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Martin Kaupp

Publications and source records attributed to Martin Kaupp.

At least 19 recordsLinked to original sources
PubMed2007

The role of radial nodes of atomic orbitals for chemical bonding and the periodic table.

The role of radial nodes, or of their absence, in valence orbitals for chemical bonding and periodic trends is discussed from a unified viewpoint. In particular, we emphasize the special role of the absence of a radial node whenever a shell with angular quantum number l is occupied for the first time (lack of "primogenic repulsion"), as with the 1s, 2p, 3d, and 4f shells. Although the consequences of the very compact 2p shell (e.g. good isovalent hybridization, multiple bonding, high electronegativity, lone-pair repulsion, octet rule) are relatively well known, it seems that some of the aspects of the very compact 3d shell in transition-metal chemistry are less well appreciated, e.g., the often weakened and stretched bonds at equilibrium structure, the frequently colored complexes, and the importance of nondynamical electron-correlation effects in bonding.

Journal Article↗
PubMed2007

Density functional calculations of NMR shielding tensors for paramagnetic systems with arbitrary spin multiplicity: validation on 3d metallocenes.

The calculation of nuclear shieldings for paramagnetic molecules has been implemented in the ReSpect program, which allows the use of modern density functional methods with accurate treatments of spin-orbit effects for all relevant terms up to order Omicron(alpha4) in the fine structure constant. Compared to previous implementations, the methodology has been extended to compounds of arbitrary spin multiplicity. Effects of zero-field splittings in high-spin systems are approximately accounted for. Validation of the new implementation is carried out for the 13C and 1H NMR signal shifts of the 3d metallocenes 4VCp2, 3CrCp2, 2MnCp2, 6MnCp2, 2CoCp2, and 3NiCp2. Zero-field splitting effects on isotropic shifts tend to be small or negligible. Agreement with experimental isotropic shifts is already good with the BP86 gradient-corrected functional and is further improved by admixture of Hartree-Fock exchange in hybrid functionals. Decomposition of the shieldings confirms the dominant importance of the Fermi-contact shifts, but contributions from spin-orbit dependent terms are frequently also non-negligible. Agreement with 13C NMR shift tensors from solid-state experiments is of similar quality as for isotropic shifts.

Journal Article↗
PubMed2007

Hyperfine coupling tensors of the benzosemiquinone radical anion from Car-Parrinello molecular dynamics.

Based on Car-Parrinello ab initio molecular dynamics simulations of the benzosemiquinone radical anion in both aqueous solution and the gas phase, density functional calculations provide the currently most refined EPR hyperfine coupling (HFC) tensors of semiquinone nuclei and solvent protons. For snapshots taken at regular intervals from the molecular dynamics trajectories, cluster models with different criteria for inclusion of water molecules and an additional continuum solvent model are used to analyse the HFCs. These models provide a detailed picture of the effects of dynamics and of different intermolecular interactions on the spin-density distribution and HFC tensors. Comparison with static calculations allows an assessment of the importance of dynamical effects, and of error compensation in static DFT calculations. Solvent proton HFCs depend characteristically on the position relative to the semiquinone radical anion. A point-dipolar model works well for in-plane hydrogen-bonded protons but deviates from the quantum chemical values for out-of-plane hydrogen bonding.

Journal Article↗
PubMed2007

A thermochemically competitive local hybrid functional without gradient corrections.

Following the suggestion of local hybrid functionals with position-dependent exact-exchange admixture [J. Jaramillo, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 1068 (2003)], a functional that mixes only local and exact exchange plus local correlation has been constructed. With a simple local mixing function for the position dependence, this Lh-SVWN functional provides atomization energies for the G2-1 set that are competitive with currently available state-of-the-art functionals like, e.g., B3LYP. This is achieved without generalized gradient approximations for exchange or correlation.

Journal Article↗
PubMed2006

Where is the limit of highly fluorinated high-oxidation-state osmium species?

