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A series of rare-earth metal diisopropylamide complexes has been obtained via salt metathesis employing LnCl3(THF)x and lithium (LDA) or sodium diisopropylamide (NDA) in n-hexane. Reactions with AM : Ln ratios ≥3 gave ate complexes (AM)Ln(NiPr2)4(THF)n (n = 1, 2; Ln = Sc, Y, La, Lu; AM = Li, Na) in good yields. For smaller rare-earth metal centres such as scandium and lutetium, a Li : Ln ratio = 2.5 accomplished ate-free tris(amido) complexes Ln(NiPr2)3(THF). The chloro-bridged dimeric derivatives [Ln(NiPr2)2(μ-Cl)(THF)]2 (Ln = Sc, Y, La, Lu) could be obtained in high yields for Li : Ln = 1.6-2. The product resulting from the Li : La = 1 : 1.6 reaction revealed a crystal structure containing two different molecules in the crystal lattice, [La(NiPr2)2(THF)(μ-Cl)]2·La(NiPr2)3(THF)2. Recrystallization of the chloro-bridged dimers led to the formation of the monomeric species Ln(NiPr2)2Cl(THF)2 (Ln = Sc, Lu) and La(NiPr2)3(THF)2. The reaction of YCl3 and LDA with Li : Y = 2 in the absence of THF gave a bimetallic ate complex LiY(NiPr2)4 with a chain-like structure. For scandium, the equimolar reactions with LDA or NDA yielded crystals of tetrametallic mono(amido) species, {[Sc(NiPr2)Cl2(THF)]2(LiCl)}2 and [Sc(NiPr2)Cl2(THF)]4, respectively. Depending on the Ln(iii) size, AM, and presence of a donor solvent, ate complexes (AM)Ln(NiPr2)4(THF)n show distinct dynamic behaviour as revealed by variable temperature NMR spectroscopy. The presence of weak LnCH(iPr) β-agostic interactions, as indicated by Ln-N-C angles

In this contribution the development, definition and selected applications of a new force field (FF) for metal-organic frameworks MOF-FF is presented. MOF-FF is fully flexible and is parameterized in a systematic and consistent fashion from first principles reference data. It can be used for a variety of different MOF-families and in particular – due to the reparametrization of a variety of organic linkers – also to explore isoreticular series of systems. The history of the development, leading to the final definition of MOF-FF is reviewed along with the application of the previous incarnations of the FF. In addition, the parametrization approach is explained in a tutorial fashion. The currently parametrized set of inorganic building blocks is constantly extended. Formate models of currently covered inorganic building blocks.

Hydrogen bond directionality in the water dimer is explained on the basis of symmetry-adapted intermolecular perturbation theory which directly separates the intermolecular interaction energy into four physically interpretable components: electrostatics, exchange-repulsion, dispersion, and induction. Analysis of these four main contributions to the binding energy allows a deeper understanding of the dominant factors ruling the mutual arrangement of the two monomers. A preference for the linear configuration is shown to be due to a subtle interplay of all four energy components. While the first-order terms, electrostatic and exchange-repulsion, almost perfectly cancel each other near the equilibrium geometry of the dimer, the importance of the second- and higher-order terms, induction and dispersion, becomes evident.

We present a fully flexible and ab initio-derived molecular mechanics force field for the ubiquitous copper paddle-wheel building block Cu2(O2C)4 in metal−organic frameworks. The force field expression is based on the established MM3 force field, extended by additional cross terms and specific bond-stretching and angle-bending terms for the square-planar CuO4 coordination environment. Using reference data computed at the DFT level for nonperiodic reference systems, the parametrization is performed using an automated genetic algorithm optimization strategy in order to reproduce structure and low normal modes of the model systems. It is validated on the much investigated Cu-btc (HKUST-1) metal−organic framework. Beyond the structure, lattice-dynamic-dependent properties such as the bulk modulus and the observed negative thermal expansion effect of Cu-btc are quantitatively predicted by the force field without recourse with respect to experimental data. In connection with available parametrizations of variou...

