Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB.

Case studies of 1T-TiSe and YBaCuO have demonstrated that X-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. However, the utility of XRD in the investigation of superconductors like MgB lacking an additional symmetry-breaking structural phase transition is not immediately evident. Nevertheless, high-resolution powder XRD experiments on MgB in combination with maximum entropy method analyses hinted at differences between the electron density distributions at room temperature and 15 K, that is, below the of approx.

HTD2: a single-crystal X-ray diffractometer for combined high-pressure/low-temperature experiments at laboratory scale.

High-pressure (HP) X-ray diffraction experiments at low temperature (LT) require dedicated instruments as well as non-standard sample environments and measuring strategies. This is especially true when helium cryogenic temperatures below 80 K are targeted. Furthermore, only experiments on single-crystalline samples provide the prerequisites to study subtle structural changes in the - phase diagram under extreme LT and HP conditions in greater detail.

Ultrafast Vibrational Response of Activated C-D Bonds in a Chloroform-Platinum(II) Complex.

The vibrational response of the activated C-D bond in the chloroform complex [Pt(CH)(btz-,')·CDCl, where btz = 2,2'-bi-5,6-dihydro-4-1,3-thiazine] is studied by linear and nonlinear two-dimensional infrared (2D-IR) spectroscopy. The change of the C-D stretching vibration of metal-coordinated CDCl relative to the free solvent molecule serves as a measure of the non-classical Pt···D-C interaction strength. The stretching absorption band of the activated C-D bond displays a red shift of 119 cm relative to uncoordinated CDCl, a strong broadening, and an 8-fold enhancement of spectrally integrated absorption.

Acenaphtho[1,2-d][1,2,3]triazole and Its Kuratowski Complex: A π-Extended Tecton for Supramolecular and Coordinative Self-Assembly.

π-Extended acenaphtho[1,2-d][1,2,3]triazoles, the unsubstituted Anta-H and its di-tert-butyl derivative Dibanta-H, as well as 5,6,7,8-tetrahydro-1H-naphtho[2,3-d][1,2,3]triazole Cybta-H were obtained in concise syntheses. In the solid state, Dibanta-H forms an unprecedented hydrogen-bonded cyclic tetrad, stabilized by dispersion interactions of the bulky tBu substituents, whereas a cyclic triad was found in the crystal structure of Anta-H. These cyclic assemblies form infinite slipped stacks in the crystals.

Pressure-Induced Excitations in the Out-of-Plane Optical Response of the Nodal-Line Semimetal ZrSiS.

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density-functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations.

The Effects of Chemical Bonding at Subatomic Resolution: A Case Study on α-Boron.

Similar to classical , aspherical deformations of the electron density in the atomic core region can result in in refinements using a Hansen-Coppens multipolar model (HCM), especially when highly precise experimental datasets with resolutions far beyond sin(θ)/λ ≤ 1.0 Å are employed. These shifts are about two orders of magnitude smaller than their counterparts caused by valence shell deformations, and their underlying deformations are mainly of dipolar character for 1st row atoms.

Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity.

We report the observation of an antipolar phase in cubic GaNb_{4}S_{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb_{4}S_{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T_{JT}=31  K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T_{JT}.

J(Si,H) Coupling Constants of Activated Si-H Bonds.

We outline in this combined experimental and theoretical NMR study that sign and magnitude of J(Si,H) coupling constants provide reliable indicators to evaluate the extent of the oxidative addition of Si-H bonds in hydrosilane complexes. In combination with experimental electron density studies and MO analyses a simple structure-property relationship emerges: positive J(Si,H) coupling constants are observed in cases where M → L π-back-donation (M = transition metal; L = hydrosilane ligand) dominates. The corresponding complexes are located close to the terminus of the respective oxidative addition trajectory.

Complex magnetic order from multiple Ce-sites in CePdSn.

Polycrystalline [Formula: see text] [Formula: see text] [Formula: see text] samples were synthesized by arc-melting and subsequent annealing at 970 K. Specific heat, electrical resistivity and magnetic susceptibility measurements are performed over a wide range in temperature and provide hints for the presence of a complex magnetic ordering below 3 K arising from three crystallographically independent Ce sites. This behaviour is driven by a complex interplay between ferro-, ferri-, and antiferromagnetic correlations among the Ce atoms.

J(Si,H) Coupling Constants in Nonclassical Transition-Metal Silane Complexes.

We will outline that the sign and magnitude of J(Si,H) coupling constants provide a highly sensitive tool to measure the extent of Si-H bond activation in nonclassical silane complexes. Up to now, this structure-property relationship was obscured by erroneous J(Si,H) sign determinations in the literature. These new findings also help to identify the salient control parameters of the Si-H bond activation process in nonclassical silane complexes.

Communication: Determination of relativistic effects from X-ray structure factors.

In this communication, a procedure is presented which allows for the determination of the scalar-relativistic contraction of individual electronic shells of transition metal atoms from X-ray structure factor data. The procedure is verified and benchmarked employing theoretical and experimental F(hkl) data, revealing an overall good agreement between the experimentally determined results and the theoretical reference values. From the experimental data, the relativistic contraction of the n = 2 shell of a cerium atom is, for example, determined as 0.

1D to 3D dimensional crossover in the superconducting transition of the quasi-one-dimensional carbide superconductor Sc3CoC4.

The transition metal carbide superconductor Sc(3)CoC(4) may represent a new benchmark system of quasi-one-dimensional (quasi-1D) superconducting behavior. We investigate the superconducting transition of a high-quality single crystalline sample by electrical transport experiments. Our data show that the superconductor goes through a complex dimensional crossover below the onset T(c) of 4.

