Publications (260)782.24 Total impact
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ABSTRACT: The asymmetric catalysis of the intramolecular enone [2+2] photocycloaddition has been subject of extensive experimental studies, however theoretical insight to its regulatory mechanism is still sparse. Accurate quantum chemical calculations at the CASPT2//CASSCF level of theory associated with energyconsistent relativistic pseudopotentials provide a basis for the first regulation theory that the enantioselective reaction is predominantly controlled by the presence of relativistic effects, that is, spinorbit coupling resulting from heavy atoms in the chiral Lewis acid catalyst.  [Show abstract] [Hide abstract]
ABSTRACT: The sandwich complex bis(η8pentalene)cerium Ce(C8H6)2 was investigated by relativistic quantum chemical ab initio methods and the nature of the interaction between the Ce 4f and ligand π orbitals was analyzed. It is shown for a multiconfigurational ground state wavefunction of the complete active space variety that by means of an orbital rotation in the space of the strongly interacting orbitals it is possible to generate nearly pure Ce 4f and ligand π orbitals without changing the total energy. These orbitals are occupied by one electron each, leading to an openshell 4f1π1 ground state configuration. The admixture of the energetically higher closedshell 4f0π2 configuration leads to a singlet ground state, making the complex another example for a molecular analog of a Ce(III)based lattice Kondo system. A further analysis using local spin and occupation number fluctuations in a twoelectron twoorbital description reveals that the Ce 4fligand π interaction corresponds to a significantly stretched covalent bond: orbital and configuration mixing coefficients as well as local spin and occupation number fluctuations are very similar to those of hydrogen dimer H2 at an interatomic distance of about two times the equilibrium distance.  [Show abstract] [Hide abstract]
ABSTRACT: The geometric and electronic structures of bis(cyclopentadienyl)cerium compounds of the type Cp2CeZ (Z=CH2, CH, NH, O, F+) are analyzed paying special attention to the terminal metalligand multiple bonding in the CeZ units. Complete active space selfconsistent field calculations were performed, followed by unitary transformations in the active orbital space in order to monitor the weights of the leading configurations, as well as the Ce and Z character of the orbitals. It is shown that all compounds, except the closedshell Ce(IV) complex Cp2CeF+, have an openshell singlet ground state and the 4f orbitals are important for their electronic structure. In case of Z=CH2, CH the singlet ground state consists to more than 90% of the Cefπ1Zpπ1 configuration when using nearly pure cerium 4f and carbon 2p orbitals of the Z group, implying Ce(III) systems. In contrast, the systems containing Z=NH, O revealed a more mixed Ce(III)/Ce(IV) ground state wavefunction. The interactions of the active orbitals were characterized by calculating the expectation values of the charge fluctuation operator and the local spin operator. The results are evaluated by comparison with the values achieved by stretching the covalent bond of a H2 molecule. The CH2 and the CH complexes show, besides a CeZ σ bond, a quite weak covalent Ce fZ p orbital interaction in π symmetry, while the other two complexes (Z=NH, O) exhibit a stronger covalent interaction with a noticeable ionic character because of the higher electronegativities of nitrogen and oxygen.  [Show abstract] [Hide abstract]
ABSTRACT: Global optimization of cluster geometries is of fundamental importance in chemistry and an interesting problem in applied mathematics. In this work, we introduce a relatively new swarm intelligence algorithm, i.e. the artificial bee colony (ABC) algorithm proposed in 2005, to this field. It is inspired by the foraging behavior of a bee colony, and only three parameters are needed to control it. We applied it to several potential functions of quite different nature, i.e., the CoulombBornMayer, LennardJones, Morse, Z and Gupta potentials. The benchmarks reveal that for longranged potentials the ABC algorithm is very efficient in locating the global minimum, while for shortranged ones it is sometimes trapped into a local minimum funnel on a potential energy surface of large clusters. We have released an efficient, userfriendly, and free program "ABCluster" to realize the ABC algorithm. It is a blackbox program for nonexperts as well as experts and might become a useful tool for chemists to study clusters.  [Show abstract] [Hide abstract]