In this contribution, available atomistic theoretical models for the new class of functional porous hybrid materials are critically overviewed. These hybrid materials (including covalent- and metal–organic frameworks, COFs and MOFs) are characterized by organic linkers, allowing for conformational flexibility and connecting nodes formed by a wide variety of elements and coordination modes. The flexibility and variability of these porous materials represent one of their potentials for application, but also afford that theoretical methods, tailored for more traditional materials like zeolites, need to be extended or modified. The current status of both periodic and non-periodic quantum mechanic, as well as molecular mechanic models are considered, focusing on the peculiarities of an application on hybrid frameworks. Atomistic models are used for structural prediction, computation of materials properties and in particular for the modeling of host–guest interactions.

A systematic strategy is proposed to derive the necessary force field parameters directly from first principles calculations of nonperiodic model systems to reproduce both the structure and curvature of the reference potential energy surface. The parameters are determined using a genetic algorithm combined with a novel fitness criterion based on a representation of structure and curvature in a set of redundant internal coordinates. Due to the efficiency of this approach it is possible to abandon the need for transferability of the parameters. The method is targeted for the application on metal-organic frameworks (MOFs), where parameters for molecular mechanics force fields are often not available, because of the wide range of possible inorganic fragments involved. The scheme is illustrated for Zn4O-based IRMOF materials on the example of MOF-5. In a "building block" approach parameters are derived for the two model systems basic zinc formate (Zn4O(O2CH)6), and dilithium terephthalate with reference data obtained from density functional theory. The resulting potential gives excellent agreement with the structure, vibrational frequencies, thermal behavior and elastic constants of the periodic MOF-5.

The extent and nature of cyclic electron delocalization in free and coordinated cyclopropenylidene carbenes has been analyzed by combined experimental and theoretical charge-density studies. The significant asymmetry of the C–C bond lengths in substituted cyclopropenylidene carbenes was identified as cooperative effect which depends on contributions of both σ- and π-bonding. We show that analyses of (i) the topology of the Laplacian of the electron density distribution and (ii) the out-of-plane atomic quadrupole moments – the charge-density analogues of pπ occupation – allow to distinguish between the influence of σ- and π-electrons on cyclic electron delocalization. These studies hint for pronounced electron localization in the carbene lone pair region which dominates the electronic structure of free cyclopropenylidene carbenes and hinders the establishment of true aromaticity. We further investigated the electron donating/withdrawing ability of cyclopropenylidene ligands relative to N-heterocyclic carbenes. The experimental benchmark systems LCr(CO)5 (L = 2,3-diphenylcyclopropenylidene and 1,2-dimethylimidazol-2-ylidene) show that the cyclopropenylidene ligand clearly displays the higher π-acceptor capability relative to N-heterocyclic carbenes.

It is argued here that the functional forms adopted in almost all popular force fields are too restrictive to allow for accurate and physics-based parametrization. Some important modifications are suggested based on symmetry-adapted intermolecular perturbation theory, which directly separates the intermolecular interaction energy into four physically interpretable components: electrostatics, exchange-repulsion, dispersion, and induction. The exact electrostatic energy is approximated as a sum of the short-range contribution (due to charge density penetration effects), included explicitly, and the long-range part (via distributed atomic multipoles), whereas the induction energy is evaluated by means of the distributed induced damped point dipole model. The dispersion energy is fitted to a simple analytical function and the exchange-repulsion contribution is approximated by the overlap of the valence-only electron charge densities of monomers. The water dimer is used to illustrate the approach and to discuss its potential and possible improvements. Analysis of the four main contributions to the binding energy allows for a deeper understanding of the hydrogen bond directionality. It is found that a notorious geometrical preference in the water dimer results mainly from large polarization contributions, including induction and dispersion.