The structures and stabilities of various osmium fluorides and oxyfluorides in high oxidation states have been studied by quantum-chemical calculations at DFT (B3LYP), MP2, CCSD, and CCSD(T) levels. The calculations indicate that the homoleptic fluorides all the way up to OsF8 may exist, even though OsF8 will be difficult to prepare. The last missing osmium oxyfluoride, OsOF6, is computed to be thermochemically stable against mononuclear gas-phase elimination reactions. The problem with the nonexistence of such highly fluorinated complexes appears thus to be mainly in difficult synthetic access under typical condensed-phase conditions. Matrix-isolation techniques might provide a means to characterize the highly fluorinated OsVIII and OsVII species.

Journal Article↗
PubMed2006

Calculation of zero-field splitting parameters: comparison of a two-component noncolinear spin-density-functional method and a one-component perturbational approach.

Two different sets of approaches for the density-functional calculation of the spin-orbit contributions to zero-field splitting (ZFS) parameters of high-spin systems have been implemented within the same quantum chemistry code ReSpect and have been validated and compared for a series of model systems. The first approach includes spin-orbit coupling variationally in a two-component calculation, using either an all-electron Douglas-Kroll-Hess ansatz or two-component relativistic pseudopotentials. The ZFS parameters are computed directly from energy differences between different relativistic states. Additionally, an approximate second-order perturbation theory approach has been implemented, based on nonrelativistic or scalar relativistic wave functions. For a series of group 16 triplet diatomics and for the octet GdH3 molecules, two-component density functional calculations underestimate the zero-field splitting D systematically by a factor of 2. This may be rationalized readily by the incomplete description of states with absolute value MJ < J by a single-determinantal wave function built from two-component spinors. In the case of two 3d transition metal complexes and for GdH3, the results depend furthermore sensitively on exchange-correlation functional. Results of the alternative one-component approach agree strikingly with the two-component data for systems with small spin-orbit effects and start to deviate from them only for heavier systems with large spin-orbit effects. These results have fundamental implications for the achievable accuracy of one-component density-functional approaches used widely to compute ZFS parameters in the field of molecular magnetism. Possible refinements of both one-and two-component approaches are discussed.

Journal Article↗
PubMed2006

Scalar relativistic calculations of hyperfine coupling tensors using the Douglas-Kroll-Hess method with a finite-size nucleus model.

A scalar relativistic method to calculate hyperfine coupling tensors at the Douglas-Kroll-Hess level has been extended to incorporate a finite-size nucleus model using a Gaussian charge and magnetic moment distribution. Density functional calculations at gradient-corrected and hybrid functional levels have been carried out for the group 11 atoms and for a set of small group 12 molecules, comparing nonrelativistic as well as scalar relativistic results at second-order Douglas-Kroll-Hess level with and without finite-size nucleus. While nonrelativistic calculations underestimate isotropic hyperfine couplings increasingly with increasing nuclear charge, scalar relativistic calculations with point nucleus provide somewhat overestimated values. Inclusion of the finite-size nuclear model in the calculation of the wavefunction, and in the transformed hyperfine operators both decrease the magnitude of the hyperfine couplings. The effects, which are cumulative, improve agreement with experiment.

Journal Article↗
PubMed2006

From local hybrid functionals to "localized local hybrid" potentials: formalism and thermochemical tests.

Hybrid exchange-correlation functionals with position-dependent exact-exchange admixture (local hybrid functionals) have been implemented self-consistently for the first time. Functional derivatives with respect to the occupied orbitals have been derived and were subsequently transformed into local and multiplicative potentials within the framework of the optimized effective potential. The resulting local and multiplicative Kohn-Sham potentials are termed "localized local hybrid" (LLH) potentials. They have been evaluated in calculations of atomization energies for a series of main-group molecules. It is shown that LLH potentials yield somewhat better thermochemical results than non-self-consistent post-GGA calculations with the same local hybrid energy functionals for orbitals obtained with a different potential. The choice of the "local mixing function" (LMF) is discussed. This is the key quantity for the performance of local hybrid functionals that determines the amount of exact-exchange admixture at a given point in space. Careful analyses of average exact-exchange admixtures and of the spatial variation of two different LMFs for various molecules provide insight into the shortcomings of the currently used local hybrid functionals. Beyond a too large average exact-exchange admixture, both LMFs used appear to provide an unbalanced description of exact-exchange admixture across bonds to hydrogen. LLH potentials open the way for property calculations with local hybrid functionals.