ABSTRACT: The experimentally observed extraction complexes of trivalent lanthanide Eu(3+) and actinide Am(3+)/Cm(3+) cations with Cyanex272 [bis(2,4,4trimethylpentyl) phosphinic acid, denoted as HC272] and Cyanex301 [bis(2,4,4trimethylpentyl) dithiophosphinic acid, denoted as HC301] have been studied by using relativistic energyconsistent 4f and 5fincore pseudopotentials for trivalent f elements, combined with density functional theory and a continuum solvation model. It has been found that, as a result of hydrogen bonding, HC272 exists primarily as a selfassociated species, whereas HC301 is preferably a monomer. The calculations show that in case of all three M(3+) (M=Eu, Am, Cm) ions for HC272 the extraction complexes M[H(C272)2]3 are formed prior to M(C272)3, whereas for HC301 the extraction complexes M(C301)3 have priority over M[H(C301)2]3. The calculated MO and MS bond lengths and the MP distances of these prefered extraction complexes agree very well with the available experimental data. The obtained changes of the Gibbs free energies in the liquidliquid extraction reactions (1): M(3+)aqu + 3(HC272)2,org > M[H(C272)2]3,org + 3H(+)aqu} and (2): M(3+)aqu} + 3HC301,org > M(C301)3,org + 3H(+)aqu agree with the experimentally observed thermodynamical priority of HC272 and HC301, i.e., HC272 prefers Eu(3+) over Am(3+)/Cm(3+) and HC301 prefers Am(3+)/Cm(3+) over Eu(3+). The obtained changes of Gibbs free energies in reaction (2) (Eu, 68.10 kJ/mol; Am, 46.50 kJ/mol) agree quite well with the experimental findings (Eu, 63.3 kJ/mol; Am, 44.1 kJ/mol).  [Show abstract] [Hide abstract]
ABSTRACT: A detailed theoretical study of the mechanism and energetics of an organocatalysis based on CN activation by halogenbonding is presented for the hydrocyanation of Nbenzylidenemethylamine. The calculations at the level of scalarrelativistic gradientcorrected density functional theory give an insight in this catalytic concept and provide information on the characteristics of four different monodentate catalyst candidates acting as halogenbond donors during the reaction. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc. 
Article: ThirdOrder Incremental DualBasis Set ZeroBuffer Approach for Large HighSpin OpenShell Systems
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ABSTRACT: The thirdorder incremental dualbasis set zerobuffer approach (inc3dbB0) is an efficient, accurate and blackbox quantum chemical method for obtaining correlation energies of large systems, and has been successfully applied to many real chemical problems. In this work we extend this approach to highspin openshell systems. In the openshell approach we will first decompose the occupied orbitals of a system into several domains by a Kmeans clustering algorithm. The essential part is that we preserve the active (singly occupied) orbitals in all the calculations of the domain correlation energies. The duplicated contributions of the active orbitals to the correlation energy are subtracted from the incremental expansion. All techniques of truncating the virtual space such as the B0 approximation can be applied. This openshell inc3dbB0 approach is combined with the CCSD and CCSD(T) methods and applied to the computations of a singlettriplet gap and an electron detachment process. Our approach exhibits an accuracy better than 0.6 kcal/mol or 0.3 eV compared with the standard implementation, while it saves a large amount of the computational time and can be efficiently parallelized.  [Show abstract] [Hide abstract]
ABSTRACT: This chapter describes some important aspects of the shell structure, relativistic effects, and electron correlation effects for lanthanide and actinide atoms and molecules, mainly using the example of Cerium (Ce) and Thorium (Th). It turns to a discussion of the electronic structure of ceriumbis(η8cyclooctatetraene), cerocene, which fascinated this author for more than two decades, as well as a related cerium (III) based molecular Kondo system, i.e., bis(η8pentalene) cerium. These systems feature many problems of an accurate relativistic correlated description of their electronic structure and moreover leave plenty of room for alternative interpretations of their electronic ground state as well as the involvement of 4f orbitals in chemical bonding. In order to describe accurately the electronic structure of manyelectron systems, highly correlated approximate solutions of the Schrödinger equation built from the best possible relativistic Hamiltonian are needed.  [Show abstract] [Hide abstract]
ABSTRACT: For a wide range of trivalent lanthanide ion coordination complexes of tricapped trigonal prism or monocapped square antiprism configurations, the bonds between the central lanthanide ions and the capping ligands are found to violate Badger's rule: they can get weaker as they get shorter. We demonstrate that this observation originates from the screening and repulsion effect of the prism ligands. Both effects enhance as the electric field of the central ion or the softness of the prism ligands increases. Thus for heavier lanthanides despite that the capping bond could be shorter, it is more efficient to be weakened by the prism ligands, being inherently labile. This concept of "labile capping bonds phenomenon" is then successfully used to interpret many problems in lanthanide(III) hydration, e.g. why the water exchange rate of a lanthanide(III) complex is much higher in twisted square antiprism than in square antiprism configuration. Thus the theory proposed in this paper offers new insights in understanding chemical problems.  [Show abstract] [Hide abstract]
