The leading overall motivation is to unravel reaction mechanisms of complex, transition metal catalyzed reactions at the electronic structure level. As the experimental means of addressing electronic structure involves various forms of spectroscopy, a thorough understanding of structure/spectra relationships is of paramount importance (and clearly branches into the area of material science). Furthermore, the characterization of reaction intermediates can in almost all cases only proceed through a thorough interpretation of spectra taken under transient or quench conditions.

(Figure 1): In our department theory development, application and advanced spectroscopy are combined

The activation of small molecules by transition metals is of paramount importance in the active sites of metalloproteins, in homogenous and in heterogeneous catalysis. In fact, the chemistry of sustainable energy, the coming focus of the future institute involves a number of elementary reactions:

While all of these reactions are catalyzed in a highly efficient manner by metalloproteins, the search for suitable low molecular weight catalysts represents an active area of research in all cases. In our department, we are interested in all of these reactions, the associated enzymes (in cooperation with the department of Prof. Wolfgang Lubitz), low-molecular weight catalysts with potential for large scale applications, as well as heterogeneous processes (in cooperation with the department Prof. Robert Schlögl).

The overall strategy of the work involves a careful combination of theoretical and experimental techniques. Where necessary, theoretical or experimental methods will be developed in house. Furthermore, the department is involved in a large number of coordinated research programs and collaborations with scientists worldwide.

(1) Quantum chemical method development
In our group, the large-scale quantum chemistry program ORCA is developed. ORCA is a highly-efficient, flexible and user friendly quantum chemistry program that is intensely used by a quickly growing user community of about 10,000 researchers worldwide. Its features are fully described elsewhere

http://www.mpibac.mpg.de/bac/index_en.php (Download area)

(Figure 2): The quantum chemistry program system ORCA is a powerful general purpose tool with emphasis on open shell transition metals and spectroscopy.

ORCA features all common standard functionality involving density functional theory (DFT), correlated single- (CCSD(T)) and multireference (MR-CI, SORCI, NEVPT2) ab initio wavefunctionmethods, as well as semi-empirical methods. ORCA is particularly well-suited for the calculation of molecular spectra and is widely used by spectroscopists in various areas of research ranging from solid state chemistry to pharmacology.

One obvious goal of the theoretical method development is to enhance the efficiency and accuracy of theoretical methods and thereby push the boundaries of what is possible with computational chemistry. In recent years, we have actively pursued ways to improve the speed of exchange term evaluations, ^1-4 new generation of double hybrid density functionals (DHDFs, in cooperation with the group of Prof. Grimme), ^5,6 the development of low-order scaling coupled-cluster methods on the basis of pair natural orbitals (LPNO-CCSD)^7,8 as well as highly efficient multireference approaches on the basis of configuration interaction^9,10 or N-electron valence perturbation theory (NEVPT2, in collaboration with the groups of Profs. DeAngeli and Malrieu).¹¹

(Figures 3,4): Low order scaling local coupled cluster methods based on the pair natural orbital concept allow application to real life molecules in reasonable turnaround times. The physical basis of these calculation is the fast decay of the electron pair correlation energies with interelectronic distance

However, a major focus of method development is the design of suitable methods for the prediction of spectroscopic properties throughout all regions of the electromagnetic spectrum. An overview can be found in refs ^12,13. Being based on elementary to highly advanced theoretical concepts, methods to calculate Mössbauer parameters,¹⁴ X-ray absorption and emission, ^15-17 electron paramagnetic resonance (EPR^18-22), resonance Raman (rR²³) and magnetic circular dichroism (MCD²⁴) spectra have found widespread application in various communities.

(Figures 5): Theoretical spectroscopy allows for the detailed assignment of complicated molecular spectra and thereby obtain deep insight into geometric and electronic structure. In this case the X-ray absorption and emission spectra of a six-iron carbide cluster.