Anagostic interactions under pressure: attractive or repulsive?

Square-planar d(8)-ML4 complexes might display subtle but noticeable local Lewis acidic sites in axial direction in the valence shell of the metal atom. These sites of local charge depletion provide the electronic prerequisites to establish weakly attractive 3c-2e M⋅⋅⋅H-C agostic interactions, in contrast to earlier assumptions. Furthermore, we show that the use of the sign of the (1)H NMR shifts as major criterion to classify M⋅⋅⋅H-C interactions as attractive (agostic) or repulsive (anagostic) can be dubious.

Charge-scaling effect in ionic liquids from the charge-density analysis of N,N'-dimethylimidazolium methylsulfate.

The charge scaling effect in ionic liquids was explored on the basis of experimental and theoretical chargedensity analyses of [C1MIM][C1SO4] employing the quantum theory of atoms in molecules (QTAIM) approach. Integrated QTAIM charges of the experimental (calculated) charge density of the cation and anion resulted in non-integer values of ±0.90 (±0.

Probing the Zintl-Klemm concept: a combined experimental and theoretical charge density study of the Zintl phase CaSi.

The nature of the chemical bonds in CaSi, a textbook example of a Zintl phase, was investigated for the first time by means of a combined experimental and theoretical charge density analysis to test the validity of the Zintl-Klemm concept. The presence of covalent Si-Si interactions, which were shown by QTAIM analysis, supports this fundamental bonding concept. However, the use of an experimental charge density study and theoretical band structure analyses give clear evidence that the cation-anion interaction cannot be described as purely ionic, but also has partially covalent character.

Topology of the electron density of d0 transition metal compounds at subatomic resolution.

Accurate X-ray diffraction experiments allow for a reconstruction of the electron density distribution of solids and molecules in a crystal. The basis for the reconstruction of the electron density is in many cases a multipolar expansion of the X-ray scattering factors in terms of spherical harmonics, a so-called multipolar model. This commonly used ansatz splits the total electron density of each pseudoatom in the crystal into (i) a spherical core, (ii) a spherical valence, and (iii) a nonspherical valence contribution.

Interplay between crystal field splitting and Kondo effect in CeNi9Ge(4-x)Si(x).

The pseudo-ternary solid solution CeNi(9)Ge(4-x)Si(x) (0 ≤ x ≤ 4) has been investigated by means of x-ray diffraction, magnetic susceptibility, specific heat, electrical resistivity, thermopower and inelastic neutron scattering studies. The isoelectronic substitution of germanium by silicon atoms causes a dramatic change of the relative strength of competing Kondo, RKKY and crystal field (CF) energy scales. The strongest effect is the continuous elevation of the Kondo temperature T(K) from approximately 3.

Reversible gas-phase redox processes catalyzed by Co-exchanged MFU-4l(arge).

Postsynthetic metal ion exchange in a benzotriazolate-based MFU-4l(arge) framework leads to a Co(II)-containing framework with open metal sites showing reversible gas-phase oxidation properties.

Experimental and theoretical charge density studies at subatomic resolution.

Analysis of accurate experimental and theoretical structure factors of diamond and silicon reveals that the contraction of the core shell due to covalent bond formation causes significant perturbations of the total charge density that cannot be ignored in precise charge density studies. We outline that the nature and origin of core contraction/expansion and core polarization phenomena can be analyzed by experimental studies employing an extended Hansen-Coppens multipolar model. Omission or insufficient treatment of these subatomic charge density phenomena might yield erroneous thermal displacement parameters and high residual densities in multipolar refinements.

Borane and borohydride complexes of the rare-earth elements: synthesis, structures, and butadiene polymerization catalysis.

The reaction of potassium 2,5-bis[N-(2,6-diisopropylphenyl)iminomethyl]pyrrolyl [(dip(2)-pyr)K] with the borohydrides of the larger rare-earth metals, [Ln(BH(4))(3)(thf)(3)] (Ln=La, Nd), afforded the expected products [Ln(BH(4))(2)(dip(2)-pyr)(thf)(2)]. As usual, the trisborohydrides reacted like pseudohalide compounds forming KBH(4) as a by-product. To compare the reactivity with the analogous halides, the dimeric neodymium complex [NdCl(2)(dip(2)-pyr)(thf)](2) was prepared by reaction of [(dip(2)-pyr)K] with anhydrous NdCl(3).

Unusual reactivity of lanthanide borohydride complexes leading to a borane complex.

Depending on the ionic radius of the central metal atom the BH(4)(-) group, which usually reacts in lanthanide chemistry like a pseudo halide, can be involved in redox chemistry and the resulting product contains an N-BH(3) unit, which binds in an unusual eta(2)-fashion onto the metal atom.

High- and low-temperature modifications of Sc3RuC4 and Sc3OsC4--relativistic effects, structure, and chemical bonding.

Sc(3)RuC(4) and Sc(3)OsC(4) were synthesized by arc-melting and subsequent annealing. At room temperature, they crystallize with the Sc(3)CoC(4) structure, space group Immm. At 223 and 255 K, Sc(3)RuC(4) and Sc(3)OsC(4), respectively, show a monoclinic distortion caused by a pair-wise displacement of the one-dimensional [Ru(C(2))(2)](delta-) and [Os(C(2))(2)](delta-) polyanions, which are embedded in a scandium matrix.