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Florian Weigend

Publications and source records attributed to Florian Weigend.

13 recordsLinked to original sources
PubMed2006

Basis-set extensions for two-component spin-orbit treatments of heavy elements.

The accuracy of standard basis sets of quadruple-zeta and lower quality for the use in two-component self-consistent field procedures including spin-orbit coupling is investigated for the elements In-I and Au-At. Spin-orbit coupling leads to energetic and spatial splittings of inner shells, which are not described accurately with standard basis sets optimized for scalar relativistic calculations. This results in large errors in total atomic energies and significant errors in atomization energies of compounds containing these atoms. We show how these errors can be corrected by adding just a few steep sets of basis functions and demonstrate the quality of the resulting extended basis sets.

Astatine↗
PubMed2006

Reactions of mixed silver-gold cluster cations AgmAun + (m+n=4,5,6) with CO: radiative association kinetics and density functional theory computations.

Near thermal energy reactive collisions of small mixed metal cluster cations Ag(m)Au(n) (+) (m+n=4, 5, and 6) with carbon monoxide have been studied in the room temperature Penning trap of a Fourier transform ion-cyclotron-resonance mass spectrometer as a function of cluster size and composition. The tetrameric species AgAu(3) (+) and Ag(2)Au(2) (+) are found to react dissociatively by way of Au or Ag atom loss, respectively, to form the cluster carbonyl AgAu(2)CO(+). In contrast, measurements on a selection of pentamers and hexamers show that CO is added with absolute rate constants that decrease with increasing silver content. Experimentally determined absolute rate constants for CO adsorption were analyzed using the radiative association kinetics model to obtain cluster cation-CO binding energies ranging from 0.77 to 1.09 eV. High-level ab initio density functional theory (DFT) computations identifying the lowest-energy cluster isomers and the respective CO adsorption energies are in good agreement with the experimental findings clearly showing that CO binds in a "head-on" fashion to a gold atom in the mixed clusters. DFT exploration of reaction pathways in the case of Ag(2)Au(2) (+) suggests that exoergicities are high enough to access the minimum energy products for all reactive clusters probed.

Journal Article↗
PubMed2006

Accurate Coulomb-fitting basis sets for H to Rn.

A series of auxiliary basis sets to fit Coulomb potentials for the elements H to Rn (except lanthanides) is presented. For each element only one auxiliary basis set is needed to approximate Coulomb energies in conjunction with orbital basis sets of split valence, triple zeta valence and quadruple zeta valence quality with errors of typically below ca. 0.15 kJ mol(-1) per atom; this was demonstrated in conjunction with the recently developed orbital basis sets of types def2-SV(P), def2-TZVP and def2-QZVPP for a large set of small molecules representing (nearly) each element in all of its common oxidation states. These auxiliary bases are slightly more than three times larger than orbital bases of split valence quality. Compared to non-approximated treatments, computation times for the Coulomb part are reduced by a factor of ca. 8 for def2-SV(P) orbital bases, ca. 25 for def2-TZVP and ca. 100 for def2-QZVPP orbital bases.

Journal Article↗
PubMed2006

Structural relaxation in charged metal surfaces and cluster ions.

Space-charge layers can noticeably affect the properties of metal electrode surfaces, for instance by modifying the surface dielectric response or indirectly via the induced atomic relaxations. While there are efforts to exploit this concept for designing novel functional nanomaterials, the underlying microscopic processes are poorly understood. Here, we report on a density functional theory (DFT) study of atomic relaxation in Au cluster ions comprising up to 309 atoms. Suitable averages over atomic displacements respond to charging consistent with experimental observation on macroscopic Au single-crystal surfaces. Moreover, the overall DFT response is also consistent with predictions of a simple phenomenological model. Motivated by our observations, we propose a scenario in which the surface relaxation ("stretch") results from out-of-plane Hellman-Feynman forces exerted on the surface atoms by the excess charge, and where the in-plane surface stress represents essentially an elastic transverse contraction tendency of the surface layer in response to stretch.