Journal Article↗
PubMed2006

Protein-cofactor interactions and EPR parameters for the Q(H) quinone binding site of quinol oxidase. A density functional study.

Recent multifrequency EPR studies of the "high-affinity" quinone binding site of quinol oxidase (Q(H) site) have suggested a very asymmetric hydrogen-bonding environment for the semiquinone radical anion state. Single-sided hydrogen bonding to the O1 carbonyl position was one of the proposals, which contrasts with some previous experimental indications. Here density functional calculations of the EPR parameters (g-tensors, 13C, 1H, and 17O hyperfine tensors) for a wide variety of supermolecular model complexes have been used to provide insight into the detailed relations among structure, environment, and EPR parameters of ubisemiquinone radical anions. A single-sided binding model is not able to account for the experimentally observed low g(x) component of the g-tensor or for the observed magnitude of the asymmetry of the 13C carbonyl HFC tensors. Based on the detailed comparison between computation and experiment, a model with two hydrogen bonds to O1 and one hydrogen bond to O4 is suggested for the Q(H) site, but a model with one more hydrogen bond on each side cannot be excluded. Several general conclusions on the interrelations between EPR parameters and hydrogen bond patterns of ubisemiquinones in proteins are provided.

Benzoquinones↗
PubMed2006

Molecular mechanical devices based on quinone-pyrrole and quinone-indole dyads: a computational study.

A set of intramolecularly connected dyads consisting of a quinone unit and a pyrrole or indole moiety have been designed and evaluated in quantum-chemical calculations. It is shown computationally for several systems, depending on the length and attachment points of the interconnecting chains, that a reduction of the quinone to the semiquinone radical anion or quinolate dianion state leads to a reversible intramolecular reorientation from a pi-stacked to a T-stacked arrangement. In the rearranged structures, a hydrogen bond from the pyrrole or indole N-H function to the semiquinone or quinolate pi-system is created upon reduction. In some systems, hydrogen bonds to the semiquinone or quinolate oxygen atoms are partly feasible and will be preferred over T-stacking. The choice of systems has been based on recent computational observations related to photosystem I. Systems with pyrrole or indole units should provide a better basis for the envisioned molecular motor than recently proposed quinone-benzene dyads. The intramolecular interactions modify the quinone redox potentials. Electronic g-tensors have been computed for the semiquinone states. These reflect characteristically the presence and nature of hydrogen bonds to the semiquinone and represent suitable electron paramagnetic resonance spectroscopic probes for the preferred structures. Intramolecular proton transfer is possible in the dianionic state.

Electron Spin Resonance Spectroscopy↗
PubMed2006

Density functional study of electron paramagnetic resonance parameters and spin density distributions of dicopper(I) complexes with bridging azo and tetrazine radical-anion ligands.

There exists a growing class of dinuclear complexes with bridging radical-anion ligands that is of interest both for bioinorganic and for supermolecular chemistry. Their bonding situation as well as chemical and spectroscopic properties are not described adequately by standard models such as the ligand-field theory. For rational design of complexes with desired properties, it is thus necessary to understand better the interrelations between electronic structure, spin density, and electron paramagnetic resonance (EPR) parameters in dinuclear systems with redox-active bridging ligands and to evaluate the performance of density functional methods in their description. As particularly suitable, experimentally well-characterized representatives, a series of dinuclear copper(I) complexes with azo or tetrazine bridge ligands have been studied here by different density functional methods. To reproduce the available experimental metal hyperfine couplings, the inclusion of spin-orbit effects into the calculations is necessary. An unusual direction of the dependence of computed hyperfine couplings on an exact-exchange admixture into the exchange-correlation functional may be understood from a McConnell-type spin polarization of the sigma-framework of the bridge. Ligand nitrogen hyperfine couplings are also compared with experiment where available. Electronic g-tensors are reproduced well by the calculations and have been analyzed in detail in terms of atomic spin-orbit contributions and electronic excitations.

Journal Article↗
PubMed2006

Resolution of identity Dirac-Kohn-Sham method using the large component only: Calculations of g-tensor and hyperfine tensor.