The ring opening is proposed to be the initial step in the unimolecular decomposition of a single-molecule precursor bisazido(dimethylaminopropyl)gallium (BAZIGA) used in a CVD process of gallium nitride. An efficient Monte Carlo importance sampling technique is developed to estimate the canonical partition functions and absolute entropies therefrom by taking the internal rotor hindrance and all coupling arising from the external and internal rotational degrees of freedom into account. This allows the estimation of the free energy difference in the ring opening process of BAZIGA to be made. Canonical flexible transition state theory is applied to the radical Ga–C bond homolysis in the unimolecular decomposition of BAZIGA. The limiting high-pressure rate constant is calculated using a simple model for the potential energy surface based on density functional calculations.

Small stoichiometric, as well as some nonstoichiometric, gallium nitride clusters of the type GanNm (n + m ≤ 8) are investigated by density functional theory (DFT), MP2, and coupled cluster, CCSD(T), calculations. The lowest energy structures of the stoichiometric clusters and the corresponding products after the loss of a nitrogen molecule have been determined. It is shown that, above room temperature, they are unstable against the loss of nitrogen molecules and therefore are even less stable than the bulk material. Thus, these clusters can be ruled out as direct intermediates for the formation of bulk GaN. This has been ignored up to now and is of crucial importance for the decomposition mechanism of single-molecule precursor systems. It is argued that the azide group can stabilize a tetrahedral arrangement of four gallium atoms around the N atom, and a nonstoichiometric gallium nitrogen cluster like gallium azide, GaN3, can play an important role as an intermediate during the formation of bulk GaN in CVD.

In the light of the important role played by the carboxylate group in bio- and coordination chemistry, its consistent and reliable parametrization for molecular simulations is crucial. The experimental vibrational spectra of three carboxylate anions (formate, acetate, and benzoate) both in the gas phase and in the condensed phase (as sodium salts) are interpreted on the basis of high-quality ab initio calculations. The interaction with the counterion (metal cation) is shown to be of major importance in the interpretation of the spectral features of the carboxylate group both in the solid state and in aqueous solution. Previous attempts to parametrize the carboxylate group within the molecular mechanics approach is critically reviewed, and a new set of the consistent valence force field parameters based on first principles calculations is proposed, which is able to reproduce accurately both the structure and the dynamics of the carboxylate moiety both free and coordinated with metal cations.

A strategy is proposed to include the missing charge penetration energy term directly into a force field using a sum over pairwise electrostatic energies between spherical atoms as originally suggested by Spackman. This important contribution to the intermolecular potential can be further refined to reproduce the accurate electrostatic energy between monomers in a dimer by allowing for the radial contraction–expansion of atomic charge densities. The other components of a force field (exchange-repulsion and dispersion) are parametrized to reproduce the accurate data calculated by symmetry-adapted perturbation theory (SAPT). As a proof-of-concept, we have derived the force field parameters suitable for modeling intermolecular interactions between polycyclic aromatic hydrocarbons (PAH). It is shown that it is possible to have a balanced force field suitable for molecular simulations of large molecules avoiding error cancellation to a large extent.

The surface morphology and termination of metal-organic frameworks (MOF) is of critical importance in many applications, but the surface properties of these soft materials are conceptually different from those of other materials like metal or oxide surfaces. Up to now, experimental investigations are scarce and theoretical simulations have focused on the bulk properties. The possible surface structure of the archetypal MOF HKUST-1 is investigated by a first-principles derived force field in combination with DFT calculations of model systems. The computed surface energies correctly predict the [111] surface to be most stable and allow us to obtain an unprecedented atomistic picture of the surface termination. Entropic factors are identified to determine the preferred surface termination and to be the driving force for the MOF growth. On the basis of this, reported strategies like employing “modulators” during the synthesis to tailor the crystal morphology are discussed.

Die metallorganische Gasphasenepitaxie (MOVPE) ist ein wichtiger Verarbeitungsschritt in der Halbleiterprozesstechnik zur Herstellung von III-V-Verbindungshalbleitern, auf deren Grundlage moderne elektronische und optoelektronische Bauelemente hergestellt werden. Daher besteht ein groser Bedarf hinsichtlich der Optimierung bestehender Verfahren bzw. der Untersuchung des Potenzials von neuen Prozessen, beispielsweise durch den Einsatz von alternativen Prekursoren. Ziel ist es dabei, eine Verbesserung der Qualitat der abgeschiedenen kristallinen Schichten zu erhalten. Hierbei spielt in mehrfacher Hinsicht die numerische Simulation eine zunehmende Rolle. Zum einen lassen sich auf diesem Wege wichtige Prozessparameter vorab berechnen, vorausgesetzt die dem Prozess zugrunde liegenden physikalischen und chemischen Vorgange lassen sich hinreichend genau quantifizieren. Zum anderen bietet die numerische Simulation Ansatze, die zu einem tieferen Verstandnis dieser physikalischen und chemischen Vorgange fuhren konnen.