Electrons↗
PubMed2005

Unusual syntheses, structures, and electronic properties of compounds containing ternary, T3-type supertetrahedral M/Sn/S anions [M5Sn(mu3-S)4(SnS4)4](10- (M = Zn, Co).

A recently discovered series of quaternary compounds of the general type [K(m)(ROH)(n)()][M(x)Sn(y)()Se(z)] (R = H, Me), containing ternary anions with [SnSe(4)](4-)-coordinated transition metal centers (M = Co, Mn, Zn, Cd, Hg) has now been extended by the synthesis and characterization of the two ortho-thiostannate-coordinated species, [Na(10)(H(2)O)(32)][M(5)Sn(mu(3)-S)(4)(SnS(4))(4)].2H(2)O (M = Zn (1), Co (2)). The central structural motifs of compounds 1 and 2 are highly charged [M(5)Sn(mu(3)-S)(4)(SnS(4))(4)](10-) anions, being the first T3-type supertetrahedral ternary anions reported to date. The exposure of single crystals of 2 to a dynamic vacuum for several hours resulted in the reversible formation of a partially dehydrated, but still monocrystalline material of the composition [Na(10)(H(2)O)(6)][Co(5)Sn(mu(3)-S)(4)(SnS(4))(4)] (3). The loss of 28 of the 34 water molecules only slightly affects the internal structure of the ternary anion in 3 and leads to a significant compacting of the crystal structure with closer linkage of the [Co(5)Sn(5)S(20)](10-) cluster units via the Na(+) cations. Magnetic measurements on 3 show that the ground state of the Co/Sn/S cluster is S = 1/2, indicating a significant antiferromagnetic coupling between the Co centers, which has also been rationalized by DFT investigations of the electronic situation in the ternary subunits of 1-3.

Journal Article↗
PubMed2005

Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy.

Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 molecules representing (nearly) all elements-except lanthanides-in their common oxidation states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, density functional theory and correlated methods, for which we had chosen Møller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.

Algorithms↗
PubMed2005

A single-molecule diode.

We have designed and synthesized a molecular rod that consists of two weakly coupled electronic pi -systems with mutually shifted energy levels. The asymmetry thus implied manifests itself in a current-voltage characteristic with pronounced dependence on the sign of the bias voltage, which makes the molecule a prototype for a molecular diode. The individual molecules were immobilized by sulfur-gold bonds between both electrodes of a mechanically controlled break junction, and their electronic transport properties have been investigated. The results indeed show diode-like current-voltage characteristics. In contrast to that, control experiments with symmetric molecular rods consisting of two identical pi-systems did not show significant asymmetries in the transport properties. To investigate the underlying transport mechanism, phenomenological arguments are combined with calculations based on density functional theory. The theoretical analysis suggests that the bias dependence of the polarizability of the molecule feeds back into the current leading to an asymmetric shape of the current-voltage characteristics, similar to the phenomena in a semiconductor diode.

Computers, Molecular↗
PubMed2005

Atom-type assignment in molecules and clusters by perturbation theory-A complement to X-ray structure analysis.

An approach to distinguish elements with similar atomic numbers in molecules and clusters is presented and applied to experimentally synthesized and structurally characterized mixed-metallic compounds. By first treating a homogenized reference system constructed from the original compound and applying first-order perturbation theory it is possible to find the most stable distribution of the atom types to the atomic sites in a very efficient way. This work is focused on the appropriate choice of homogenized reference systems and on applications treating experimentally synthesized compounds. With these examples is shown that the method is a helpful complement to X-ray crystal structure analysis.

Crystallography, X-Ray↗
PubMed2005

Binding energies of CO on gold cluster cations Au n + (n=1-65): a radiative association kinetics study.

Room temperature CO adsorption on isolated gold cluster cations is studied over a wide size range (Au(n) (+),1 26), with notable exceptions at n=30, 31 and 48, 49 which manifest local binding energy maxima. For the smallest sizes (3<n<9 and n=20), the experimental CO-Au(n) (+) binding energies are compared with those found for the most stable isomers by density functional theory based ab initio computations. While the overall size trend can be understood in terms of classical electrostatics, the atom by atom variations are likely due to differently coordinated binding sites available on the gold clusters.