The development efforts in Mülheim are coordinated jointly by Prof. Neese and Dr. Frank Wennmohs, who heads the ORCA development team. We are very grateful to our collaborators all over the world who contribute their expertise, energy and enthusiasm to the project. The group of Prof. Alexander Auer is involved in theoretical method development. The focus is on accurate electronic structure methods of the coupled cluster type

(2) Computational Chemistry
Our computational chemistry applications center around the reactions depicted above. Areas of recent interest are centered around:

(a) The oxidation of water by the oxygen evolving complex (OEC) of Photosystem II (PSII). This research area is led by Dr. Dimitrios Pantazis and is carried out in close collaboration with the department of Prof. Wolfgang Lubitz. A recent paper summarizes the efforts that have led to the proposal of a refined structure for the OEC that is consistent with all crystallographic and spectroscopic data.^25,26 Our desire to understand the reaction mechanism of the OEC on the basis of its spectroscopic properties has led us to consider the properties of manganese complexes in greater detail and has led to a series of systematic investigations on manganese monomers, e.g. ^27-29 dimers^30,31 and oligomers.³²

(Figure 6): Refined structure of the oxygen evolving complex as arising from a combination of crystallography, spectroscopy and quantum chemistry.

(b) The activation of dinitrogen, one of the most inert molecules known in chemistry, by the enzyme nitrogenase is another focus of research in the group. This research area is headed by Prof. Serena DeBeer. Despite intense research efforts, even the structural basis for biological nitrogen fixation has been proven elusive. Two recent highlights include the identification of the central atom in the active site of nitrogenase to be a carbide through the combination of X–ray emission spectroscopy with quantum chemistry.³³ as well as the characterization of a nitrogen activating trinuclear iron complex (in collaboration with the group of Prof. Patrick Holland, Rochester, USA).³⁴

(Figure 7): A combination of X-ray emission and quantum chemistry reveals that the active site of nitrogenase contains a central carbide ion.

(c) The spectroscopy and reactivity of high-valent iron centers in iron enzymes and low-molecular weight catalysts. These research efforts are coordinated by Dr. Eckhard Bill (spectroscopy) and Dr. Shengfa Ye (Theory).
Highlights include the characterization of Fe(V)³⁵ and Fe(VI)³⁶ complexes (in collaboration with the former director, Prof. Karl Wieghardt), the detailed analysis of C-H bond activation reactions^37,38 and the fascinatingly complex chemistry of iron-nitrosyls.^39,40

(Figure 8): The electronic structure analysis of the C-H bond activation catalyzed by high-valent iron(IV)-oxo species reveals that en route to the transition state an oxyl radical is formed that acts as a strong electrophile capable of attacking the C-H bond.

(3) Molecular spectroscopy
The department is involved in a wide range of advanced spectroscopic experiments that are aimed at obtaining geometric and electronic structure information on stable as well as transient open-shell transition metal species. Apart from standard laboratory equipment UV/vis, IR Raman, Fluorescence and NMR spectroscopy) the department focuses on the following techniques:

(a) X-ray Absorption and Emission spectroscopy.Modern synchrotron based techniques allow for many exciting, element specific experiments to be performed. The group of Prof. DeBeer) is actively involved in the development and application of new X-ray based techniques.^17,27,41-44

(Figure 9): A systematic study on a series of monomeric manganese complexes reveals that density functional theory is a powerful tool for the prediction of these spectra in the core-to-valence region.

(b) Mößbauer spectroscopy is one of the most powerful tools for the investigation of iron containing enzymes, coordination complexes and materials. The group of Dr. Bill has a long term tradition on performing and analyzing Mößbauer spectra with and without an applied external magnetic field.^45-48

(Figure 10): Mößbauer spectroscopy can be used to investigate reaction intermediates. In this example three different intermediates have been observed in the course of an enzymatic reaction.