A Car-Parrinello molecular dynamics implementation using real space discretized wavefunctions on equidistant Cartesian grids is presented. By using numerically discretized Laplace operators of higher order for the evaluation of the electronic kinetic energy as well as for the Poisson equation the total energy expression can be evaluated purely with local operations. Details of our implementation of microcanonical and canonical Car-Parrinello joint dynamics of nuclei and wavefunctions are described. For energy conserving molecular dynamics the exact partial derivatives of the energy expression with respect to the numerical wavefunctions must be available. This aspect is discussed for all contributions to the electronic energy. The crucial point is the efficient iterative solution of the Poisson equation. It is shown that energy conserving dynamics with real space wavefunctions is possible. Our object oriented code is fully parallelized and efficient also for the investigation of systems of realistic size.

A multiscale simulation approach, including atomistic quantum mechanical calculations on the molecular scale and physical continuum mechanical process simulations on the macroscopic scale, has been used to study the initial gas-phase decomposition of a single-molecule precursor. The compound bisazido(dimethylaminopropyl)gallium (BAZIGA), that forms gallium nitride in the absence of other nitrogen sources, is proposed to decompose by a multistep reaction mechanism to the key intermediate. GaN 3 . The reaction rates for the effective overall reaction were derived on the basis of transition state theory in an ab initio fashion from the results of density functional theory-type calculations. Methods to accurately approximate the entropic contributions to the free energies of activation for the elementary steps were developed and employed. The calculated rates were fit to an Arrhenius type rate law with an apparent energy of activation of E a = 321 kJ mol -1 (A = 9.90×10 20 s -1 ). Process simulations were performed for a real vertical stagnation flow lest reactor. The inclusion of radiative heat transfer was found to be crucial to reproduce the experimentally determined temperature distribution on the reactor wall. By inclusion of the gas phase decomposition to GaN 3 in terms of an effective one-step reaction, only the intermediate GaN 3 was found to contribute to film growth. It is concluded that all parent precursor molecules decompose in the hot zone above the substrate and do not reach the surface under the given conditions.

In this paper we describe a range of model d(0) metal ethyl compounds and related complexes, studied by DFT calculations and high resolution X-ray diffraction. The concept of ligand-opposed charge concentrations (LOCCs) for d(0) metal complexes is extended to include both cis-and trans-ligand-induced charge concentrations (LICCs) at the metal, which arise as a natural consequence of covalent metal-ligand bond formation in transition metal alkyl complexes. The interplay between locally induced sites of increased Lewis acidity and an ethyl ligand is crucial to the development of a beta-agostic interaction in d(0) metal alkyl complexes, which is driven by delocalization of the M-C bonding electrons. Topological analysis of theoretical and experimental charge densities reveals LICCs at the metal atom, and indicates delocalization of the M-C valence electrons over the alkyl fragment, with depletion of the metal-directed charge concentration (CC) at the alpha-carbon atom, and a characteristic ellipticity profile for the C(alpha)-C(beta) bond. These ellipticity profiles and the magnitude of the CC values at C(alpha) and C(beta) provide experimentally observable criteria for assessing quantitatively the extent of delocalization, with excellent agreement between experiment and theory. Finally, a concept is proposed which promises systematic control of the extent of C-H activation in agostic complexes.