Journal Article↗
PubMed2004

Atom distributions in binary atom clusters: a perturbational approach and its validation in a case study.

An approach to describe heteroatomic clusters A(n)B(N-n) as perturbed homoatomic ones is presented. By first treating the homoatomic systems A(N) or/and B(N) and subsequent application of first-order perturbation theory it is possible to estimate relative stabilities of the 2(N) possible distributions of the atom types A and B at the N atomic sites in a very efficient manner. The approach was tested considering Ir(n)Pt(13-n) as an example (treated with density functional methods). One observes good correlation between relative stabilities estimated from the homoatomic cases and those obtained from explicit treatments of the binary systems. Moreover we rationalize the observed correlation of atom type and atom position in Ir(n)Pt(13-n).

Journal Article↗
PubMed2004

Ortho-chalcogenostannates as ligands: syntheses, crystal structures, electronic properties, and magnetism of novel compounds containing ternary anionic substructures [M4(mu4-Se)(SnSe4)4]10- (M=Mn, Zn, Cd, Hg), 3(infinity)[[Hg4(mu4-Se)(SnSe4)3]6-], or 1(infinity)[[HgSnSe4]2-].

By reaction of K4[SnSe4].1.5 MeOH with CdCl2 or Hg(OAc)2 in water/methanol it was possible to prepare single crystals of four novel compounds that contain ternary anionic coordination oligomers and polymers: [K10(H2O)16(MeOH)(0.5)][M4(mu4-Se)(SnSe4)4] (4: M=Cd, 5: M=Hg), [K6(H2O)3][Hg4(mu4-Se)(SnSe4)3].MeOH (6), and K2[HgSnSe4] (7), which were structurally characterized by single-crystal X-ray diffraction. The optical absorption properties of the isostructural compounds 4 and 5, as well as those of the recently reported Zn (2) and Mn (3) analogues, were studied by UV-visible spectroscopy. These investigations showed the quaternary phases to have relatively small optical gaps for their molecular size (2.2-2.6 eV), which are similar to the excitation energies that were observed for mesostructured solids of the respective combination of elements. According to DFT investigations on the ternary anions, an experimentally observed difference between the absorption behavior of the d10 compounds 2, 4, and 5 and the open-shell d(5) compound 3 is in line with different characters of the frontier orbitals in the two cases. Both the calculations and a magnetic measurement on 3 demonstrated antiferromagnetic coupling between the mu(4)-Se-bridged Mn centers.

Journal Article↗
PubMed2004

Syntheses of the 47 electron clusters [(Cp*Fe)3(mu3-X)2] (X = S, Se) and the First Fe/Sn/Se Heterocubane Cluster [(CpFe)3(SnCl3)(mu3-Se)4] x DME by the use of chalcogenostannate salts.

By reacting 1-aminoethylammonium (H2NCH2CH2NH3+ = enH+) salts of [Sn2E6]4- anions (E = S, Se), [enH]4[Sn2S6] (1) and [enH]4[Sn2Se6] x en (2), with FeCl2/LiCp, three novel (partly) oxidized, Cp* ligated iron chalcogenide clusters were synthesized. Two of them, [(CpFe)3(mu3-S)2] (3) and [(Cp*Fe)3(mu3-Se)2] (4), contain formally 47 valence electrons. [(Cp*Fe)3(SnCl3)(mu3-Se)4] x DME (5) represents the first known mixed metal Fe/Sn/Se heterocubane type cluster. Compounds 3-5 were structurally characterized by single-crystal X-ray diffraction, and the odd valence electron number of the [Fe3E2] clusters (E = S, Se) was confirmed by density functional (DFT) investigations, mass spectrometry, cyclic voltammetry and a susceptibility measurement of 3.

Journal Article↗