A new relativistic two-component density functional approach, based on the Dirac-Kohn-Sham method and an extensive use of the technique of resolution of identity (RI), has been developed and is termed the DKS2-RI method. It has been applied to relativistic calculations of g and hyperfine tensors of coinage-metal atoms and some mercury complexes. The DKS2-RI method solves the Dirac-Kohn-Sham equations in a two-component framework using explicitly a basis for the large component only, but it retains all contributions coming from the small component. The DKS2-RI results converge to those of the four-component Dirac-Kohn-Sham with an increasing basis set since the error associated with the use of RI will approach zero. The RI approximation provides a basis for a very efficient implementation by avoiding problems associated with complicated integrals otherwise arising from the elimination of the small component. The approach has been implemented in an unrestricted noncollinear two-component density functional framework. DKS2-RI is related to Dyall's [J. Chem. Phys. 106, 9618 (1997)] unnormalized elimination of the small component method (which was formulated at the Hartree-Fock level and applied to one-electron systems only), but it takes advantage of the local Kohn-Sham exchange-correlation operators (as, e.g., arising from local or gradient-corrected functionals). The DKS2-RI method provides an attractive alternative to existing approximate two-component methods with transformed Hamiltonians (such as Douglas-Kroll-Hess [Ann. Phys. 82, 89 (1974); Phys. Rev. A 33, 3742 (1986)] method, zero-order regular approximation, or related approaches) for relativistic calculations of the structure and properties of heavy-atom systems. In particular, no picture-change effects arise in the property calculations.

Journal Article↗
PubMed2006

Has AuF7 been made?

Quantum chemical calculations at DFT (BP86, B3LYP, BHLYP), MP2, CCSD, and CCSD(T) levels have been carried out on various fluoro complexes of gold in oxidation states +V through +VII to evaluate the previously claimed existence of AuF7. The calculations indicate clearly that elimination of F2 from AuF7 is a strongly exothermic reaction with a low activation barrier. This is inconsistent with the reported stability of AuF7 up to room temperature. A reported experimental vibrational frequency at 734 cm(-1) for AuF7 could not be verified computationally. It is concluded that the reported observation of AuF7 was probably erroneous. As the calculations indicate also an extremely large electron affinity and little stability for AuF6, Au(V) remains the highest well-established gold oxidation state.

Journal Article↗
PubMed2005

Relativistic two-component calculations of electronic g-tensors that include spin polarization.

The first two-component relativistic density-functional approach for the calculation of electronic g-tensors is reported that includes spin polarization using noncollinear spin-density functionals. The method is based on the relativistic Douglas-Kroll-Hess Hamiltonian and has been implemented into the ReSpect program package. Using three self-consistent-field calculations with orthogonal orientations of total magnetization J, the full g-matrix may be obtained. In contrast to previous spin-restricted two-component treatments, results with the new approach agree excellently with spin-polarized one-component calculations for light-atom radicals. Additionally, unlike one-component approaches, the method also reproduces successfully the negative deltag(parallel)-values of heavy-atom 2sigma radicals and the negative deltag(perpendicular) components in cysteinyl. The new method removes effectively the dilemma existing up to now regarding the simultaneous inclusion of spin polarization and higher-order spin-orbit effects in g-tensor calculations. It is straightforwardly applicable to higher than doublet spin multiplicities and has been implemented with hybrid functionals.

Journal Article↗
PubMed2005

New route to 1-thia-closo-dodecaborane(11), closo-1-SB11H11, and its halogenation reactions. The effect of the halogen on the dipole moments and the NMR spectra and the importance of spin-orbit coupling for the 11B chemical shifts.