(c) High resolution electron paramagnetic resonance is the most powerful technique to investigate paramagnetic molecules. In addition to our collaboration with the department of Prof. Lubitz this technique is implemented in our department in the group of Dr. Maurice van Gastel who is exploring novel techniques as well as applications in the fields of bioinorganic chemistry and energy research.⁴⁹

(Figure 11): High-resolution EPR combined with modern quantum chemistry provides extended insight into fine structural and electronic details. In this example, the HYSCORE spectra of nitrosyl-myoglobin have been successfully assigned on the basis of QM/MM calculations.

(d) Resonance Raman spectroscopy is a particularly powerful technique for the investigation of chromophores. This technique is represented in our department by Dr. Taras Petrenko who is developing the instrumental as well as theoretical aspects of the technique.^12,23,50,51 Using resonance Raman spectroscopy one obtains highly and selectively vibrationally resolved information about absorbing species. Besides carrying a wealth of electronic structure information, the enormous enhancement of the inelastic response of a system once excited in the area of an absorption band provides extremely powerful fingerprints that allow for the characterization of elusive species.

(Figure 12): Resonance Raman spectroscopy provides a detailed electronic structure picture of the excited state that is excited. The vibrational pattern is characteristic for the chromophore and type of excitation.

(e) Magnetic Circular Dichroism spectroscopy is a powerful technique that bridges the fields of optical and magnetic spectroscopy. MCD, as applied to paramagnetic substances, provides a wealth of electronic structure information. In addition, variation of applied field and temperature allows for the optical measurement of the ground state magnetic susceptibility even in the presence of mixtures or impurities. The MCD laboratory is also headed by Dr. Bill using a home-designed setup that allows for spectra to be taken all the way from the deep UV to the near-IR regions.^52-54

(Figure 13): MCD spectroscopy provides high-resolution electronic structure insight as well as an optical probe of the ground state magnetic properties. In this example the magnetism of an exchange coupled transition metal dimer has been revealed by MCD spectroscopy (right). The MCD spectra are superpositions of the individual ion spectra with the signs being characteristic of the magnetic coupling pattern.

1. Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient, approximate and parallel Hartree-Fock and hybrid DFT calculations. A 'chain-of-spheres' algorithm for the Hartree-Fock exchange Chemical Physics 2009, 356, 98-109.
2. Kossmann, S.; Neese, F. Efficient Structure Optimization with Second-Order Many-Body Perturbation Theory: The RIJCOSX-MP2 Method Journal of Chemical Theory and Computation 2010, 6, 2325-2338.
3. Petrenko, T.; Kossmann, S.; Neese, F. Efficient time-dependent density functional theory approximations for hybrid density functionals: Analytical gradients and parallelization J. Chem. Phys. 2011, 134, 14.
4. Izsak, R.; Neese, F. An overlap fitted chain of spheres exchange method J. Chem. Phys. 2011, 135, 11.
5. Neese, F.; Schwabe, T.; Grimme, S. Analytic derivatives for perturbatively corrected "double hybrid" density functionals: Theory, implementation, and applications J. Chem. Phys. 2007, 126, 124115.
6. Grimme, S.; Neese, F. Excited states with double hybrid functionals J. Chem. Phys. 2007, 127, 154116.
7. Neese, F.; Wennmohs, F.; Hansen, A. Efficient and accurate local approximations to coupled-electron pair approaches: An attempt to revive the pair natural orbital method J. Chem. Phys. 2009, 130, 18.
8. Neese, F.; Hansen, A.; Liakos, D. G. Efficient and accurate approximations to the local coupled cluster singles doubles method using a truncated pair natural orbital basis J. Chem. Phys. 2009, 131, 15.
9. Neese, F. A Spectroscopy Oriented Configuration Interaction Procedure J. Chem. Phys. 2003, 119, 9428-9443.
10. Ganyushin, D.; Neese, F. First Principle Calculation of Zero-Field Splittings J. Chem. Phys. 2006, 125, 024103.
11. Atanasov, M.; Ganyushin, D.; Pantazis, D. A.; Sivalingam, K.; Neese, F. Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes Inorg. Chem. 2011, 50, 7460-7477.
12. Neese, F.; Petrenko, T.; Ganyushin, D.; Olbrich, G. Advanced aspects of ab initio theoretical optical spectroscopy of transition metal complexes: Multiplets, spin-orbit coupling and resonance Raman intensities Coord. Chem. Rev. 2007, 251, 288-327.
13. Neese, F. Prediction of molecular properties and molecular spectroscopy with density functional theory: From fundamental theory to exchange-coupling Coordination Chemistry Reviews 2009, 253, 526-563.
14. Neese, F. Prediction and Interpretation ofIsomer Shifts in ⁵⁷Fe Mössbauer Spectra by Density Functional Theory Inorg. Chim. Acta 2002, 337C, 181-192.
15. DeBeer George, S.; Petrenko, T.; Neese, F. XAS + DFT Inorg. Chim. Acta 2007, 361 965-972
16. DeBeer George, S.; Petrenko, T.; Neese, F. Prediction of Iron- K-edge Absorption Spectra using Time-Dependent Density Functional Theory J. Phys. Chem. A 2008, 112, 12936–1294.
17. Lee, N.; Petrenko, T.; Bergmann, U.; Neese, F.; DeBeer, S. Probing Valence Orbital Composition with Iron K beta X-ray Emission Spectroscopy J. Am. Chem. Soc. 2010, 132, 9715-9727.
18. Neese, F.; Solomon, E. I. Calculation of Zero-Field Splittings, g-values and the Relativistic Nephelauxetic Effect in Transition Metal Complexes. Application to High Spin Ferric Complexes Inorg. Chem. 1998, 37, 6568-6582.
19. Neese, F. Configuration interaction calculation of electronic g tensors in transition metal complexes Int. J. Quant. Chem. 2001, 83, 104-114.
20. Neese, F. Metal and Ligand Hyperfine Couplings in Transition Metal Complexes. The Effect of Spin-Orbit Coupling as Studied by Coupled Perturbed Kohn-Sham Theory J. Chem. Phys. 2003, 118, 3939.
21. Neese, F. Correlated Ab Initio Calculation of Electronic g-Tensors Using a Sum Over States Formulation Chem. Phys. Lett. 2003, 380, 721-728.
22. Neese, F. Analytic Derivative Calculation of Zero-Field Splittings J. Chem. Phys. 2007, 127, 164112.
