Mikko Linnolahti

Structural Principles of Semiconducting Group 14 Clathrate Frameworks

We have performed a comprehensive theoretical investigation of the structural principles of semiconducting clathrate frameworks composed of the Group 14 elements carbon, silicon, germanium, and tin. We have investigated the basic clathrate frameworks, together with their polytypes, intergrowth clathrate frameworks, and extended frameworks based on larger icosahedral building blocks. Quantum chemical calculations with the PBE0 hybrid density functional method provided a clear overview of the structural trends and electronic properties among the various clathrate frameworks. In agreement with previous experimental and theoretical studies, the clathrate II framework proved to be the energetically most favorable, but novel hexagonal polytypes of clathrate II also proved to be energetically very favorable. In the case of silicon, several of the studied clathrate frameworks possess direct and wide band gaps. The band structure diagrams and simulated powder X-ray patterns of the studied frameworks are provided and systematic preliminary evaluation of guest-occupied frameworks is conducted to shed light on the characteristics of novel, experimentally feasible clathrate compositions.

High-performance light-emitting diodes based on carbene-metal-amides

Adding a twist for enhanced performance The efficiency of organic light-emitting diodes (OLEDs) is fundamentally governed by the ratio of emissive singlet to dark triplet excitons that are formed from spin-polarized electron and hole currents within the material. Typically, this has set an upper limit of 25% internal quantum efficiency for OLEDs. Di et al. manipulated the ratio of spin states through a modification of process chemistry. They introduced a rotation of the molecular structure, which inverted the spin-state energetics and enhanced OLED performance. Science, this issue p. 159 Spin-state inversion via intramolecular rotation can enhance the performance of solution-processed organic light-emitting diodes. Organic light-emitting diodes (OLEDs) promise highly efficient lighting and display technologies. We introduce a new class of linear donor-bridge-acceptor light-emitting molecules, which enable solution-processed OLEDs with near-100% internal quantum efficiency at high brightness. Key to this performance is their rapid and efficient utilization of triplet states. Using time-resolved spectroscopy, we establish that luminescence via triplets occurs within 350 nanoseconds at ambient temperature, after reverse intersystem crossing to singlets. We find that molecular geometries exist at which the singlet-triplet energy gap (exchange energy) is close to zero, so that rapid interconversion is possible. Calculations indicate that exchange energy is tuned by relative rotation of the donor and acceptor moieties about the bridge. Unlike other systems with low exchange energy, substantial oscillator strength is sustained at the singlet-triplet degeneracy point.

Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single‐Site α‐Olefin Polymerization Catalysis

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n (AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis-acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2(+) cation. Thermodynamics suggest that activation through AlMe2(+) transfer is the dominant mechanism because sites that are able to release AlMe2(+) are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.

Screening the thermodynamics of trimethylaluminum-hydrolysis products and their co-catalytic performance in olefin-polymerization catalysis.

Herein, we introduce an approach for the computational screening of stoichiometric reactions between trimethylaluminum (TMA) and water. The thermodynamic products of these reactions are methylaluminoxanes (MAOs) with different compositions, which have the general formula (AlOMe)n(AlMe3 )m, in which n describes the degree of oligomerization and m is the number of associated TMA molecules. These reaction products were thoroughly explored up to n=4, thus demonstrating the thermodynamically preferable association of up to four AlMe3 molecules, that is, TMA molecules in their monomeric form. The relative Lewis acidities of the Al sites in these MAOs were systematically explored and we found that the associated TMA molecules were a key ingredient for co-catalytic activity in olefin-polymerization catalysis. This conclusion was supported by computational studies on catalyst activation, which revealed an exergonic insertion of ethene into the metallocene/MAO complex.

Two‐, One‐, and Zero‐Dimensional Elemental Nanostructures Based on Ge9‐Clusters

We investigated the structural principles of novel germanium modifications derived by oxidative coupling of Zintl-type [Ge(9)](4-)clusters in various ways. The structures, stabilities, and electronic properties of the predicted {(2) (infinity)[Ge(9)](n)} sheet, {(1) (infinity)[Ge(9)](n)} nanotubes, and fullerene-like {Ge(9)}(n) cages were studied by using quantum chemical methods. The polyhedral {Ge(9)}(n) cages are energetically comparable with bulk-like nanostructures of the same size, in good agreement with previous experimental findings. Three-dimensional structures derived from the structures of lower dimensionality are expected to shed light on the structural characteristics of the existing mesoporous Ge materials that possess promising optoelectronic properties. Furthermore, 3D networks derived from the polyhedral {Ge(9)}(n) cages lead to structures that are closely related to the well-known LTA zeolite framework, suggesting further possibilities for deriving novel mesoporous modifications of germanium. Raman and IR spectra and simulated X-ray diffraction patterns of the predicted materials are given to facilitate comparisons with experimental results. The studied novel germanium modifications are semiconducting, and several structure types possess noticeably larger band gaps than bulk alpha-Ge.