The synthesis as well as the spectroscopic and structural characterization of mononuclear metallocene complexes of the trivalent rare earth elements yttrium and lanthanum is described. Lanthanidocene silylamide complexes were obtained ate-complex free and in high yields according to a silylamine elimination reaction from complexes Ln[N(SiHMe2)2]3(THF)2 (Ln=Y, La) and highly substituted cyclopentadiene derivatives (CH3)5C5H, (CH3)4C5H2 and (C6H5)4C5H2. Deprotonation of tetraphenylcyclopentadiene was accomplished only for the lanthanum derivative indicating steric constraints due to the size of the metal cation. IR and 1H-NMR spectroscopy reveal the presence of asymmetrically bonded silylamide ligands featuring a strong agostic interaction between the electron-deficient metal centers and the SiH moiety of the bis(dimethylsilyl)amide ligand: SiH stretching vibrations as low as 1827 cm−1 are indicative of a distinct weakening of the SiH bonding. X-ray structure analyses of complexes [(CH3)4C5H]2YN(SiHMe2)2, [(CH3)5C5]2YN(SiHMe2)2 and [(C6H5)4C5H]2LaN(SiHMe2)2 show that the structural features of the agostically bonded bis(dimethylsilyl)amide moiety depend on the steric crowding of the ancillary cyclopentadienyl ligand: Y⋯Si and Y⋯H contacts as close as 3.0506(7) and 2.40(3) A, respectively, are detected, forcing Ln–N–Si angles as low as 99.7(1)°.

Symmetry-adapted perturbation theory (SAPT) is used to decompose the total intermolecular interaction energy between the ammonium cation and a benzene molecule into four physically motivated individual contributions: electrostatics, exchange, dispersion, and induction. Based on this rigorous decomposition, it is shown unambiguously that both the electrostatic and the induction energy components contribute almost equally to the attractive forces stabilizing the dimer with a nonnegligible contribution coming from the dispersion term. A polarizable potential model for the interaction of ammonium cation with benzene is parametrized by fitting these four energy components separately using the functional forms of the AMOEBA force field augmented with the missing charge penetration energy term calculated as a sum over pairwise electrostatic energies between spherical atoms. It is shown that the proposed model is able to produce accurate intermolecular interaction energies as compared to ab initio results, thus avoiding error compensation to a large extent.

Magnesium silylamide complexes Mg[N(SiHMe(2))(2)](2)(THF)(2) and Mg[N(SiPhMe(2))(2)](2) were synthesized according to transsilylamination and alkane elimination protocols, respectively, utilizing Mg[N(SiMe(3))(2)](2)(THF)(2) and [Mg(n-Bu)](2) as precursors. Cage-like periodic mesoporous silica SBA-1 was treated with donor solvent-free dimeric [Mg{N(SiHMe(2))(2)}(2)](2), [Mg{N(SiMe(3))(2)}(2)](2) and monomeric Mg[N(SiPhMe(2))(2)](2), producing hybrid materials [Mg(NR(2))(2)]@SBA-1 with magnesium located mainly at the external surface. Consecutive grafting of [Mg{N(SiHMe(2))(2)}(2)](2) and [Fe(II){N(SiHMe(2))(2)}(2)](2) onto SBA-1 led to heterobimetallic hybrid materials which exhibit complete consumption of the isolated surface silanol groups, evidencing intra-cage surface functionalization. All materials were characterized by DRIFT spectroscopy, nitrogen physisorption and elemental analysis.

By using an accurate molecular mechanics force field, which was parametrized on the basis of first principles calculations, the network topology of the Covalent Organic Framework COF-102 could correctly be predicted without experimental input. The ctn structure is preferred thermodynamically by 11.2 kcal mol(-1) over the bor topology due to a lower steric strain. The origin for this strain is analyzed in detail.