23. Petrenko, T.; Ray, K.; Wieghardt, K. E.; Neese, F. Vibrational markers for the open-shell character of transition metal bis-dithiolenes: An infrared, resonance Raman, and quantum chemical study J. Am. Chem. Soc. 2006, 128, 4422-4436.
24. Ganyushin, D.; Neese, F. First-principles calculations of magnetic circular dichroism spectra J. Chem. Phys. 2008, 128.
25. Ames, W.; Pantazis, D.; Krewald, V.; Cox, N.; Messinger, J.; Lubitz, W.; Neese, F. A theoretical evaluation of structural models of the S2 state in the Oxygen Evolving Complex: protonation states and magnetic interactions J. Am. Chem. Soc. 2011, in press.
26. Pantazis, D. A.; Orio, M.; Petrenko, T.; Zein, S.; Lubitz, W.; Messinger, J.; Neese, F. Structure of the oxygen-evolving complex of photosystem II: information on the S-2 state through quantum chemical calculation of its magnetic properties Physical Chemistry Chemical Physics 2009, 11, 6788-6798.
27. Beckwith, M. A.; Roemelt, M.; Collomb, M. N.; DuBoc, C.; Weng, T. C.; Bergmann, U.; Glatzel, P.; Neese, F.; DeBeer, S. Manganese K beta X-ray Emission Spectroscopy As a Probe of Metal-Ligand Interactions Inorganic Chemistry 2011, 50, 8397-8409.
28. Duboc, C.; Ganyushin, D.; Sivalingam, K.; Collomb, M. N.; Neese, F. Systematic Theoretical Study of the Zero-Field Splitting in Coordination Complexes of Mn(III). Density Functional Theory versus Multireference Wave Function Approaches Journal of Physical Chemistry A 2010, 114, 10750-10758.
29. Lassalle-Kaiser, B.; Hureau, C.; Pantazis, D. A.; Pushkar, Y.; Guillot, R.; Yachandra, V. K.; Yano, J.; Neese, F.; Anxolabehere-Mallart, E. Activation of a water molecule using a mononuclear Mn complex: from Mn-aquo, to Mn-hydroxo, to Mn-oxyl via charge compensation Energy & Environmental Science 2010, 3, 924-938.
30. Pantazis, D. A.; Krewald, V.; Orio, M.; Neese, F. Theoretical magnetochemistry of dinuclear manganese complexes: broken symmetry density functional theory investigation on the influence of bridging motifs on structure and magnetism Dalton Trans. 2010, 39, 4959-4967.
31. Sinnecker, S.; Noodleman, L.; Neese, F.; Lubitz, W. Calculation of the EPR Parameters of a Mixed Valence Mn(III)/Mn(IV) Model Complex with Broken Symmetry Density Functional Theory. J. Am. Chem. Soc. 2004, 126, 2613-2622.
32. Pantazis, D.; Orio, M.; Petrenko, T.; Messinger, J.; Lubitz, W.; Neese, F. A new quantum chemical approach to the magnetic properties of oligonuclear transition metal clusters: Application to a model for the tetranuclear manganese cluster of Photosystem II Chem. Eur. J. 2009, 15, 5108-5123.
33. Lancaster, K.; Römelt, M.; Ettenhuber, P.; Hu, Y.; Ribbe, M. W.; Neese, F.; Bergmann, U.; DeBeer, S. X-Ray Emission Spectroscopy Evidences a Central Carbon in the Nitrogenase Iron-Molybdenum Cofactor Science 2011, in press.
34. Rodriguez, M. H.; Bill, E.; Brennessel, W. W.; Holland, P. L. N2 Reduction and Hydrogenation to Ammonia by a Molecular Iron-Potassium Complex Science 2011, in press.
35. Aliaga-Alcade, N.; DeBeer George, S.; Bill, E.; Wieghardt, K.; Neese, F. The Geometric and Electronic Structure of [(Cyclam-acetato)Fe(N)]+: a Genuine Iron(V) Species with Ground State Spin S = ½. Angew. Chem. Int. Ed. 2005, 44, 2908-2912.
36. Berry, J. F.; Bill, E.; Bothe, E.; George, S. D.; Mienert, B.; Neese, F.; Wieghardt, K. An octahedral coordination complex of iron(VI) Science 2006, 312, 1937-1941.
37. Ye, S. F.; Neese, F. Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C-H bond activation transition state Proc. Natl. Acad. USA 2011, 108, 1228-1233.
38. Geng, C. Y.; Ye, S. F.; Neese, F. Analysis of Reaction Channels for Alkane Hydroxylation by Nonheme Iron(IV)-Oxo Complexes Angewandte Chemie-International Edition 2010, 49, 5717-5720.