ABSTRACT: The Gibbs energies of hydration of actinoid(III) ions are evaluated for density functional optimized geometries of [An(H2O)h]3+ complexes (h = 8, 9) at the coupled cluster singles, doubles, and perturbative triples level by means of the incremental scheme. Scalarrelativistic 5fincore pseudopotentials for actinoids and basis sets of polarized tripleζ quality were applied. The calculated Gibbs energies for the octa and nonaaquo complexes agree within 1% with the experimental values which are available only for uranium and plutonium. Compared to the hydrate complexes of the lanthanoid(III) ions those of the actinoid(III) series are slightly less stable.  [Show abstract] [Hide abstract]
ABSTRACT: The bond length and dissociation energy of the Copernicium dimer calculated at different levels of theory are presented. The coupledcluster and the density functional method are used in fully relativistic fourcomponent allelectron calculations and twocomponent calculations using effective core potentials, which include the Breit contribution and quantum electrodynamic effects. The results show the importance of the spin–orbit coupling and also the Gaunt/Breit contribution, which could be easily included in the pseudopotential calculations. Additionally, a reversal in the trend of the bonding of the group 12 dimers could be shown by analyzing the charge fluctuation and local spin expectation values. 
Dataset: JCP08734
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ABSTRACT: In this paper we present ab initio manybody calculations on the strain energy of W silica, taken as a model system for edgesharing tetrahedral SiO2 systems with respect to cornersharing ones as in a quartz. The meanfield results were obtained using the restricted Hartree–Fock approach, while the manybody effects were taken into account by the secondorder Møller–Plesset perturbation theory and the coupledcluster approach. Correlation contributions are found to play an important role to determine the stability of edgesharing units. The most sophisticated method used in our calculation, i.e., the coupledcluster approach with single and double excitations, yields a strain energy of 0.0427 a.u. per Si2O4 unit with respect to a quartz, which is even smaller than the value obtained by a previous density functional theory calculation.  [Show abstract] [Hide abstract]
ABSTRACT: By stateoftheart quantum chemical methods, we show that for bulky functional groups like cyclohexane, [20]fullerene, dodecahedrane, and C60, the attractive dispersion interaction can have a greater impact on stereochemistry than the repulsive steric effect, making the compact isomer the more stable one. In particular, for the double C60 adduct of pentacene 1, the syn isomer should be the main product instead of the anti one inferred in the original synthesis experiment (Y. Murata et al., J. Org. Chem. 1999, 64, 3483). With and without dispersion interactions taken into account, the Gibbs energy difference ΔG(syn−anti) is −6.36 and +1.15 kcal mol−1, respectively. This study reminds us that dispersion interactions as well as electrostatic or hyperconjugation effects, etc. can lead to some unusual stereochemical phenomena.  [Show abstract] [Hide abstract]
ABSTRACT: The hydration of all trivalent lanthanoid (Ln) ions is studied theoretically from two aspects: energy and wave function. With the help of the incremental scheme, for the first time the lanthanoid(III) aqua complexes are computed at the CCSD(T) level using large basis sets. These computations prove that SCSMP2 is nearly as accurate as CCSD, thus enabling us to give the most accurate first principle hydration Gibbs free energies and reliable preferred coordination numbers (CNs) of lanthanoid(III) aqua complexes: 9, 8, and both, for light, heavy, and intermediate lanthanoids, respectively. Then a series of wave function analyses were performed to explore the deeper reasons for the preference of specific CNs. An unexpected observation is that as Ln goes from samarium to lutetium, the capping LnO bonds in nonaaqua lanthanoid complexes become weaker while they get shorter. Therefore, as the capping LnO bonds are getting easier to disrupt, heavier lanthanoids will prefer a low CN, i.e., 8. On the basis of this and previous work of other groups, a model for the water exchange kinetics of lanthanoid(III) ions is proposed. This model suggests that the capping LnO bonds of moderate strength, which occur for intermediate lanthanoids, are advantageous for the formation of a bicapped trigonal prism intermediate during water exchange. This explains some NMR experiments and, more importantly, an observation which puzzled investigators for a long time, i.e., that the exchange rate reaches a maximum for the middle region but is low at the beginning and end of the lanthanoid series. This nontrivial behavior of capping LnO bonds is interpreted and is believed to determine the hydration behavior of lanthanoid(III) ions.  [Show abstract] [Hide abstract]