Reaction between nido-B10H14 (1) and elemental sulfur in CHCl3 in the presence of Et3N at room temperature, followed by treatment with Et3N.BH3 at 170-190 degrees C, resulted in the isolation of closo-1-SB11H11 (2) in 50% yield. Selected electrophilic halogenation reactions of compound led to the isolation of a series of monohalogenated derivatives of general constitution 12-X-closo-1-SB11H10 (12-X-, where X = Cl, Br, and I). The structures of 12-Cl- and 12-I- were determined by an X-ray diffraction analysis and the structures of all compounds were geometry optimised at the RMP2(fc)/6-31G* level. The constitution of all compounds is consistent with the results of mass spectrometry and multinuclear (1H and 11B) spectroscopy complemented by two-dimensional [11B-11B]-COSY and 1H{11B(selective)} NMR measurements. Experimental 11B chemical shifts generally show acceptable agreement with theoretical values calculated by GIAO methods, but spin-orbit coupling must be included for nuclei bearing heavy-atom substituents such as Br or I. The dipole moments determined for the B12-X bonds show similarities to those of aliphatic C-X bonds and confirm unambiguously the B12 --> S dipole moment orientation in the SB11 cage.

Journal Article↗
PubMed2005

Where is the spin? Understanding electronic structure and g-tensors for ruthenium complexes with redox-active quinonoid ligands.

Understanding the bonding in transition metal complexes with redox-active ligands is a major challenge, for example in redox catalysis or in bioinorganic chemistry. In this work, electronic g-tensors, spin-density distributions, and electronic structure have been studied by different density functional methods for an extended series of complexes [Ru(acac)2(L)]n (n = -1, 0, +1; L = redox-active o-quinonoid ligand). Comparison is made with experimental g-tensors and g-tensor-based oxidation-state assignments for a number of experimentally studied examples, using both gradient-corrected (BP86) and hybrid functionals (B3LYP, BHLYP) representing a range of exact-exchange admixtures. Reasonable, albeit not perfect, agreement with experimental g-tensors is obtained in one-component DFT calculations with hybrid functionals. Analyses of spin densities confirm the assignment of the cationic complexes as predominantly d5-Ru(III) with a neutral quinonoid ligand. However, this conclusion is obtained only after inclusion of the appreciable spin polarization of the unrestricted determinant, while the singly occupied molecular orbital (SOMO) is localized more on the acac ligands. The anionic complexes turn out to be approximately halfway between a d6-Ru(II)/semiquinone and a d5-Ru(III)/catecholate formulation, but again only after taking into account the extensive spin polarization. Even the previous assignment of the neutral parent systems as d5-Ru(III)/semiquinone is not accurate, as a d6-Ru(II)/quinone resonance structure contributes to some extent. Very unusual trends in the spin contamination of the Kohn-Sham determinant with increasing exact-exchange admixture in some of the cationic complexes have been traced to an interplay between spin delocalization and spin polarization.

Electrons↗
PubMed2005

Structure of the nitrogen-centered radical formed during inactivation of E. coli ribonucleotide reductase by 2'-azido-2'-deoxyuridine-5'-diphosphate: trapping of the 3'-ketonucleotide.

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides providing the monomeric precursors required for DNA replication and repair. The class I RNRs are composed of two homodimeric subunits: R1 and R2. R1 has the active site where nucleotide reduction occurs, and R2 contains the diiron tyrosyl radical (Y*) cofactor essential for radical initiation on R1. Mechanism-based inhibitors, such as 2'-azido-2'-deoxyuridine-5'-diphosphate (N(3)UDP), have provided much insight into the reduction mechanism. N(3)UDP is a stoichiometric inactivator that, upon interaction with RNR, results in loss of the Y* in R2 and formation of a nitrogen-centered radical (N*) covalently attached to C225 (R-S-N*-X) in the active site of R1. N(2) is lost prior to N* formation, and after its formation, stoichiometric amounts of 2-methylene-3-furanone, pyrophosphate, and uracil are also generated. On the basis of the hyperfine interactions associated with N*, it was proposed that N* is also covalently attached to the nucleotide through either the oxygen of the 3'-OH (R-S-N*-O-R') or the 3'-C (R-S-N*-C-OH). To distinguish between the proposed structures, the inactivation was carried out with 3'-[(17)O]-N(3)UDP and N* was examined by 9 and 140 GHz EPR spectroscopy. Broadening of the N* signal was detected and the spectrum simulated to obtain the [(17)O] hyperfine tensor. DFT calculations were employed to determine which structures are in best agreement with the simulated hyperfine tensor and our previous ESEEM data. The results are most consistent with the R-S-N*-C-OH structure and provide evidence for the trapping of a 3'-ketonucleotide in the reduction process.

Azides↗