39. Ye, S. F.; Price, J. C.; Barr, E. W.; Green, M. T.; Bollinger, J. M.; Krebs, C.; Neese, F. Cryoreduction of the NO-Adduct of Taurine:alpha-Ketoglutarate Dioxygenase (TauD) Yields an Elusive {FeNO}(8) Species J. Am. Chem. Soc. 2010, 132, 4739-4751.
40. Ye, S. F.; Neese, F. The Unusual Electronic Structure of Dinitrosyl Iron Complexes J. Am. Chem. Soc. 2010, 132, 3646.
41. George, S. D.; Neese, F. Calibration of Scalar Relativistic Density Functional Theory for the Calculation of Sulfur K-Edge X-ray Absorption Spectra Inorg. Chem. 2009, 49, 1849-1853.
42. George, S. D.; Petrenko, T.; Neese, F. Time-dependent density functional calculations of ligand K-edge X-ray absorption spectra Inorg. Chim. Acta 2008, 361, 965-972.
43. George, S. D.; Petrenko, T.; Neese, F. Prediction of Iron K-Edge Absorption Spectra Using Time-Dependent Density Functional Theory J. Phys. Chem. A 2008, 112, 12936-12943.
44. Petrenko, T.; DeBeer George, S.; Aliaga-Alcalde, N.; Bill, E.; Mienert, B.; Xiao, Y.; Guo, Y.; Sturhahn, W.; Cramer, S. P.; Wieghardt, K.; Neese, F. Characterization of a Genuine Iron(V)-Nitrido Species by Nuclear Resonant Vibrational Spectroscopy Coupled to Density Functional Calculations J. Am. Chem. Soc. 2007, 129, 11053-11060.
45. Ye, S. F.; Tuttle, T.; Bill, E.; Simkhovich, L.; Gross, Z.; Thiel, W.; Neese, F. The Electronic Structure of Iron Corroles: A Combined Experimental and Quantum Chemical Study Chem. Eur. J. 2008, 14, 10839-10851.
46. Patra, A. K.; Bill, E.; Bothe, E.; Chlopek, K.; Neese, F.; Weyhermüller, T.; Stobie, K.; Ward, M. D.; McCleverty, J. A.; Wieghardt, K. Electronic structure of mononuclear bis(1,2-diaryl-1,2-ethylenedithiolato)iron complexes containing a fifth cyanide or phosphite ligand: A combined experimental and computational study Inorg. Chem 2006, 45, 7877-7890.
47. Berry, J. F.; Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K. Octahedral non-heme oxo and non-oxo Fe(IV) complexes: An experimental/theoretical comparison J. Am. Chem. Soc. 2006, 128, 13515-13528.
48. Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. Electronic Structure of Bis(imino)pyridine Iron Dichloride, Monochloride, and Neutral Ligand Complexes: A Combined Structural, Spectroscopic, and Computational Study J. Am. Chem. Soc. 2006, 128, 13901-13912.
49. van Gastel, M. Zero-field splitting of the lowest excited triplet states of C60, C70 and benzene J. Phys. Chem. A 2010, 114, 10864-10870.
50. Petrenko, T.; Krylova, O.; Neese, F.; Sokolowski, M. Optical Absorption and Emission Properties of Rubrene: Insight by a Combined Experimental and Theoretical Study New J. Phys. 2009, 11, 015001.
51. Petrenko, T.; Neese, F. An Efficient and General Method for the Calculations of Absorption and Fluorescence Bandshapes as well as Resonance Raman Intensities J. Chem. Phys. 2007, 127, 164319.
52. Neese, F.; Solomon, E. I. MCD C-term Signs, Saturation Behavior and Determination of Band Polarizations in Randomly Oriented Systems with Spin S>=1/2. Applications to S=1/2 and S=5/2 Inorg. Chem. 1999, 38, 1847-1865.
53. Piligkos, S.; Slep, L. D.; Weyhermuller, T.; Chaudhuri, P.; Bill, E.; Neese, F. Magnetic circular dichroism spectroscopy of weakly exchange coupled transition metal dimers: A model study Coord. Chem. Rev. 2009, 253, 2352-2362.
54. Ray, K.; Begum, A.; Weyhermüller, T.; Piligkos, S.; vanSlageren, J.; Neese, F.; Wieghardt, K. The Electronic Structure of the Isoelectronic, Square Planar Complexes [FeII(L)2]2- and [CoIII(LBu)2]- (L2- and (LBu)2- = benzene-1,2-dithiolates): an Experimental and Density Functional Theoretical Study J. Am. Chem. Soc. 2005, 127, 4403-4415.