The evaluation of the classical rotational partition function represented by a configuration integral over all external and internal rotational degrees of freedom of nonrigid chain polyatomic molecules is described. The method of Pitzer and Gwinn is used to correct the classical partition function for quantum mechanical effects at low temperatures. The internal rotor hindrance and all coupling arising from the external and internal rotational degrees of freedom are explicitly taken into account. Importance sampling Monte Carlo based on the adaptive VEGAS algorithm to perform multidimensional integration is implemented within the TINKER program package. A multidimensional potential energy hypersurface is calculated with the MM3(2000) molecular mechanics force field. Numerical tests are performed on a number of small n-alkanes (from ethane to octane), for which the absolute entropies calculated at three different temperatures are compared both with the experimental values and with the previous theoretical results. The application of a more efficient importance sampling technique developed here results in a substantial reduction of statistical errors in the evaluation of the configuration integral for a given number of Monte Carlo steps. Error estimates for the calculated entropies are given, and possible sources of systematic errors, and their importance for a reliable prediction of the absolute entropy, are discussed.

Combined experimental and theoretical charge-density studies on free and metal-coordinated N-heterocyclic carbenes have been performed to investigate the extent of electron delocalization in these remarkable species. Tracing the orientation of the major axis of the bond ellipticity (the least negative curvature in the electron density distribution) along the complete bond paths distinguishes unambiguously between fully delocalized systems and those with interrupted cyclic electron delocalization. Evaluation of charge-density-based properties such as atomic quadrupole moments serves as a direct and quantitative measure of the extent of pi-electron delocalization and reveals consistency between theory and experiment. A detailed topological analysis of theoretical charge densities for two benchmark carbene systems, viz., 1,2-dimethylpyrazol-3-ylidene 1a and 1,3-dimethylimidazol-2-ylidene 2a, and their corresponding stable chromium pentacarbonyl complexes 1 and 2, highlights the advantages of charge-density-based criteria to analyze such complex electronic situations. Thus, 1a and 2a display a different extent of electron delocalization; yet nearly identical p(pi) occupations at the carbene center are computed for 1a and 2a. However, atomic quadrupoles Q(zz) - the charge-density analogues of p(pi) occupation - reveal faithfully the electronic differences in 1a and 2a and demonstrate the sensitivity of charge-density-based properties to the bonding situation. The acyclic aminocarbene (iPr(2)N)(2)CCr(CO)(4) has also been studied, and the high barrier to rotation about the C-N bond is shown not to arise solely from p(pi)-p(pi) bonding.

The homoleptic ytterbium(II) tetraalkylaluminate complexes {Yb[AlR4]2}n have been obtained according to a silylamide elimination reaction from Yb[N(SiMe3)2]2(THF)2 and excess AlR3 (R = Me, Et, iBu)...

The synthesis and the spectroscopic and structural characterization of lower-rim-silylated and rare-earth-metalated calix[4]arenes are presented. Hexamethyldisilazane, HN(SiMe3)2, reacted in a selective manner with [p-tert-buttylcalix[4]arene]H4 (1) in refluxing mesitylene to give the 1,3-silylated product [p-tert-butylcalix[4]arene(SiMe3)2]H2 (2) in high yield. The molecular structure of compound 2, as revealed by X-ray crystallography, shows the pinched cone conformation of the calixarene bowl, featuring hydrogen bonding between the phenylsilyl ether and phenolic oxygen atoms (O...O, 2.838 A). From the reaction of the sterically more crowded tetraphenyldimethyldisilazane, HN(SiMePh2)2, only starting material could be recovered. In contrast, tetramethyldisilazane, HN(SiHMe2)2, afforded the tetrakis-silylated product [p-tert-butylcalix[4]arene(SiHMe2)4] (3) in hexane solution at ambient temperature. A single-crystal X-ray diffraction study of compound 3 established the 1,2-alternate conformation, which is also present in solution, as indicated by 1H NMR spectroscopy. The yttrium complex Y[N(SiHMe2)2]3(THF)2 (4) exchanged all of its silylamide ligands when treated with an equimolar amount of 1 in toluene at ambient temperature to yield compound 5, as indicated by IR and NMR spectroscopy. The molecular structure of 5 revealed a centrosymmetric dimer of composition [Y(p-tert-butylcalix[4]arene(SiHMe2)(THF)]2. Three of the deprotonated phenolic oxygen atoms of the calixarene bowl bind to the metal center, two as terminal ligands and one in a bridging mode, while the fourth undergoes in situ silylation (nu(SiH) 2127 cm-1). The distorted-trigonal-bipyramidal coordination geometry is completed by a THF molecule. Bis-silylated 2 reacted with 4 to form the heteroleptic complex (Y[p-tert-butylcalix[4]arene(SiMe3)2][N(SiHMe2)2]) (6). Crystal data: C50H72O4Si2 (2), triclinic, P1, a = 12.8914(3) A, b = 14.9270(5) A, c = 15.1652(4) A, alpha = 77.293(2) degrees, beta = 65.019(2) degrees, gamma = 72.234(2) degrees, Z = 2; C52H80O4Si4 (3), triclinic, P1, a = 10.1774(2) A, b = 14.1680(2) A, c = 18.7206(2) A, alpha = 95.8195(8) degrees, beta = 95.5294(8) degrees, gamma = 98.1098(7) degrees, Z = 2; C100H132O10Si2Y2, 2(C6H6) (5), triclinic, P1, a = 13.2625(4) A, b = 14.5894(3) A, c = 17.0458(5) A, alpha = 65.0986(14) degrees, beta = 77.8786(8) degrees, gamma = 85.5125(13) degrees, Z = 1.