ABSTRACT: In complexes of trivalent Eu and Am standard unrestricted KohnSham density functional calculations tend to yield shorter bond distances for the AmX than for the EuX bonds, especially when X is a so called soft ligand. Since the ionic radius of AmIII is larger than the one of EuIII the reversed order of the bond distances is sometimes explained by a higher covalency of the AmX bond compared to the one of the EuX bond. A comparison of density functional with wavefunctionbased correlated calculations for several model systems reveals, however, that the energetically lowlying and spatially compact 4f shell of EuIII often is erroneously filled with significantly more than 6 electrons at the density functional theory level, thus yielding considerably too long bond distances. Particularly claims based on comparisons of structures optimized at the density functional level that the strong preference of the Cyanex 301 ligand for AmIII over EuIII is due to an increased covalency in the AmS bonds should be viewed with some reservation.  [Show abstract] [Hide abstract]
ABSTRACT: Smallcore pseudopotentials for actinium, thorium, and protactinium have been energyadjusted to multiconfiguration DiracHartreeFock reference data based on the DiracCoulombBreit Hamiltonian and the Fermi nucleus model. Corresponding optimized valence basis sets of polarized valence quadruplezeta quality are presented. Atomic test calculations for the first four ionization potentials show satisfactory results at both the HartreeFock as well as the multireference averaged coupledpair functional level. Highlycorrelated Fockspace coupled cluster calculations demonstrate that the new pseudopotentials yield ionization potentials, which are in excellent agreement with corresponding allelectron results and experimental data. The pseudopotentials and basis sets supplement a similar set previously published for uranium.  [Show abstract] [Hide abstract]
ABSTRACT: Two alternative interpretations are given for the electronic ground state of bisη8annulene[8] cerium, cerocene, based on the same relativistic DouglasKrollHess complete active space allelectron wavefunction. Rotations in the spaces of the one and manyelectron wavefunctions, leaving the total energy invariant, show that the system can be viewed as a complex of a closedshell Ce(IV) ion sandwiched by two aromatic annulene[8] dianions and bonded with a significant Ce 4f  ring π covalency, or as a Ce(III) ion with an almost atomiclike 4f1 subconfiguration, coupled to the unpaired electron in the rings highest energy occupied π orbitals in a Kondotype fashion.  [Show abstract] [Hide abstract]
ABSTRACT: The thirdorder incremental dualbasis set zerobuffer approach was combined with CCSD(T)F12x (x = a, b) theory to develop a new approach, i.e., the inc3dbB0CCSD(T)F12 method, which can be applied as a blackbox procedure to efficiently obtain the near complete basis set (CBS) limit of the CCSD(T) energies also for large systems. We tested this method for several cases of different chemical nature: four complexes taken from the standard benchmark sets S66 and X40, the energy difference between isomers of water hexamer and the rotation barrier of biphenyl. The results show that our method has an error relative to the best estimation of CBS energy of only 0.2 kcal/mol or less. By parallelization, our method can accomplish the CCSD(T)F12 calculations of about 60 correlated electrons and 800 basis functions in only several days, which by standard implementation are impossible for ordinary hardware. We conclude that the inc3dbB0CCSD(T)F12a/AVTZ method, which is of CCSD(T)/AV5Z quality, is close to the limit of accuracy that one can achieve for large systems currently.  [Show abstract] [Hide abstract]
ABSTRACT: Not only σ*σ but also σ*π: Highlevel electronic structure calculations reveal the σ*σ and σ*π bonding patterns for Au(I) Au(I) bonding in excited states and suggest two conformationdependent photophysical relaxation mechanisms for dicyanoaurate oligomers (n=25; see picture) in aqueous solution. These insights into the excitedstate electronic structure should also be relevant for other gold complexes with a similar gold scaffold.
Publication Stats
20k  Citations  
782.24  Total Impact Points  
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Institutions

20032015

University of Cologne
 Institute for Theoretical Chemistry
Köln, North RhineWestphalia, Germany


20002010

University of Bonn
 • Institute for Inorganic Chemistry
 • Institute of Physical and Theoretical Chemistry
Bonn, North RhineWestphalia, Germany


19952002

Max Planck Institute of Physics
München, Bavaria, Germany


19952000

Max Planck Institute for Dynamics of Complex Technical Systems
Magdeburg, SaxonyAnhalt, Germany


1999

Max Planck Institute for the Physics of Complex Systems
Dresden, Saxony, Germany


19831995

Universität Stuttgart
 Institute of Theoretical Chemistry
Stuttgart, BadenWuerttemberg, Germany


1989

Universität Siegen
Siegen, North RhineWestphalia, Germany
