molecular cluster models of zeolites and related materials have been used to parametrize interatomic potential functions. Early work used the force field functional form (110, 111, MSI software: cff-czeo). More recently shell model ion pair potentials have been derived. Parameters are available for SiO2 and protonated aluminosilicates (118), for their interaction with NH3 and NH4+ (125) and for Ti-substituted SiO2-modifications (de Man, Ricchiardi, Sauer, to be published). All these parameters have been derived from Hartree Fock data. Parameters derived from density funcitonal (B3LYP) data are available fo SiO2 and protonated aluminosilicates (128), for aluminium phosphates and protonated silicoaluminium phosphates (134) and for Cu+ exchange aluminosilicates (3).

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Applications

Structure and spectra of zeolite catalysts
When employed in lattice energy minimizations our ab initio parametrized interatomic potential functions reproduce the observed structures of dense and microporous SiO2 and AlPO4 modifications within narrow limits (128, 134). Reliable predictions can be made for the local structures of catalytically active Bronsted sites, SiO(H)Al, both in SiO2 and AlPO4 frameworks (127, 128, 133, 135). The quality of the simulated structures make reliable quantum mechanical calculations of NMR chemical shifts possible. Examples are the 29Si-NMR spectrum of ZSM-5 and the 1H- and 29Si-NMR spectrum of Ti-substituted zeolite catalysts.

Elementary steps of the acidic catalysis by zeolites
New results have been obtained for the initial step of the MTG and MTO processes ("Methanol to Gasoline" and "Methanol to Olefines"). Structure optimizations using the MP2 gradient module of the Turbomole code have shown that the surface complex formed on adsorption of methanol involves the neutral methanol molecule stabilized by H-bonds with the surface. In contrast, the ion-pair structure obtained when transfering a proton to the methanol yieldinga methoxonium cation is not a minimum of the potential energy surface, but a transition structure (109). Later calculations adopting periodic boundary conditions confirmed these findings (122). Previous speculations about a possible critical effect of the framework shape for this reaction could not be confirmed. Dimilar results have been obtained for the adsorption of water molecules (121). For this system the dependence on the water loading was investigated. On adsorption of two water molecules per site the H5O2+ ion can exist as stable surface species. Neutron diffraction studies on water loaded H-SAPO-34 (microporous AlPO4 material substituted with Si) seemed to contradict the ab initio cluster calculations. However, periodic ab initio calculations using the Car-Parrinello code confirmed the important role of water loading and showed that three interacting water molecules are required for the proton transfer (135).The latter yields a H7O3+ cluster which is stabilized by several h-bonds to the (internal) surface.

Structure and reactivity of sulfated zirconia (140)
The treatment of zirconium dioxide with sulfuric acid yields a catalyst of surprising activity for many industrial reactions. Periodic ab initio studies (Car-Parrinello Molecular Dynamics) of sulfuric acid adsorbed on ZrO2(101) and ZrO2(001) showed that on the surface the acid dissociates into sulfate anions and protons. The protons form surface hydroxyl groups and the sulfate anions form bidentate or tridentate surface complexes. The computed vibrational spectra for the two types of sulfate surface species show characteristic differences which allows to identify them in experiments.

Structure, dynamics and vibrational spectra of H-bonded gas phase clusters (114)
Calculations on the water dimer, (H2O)2, showed that electron correlation, basis set effects and anharmonicity of the potential energy surface all make signifcant contributions to the calculated red shift of the OH donor band in the infrared spectrum. These calculations helped to settle the assignment of the bands of this system in the OH region (110). For methanol clusters in the gas phase (dimer to hexamer), an assignment could be made for all bands observed in the CO and OH stretch region using HF and MP2 results (136). For the formic acid tetramer, DF calculations predict three structures of nearly equal stability (123). Computations of the vibrational spectra can help to find out which of these structures contribute to the many lines observed in the OH/CH region.
Molecular dynamics simulations on Born-Oppenheimer potential energy surfaces using ab initio forces computed "on the fly" can be performed for gas pahse clusters (BOMDAI). When DF methods are used, numerical integration is critical (119). Such simulations have been made for the proton bound water dimers and trimers, H5O2+ and H7O3+, and the vibrational spectra have been predicted from the dipole autocorrelation function (126). Previously this method was applied to Li4F4 clusters using the HF method (113).

Structure and stability of transition metal compounds in the gas phase
FeS (137) and Fe2S2 are studied in different charge states by CASSCF, ACPF and MRCI methods. Complications arise from the many electronic states, in particular from the different spin couplings between the d-electron of the two Fe atoms. An assignment could be made of the photoelectron spectrum of FeS (137). Test calculations using DF methods are also made with respect to larger FeS clusters.
 
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