Quantum chemical studies on elementary fragments of three-coordinated methylaluminoxanes

Abstract The molecular structures and relative stabilities of methylaluminoxanes (MAO) with three-coordinated aluminum centers were investigated by quantum chemical Hartree–Fock, MP2, and B3LYP methods. Linear and cyclic fragments of MAO were studied as a function of the degree of oligomerisation. The molecular structures are determined by the strain due to ring formation and/or by the stabilizing π-bonding interactions between the oxygen lone pairs and vacant p-orbitals of aluminums. The latter results in short AlO distances, and in an AlOAl angle of 180° for linear MAO, with adjacent methyl groups perpendicular to each other. The relative stabilities of the cyclic MAO increase with the degree of oligomerisation. Smaller oligomers are destabilized due to the strain of the ring and reduced π-conjugation. Stabilities of the linear MAO show no variation with length of the fractions.

Structural principles of polyhedral allotropes of phosphorus.

We derive the structural principles of polyhedral allotropes of phosphorus, introducing three distinct families of black phosphorus nanostructures. The predicted tetrahedral, octahedral, and icosahedral phosphorus cages can also be considered as phosphorus fullerenes. Phosphorus cages up to P(888) are systematically investigated by quantum chemical methods, and their thermodynamic stabilities are compared with the experimentally known allotropic forms of phosphorus. The tetrahedral cages are thermodynamically favored over the octahedral and icosahedral structures, although large octahedral structures become nearly as stable as the tetrahedral ones. The stability trends of the studied polyhedral families can be rationalized on the basis of their structural characteristics. The phosphorus polyhedra can be further stabilized by fitting smaller structures inside larger ones, resulting in multilayered, bulk-like cages. The synthesis of the predicted black phosphorus nanostructures is suggested to be viable from the thermodynamic point of view, and several approaches for their experimental preparation can be envisaged.

Geometry prediction of bridged zirconocene dichlorides by quantum chemical methods

The ab initio Hartree–Fock theory has been demonstrated to give accurate geometry predictions for bridged zirconocene dichlorides. Equilibrium geometries of crystallographically characterized bridged zirconocene dichlorides were optimized by Hartree–Fock, MP2, BLYP, and B3LYP methods, with basis sets ranging from 3‐21G* to 6‐311G**. Selected geometrical parameters were compared with experimental crystal structures. The least expensive HF/3‐21G* method proved to be notably accurate. The accuracy of HF/3‐21G* was verified by a comprehensive data set of 62 bridged zirconocene dichlorides. Furthermore, experimental corrections were applied to the optimized geometry parameters to eliminate systematic deviations. Corrections resulted in considerably improved accuracy for systematically overestimated metal–ligand distances, with maximum deviation falling from 0.081 to 0.039 Å, and absolute average deviations from 0.048 to 0.012 Å. Ligand–metal–ligand angles were predicted accurately with absolute average deviations of 0.7–1.3°. Zirconium–chlorine distances and chlorine–zirconium–chlorine angles are relatively constant in the studied molecules. Zirconium–cyclopentadienyl distances can be influenced mainly by modifying the ligand structure, whereas cyclopentadienyl–zirconium– cyclopentadienyl angles and cyclopentadienyl–cyclopentadienyl plane angles can be controlled by bridge modifications. The HF/3‐21G* method can be applied for the estimation of steric effects in zirconocene catalyzed polymerization reactions, therefore being suitable for the construction of structure–polymerization property correlations.

COMPUTER SIMULATIONS OF BRANCHED ALKANES : THE EFFECT OF SIDE CHAIN AND ITS POSITION ON RHEOLOGICAL BEHAVIOR

Nonequilibrium molecular dynamics simulations have been performed on model fluids representing eicosane isomers in order to investigate the effect of branching and side chain position on fluid rheology. A heterogeneous, united-atom model with 20 Lennard-Jones interaction sites located at carbon centers was used to model the fluids. Vibrations and bond rotations were frozen, but torsional rotation was included. It was found that viscosity increases significantly from the n-alkane structure to a branch on carbon 2, but the movement of the branch along the carbon backbone has a smaller increasing than decreasing effect. The size of the group in a branched position has a more substantial effect upon the viscosity.

Structural Characteristics of Graphane-Type C and BN Nanostructures by Periodic Local MP2 Approach

The structural characteristics of fully-hydrogenated carbon and boron nitride mono- and multilayer slabs, together with nanotubes derived from the slabs, are investigated mainly by means of periodic local second-order Møller-Plesset perturbation (LMP2) calculations and the results are compared with Hartree-Fock (HF), density functional theory (DFT), and dispersion function-augmented DFT (DFT-D) obtained ones. The investigated systems are structurally analogous to (111) and (110) slabs of diamond, where the hydrogenated (111) slab of diamond corresponds to the experimentally known graphane. Multilayering of monolayers and nanotubes is energetically favorable at the LMP2 level for both C and BN, while HF and DFT are not able to reproduce this behavior for CH systems. The work highlights the importance of utilizing methods capable of properly describing weak interactions in the investigation of dispersively-bound systems such as the multilayered graphanes and the corresponding nanotubes.