Sm[N(SiMe3)2]2(thf)2 reacts with 1,1,3,3-tetramethyldisilazane by a transsilylamination reaction to form the hexane-soluble Sm[N(SiHMe2)2]2(thf)x in high yield. An X-ray diffraction study reveals a novel trinuclear complex of composition Sm{[μ-N(SiHMe2)2]2Sm[N(SiHMe2)2](thf)}2 featuring a bent array of metal centers. Multiple metal···SiH β-agostic interactions sterically and electronically saturate the samarium(II) centers implying a new coordination mode of the bridging amide ligands.

We present a novel method to determine diffusion constants of small molecules within highly porous metal-organic frameworks (MOFs). The method is based on the recently proposed liquid-phase epitaxy (LPE) process to grow MOF thin films (SURMOFs) on appropriately functionalized substrates, in particular on organic surfaces exposed by thiolate-based self-assembled monolayers (SAMs). By applying the LPE-method to SAM-coated quartz crystals, the time-dependence of the mass-uptake of the MOF when exposing it to a gas is measured by a quartz-crystal microbalance (QCM). The homogenous nature of the SURMOFs together with their well-defined thickness allow to analyze the QCM-data using Fickian diffusion to yield the diffusion constant. We demonstrate the potential of this method for the case of pyridine diffusion within HKUST-1 (Cu(3)(BTC)(2)) MOF, for which the diffusion coefficient at room temperature is found to amount to 1.5 x 10(-19) m(2) s(-1). Assuming a Fickian diffusion and a hopping mechanism, we yield a binding energy of 0.78 eV of the pyridine to the Cu(2+) sites within the HKUST-1 MOF, a value in good agreement with the results of precise ab initio quantum chemistry calculations.

A scheme for the Fourier filtering of pseudopotentials in real space calculations is proposed, in order to reduce the artifact of positional energy dependence ("egg box" effect). It is based on an improved version of the mask function method poposed by Wang [Phys. Rev. B 64, 201107/1 (2001)]. It is easy to implement, efficient, and accurate. By using atom-centered compensation charges, the local part of the pseudopotential becomes short ranged and can be filtered on the same footing as the nonlocal parts. A major advantage of the approach is that a generic set of parameters can be used for different pseudopotentials. A balanced parameter set is derived and validated. In this context a strategy to monitor the extent of grid dependence is introduced. It is found that, given a sufficiently fine grid spacing is used to represent the atomic valence density, the positional energy dependence can be reduced below 0.1 mhartree for all investigated atoms. On the example of a D(3h) symmetric Si(5) cluster and the C(60) molecule it is demonstrated that the artificial symmetry breaking of both bond lengths and orbital energies can substantially be reduced by the filtering scheme.

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Topological analysis of experimental and theoretical charge densities in tetrasulfur tetranitride clarifies features of the intramolecular bonding; intermolecular charge concentrations reveal directional ‘key–lock’ interactions corresponding to molecular recognition in the solid state.