Adhesion, forces and the stability of interfaces.

Weak molecular interactions (WMI) are responsible for processes such as physisorption; they are essential for the structure and stability of interfaces, and for bulk properties of liquids and molecular crystals. The dispersion interaction is one of the four basic interactions types - electrostatics, induction, dispersion and exchange repulsion - of which all WMIs are composed. The fact that each class of basic interactions covers a wide range explains the large variety of WMIs.

Dispersion Interactions and the Stability of Amine Dimers.

Weak, intermolecular interactions in amine dimers were studied by using the combination of a dispersionless density functional and a function that describes the dispersion contribution to the interaction energy. The validity of this method was shown by comparison of structural and energetic properties with data obtained with a conventional density functional and the coupled cluster method. The stability of amine dimers was shown to depend on the size, the shape, and the relative orientation of the alkyl substituents, and it was shown that the stabilization energy for large substituents is dominated by dispersion interactions.

Chemical bonding: the orthogonal valence-bond view.

Chemical bonding is the stabilization of a molecular system by charge- and spin-reorganization processes in chemical reactions. These processes are said to be local, because the number of atoms involved is very small. With multi-configurational self-consistent field (MCSCF) wave functions, these processes can be calculated, but the local information is hidden by the delocalized molecular orbitals (MO) used to construct the wave functions.

Adsorption of glucose, cellobiose, and cellotetraose onto cellulose model surfaces.

Reliable simulation of molecular adsorption onto cellulose surfaces is essential for the design of new cellulose nanocomposite materials. However, the applicability of classical force field methods to such systems remains relatively unexplored. In this study, we present the adsorption of glucose, cellobiose, and cellotetraose on model surfaces of crystalline cellulose Iα and Iβ.

Is electrostatics sufficient to describe hydrogen-bonding interactions?

The stability and geometry of a hydrogen-bonded dimer is traditionally attributed mainly to the central moiety A-H⋅⋅⋅B, and is often discussed only in terms of electrostatic interactions. The influence of substituents and of interactions other than electrostatic ones on the stability and geometry of hydrogen-bonded complexes has seldom been addressed. An analysis of the interaction energy in the water dimer and several alcohol dimers--performed in the present work by using symmetry-adapted perturbation theory--shows that the size and shape of substituents strongly influence the stabilization of hydrogen-bonded complexes.

Dioxomolybdenum(VI) complexes with pyrazole based aryloxide ligands: synthesis, characterization and application in epoxidation of olefins.

Synthesis, characterization, and epoxidation chemistry of four new dioxomolybdenum(VI) complexes [MoO(2)(L)(2)] (1-4) with aryloxide-pyrazole ligands L = L1-L4 is described. Catalysts 1-4 are air and moisture stable and easy to synthesize in only three steps in good yields. All four complexes are coordinated by the two bidentate ligands in an asymmetric fashion with one phenoxide and one pyrazole being trans to oxo atoms, respectively.

Localization of molecular orbitals on fragments.

A non-iterative algorithm for the localization of molecular orbitals (MOs) from complete active space self consistent field (CASSCF) and for single-determinantal wave functions on predefined moieties is given. The localized fragment orbitals can be used to analyze chemical reactions between fragments and also the binding of fragments in the product molecule with a fragments-in-molecules approach by using a valence bond expansion of the CASSCF wave function. The algorithm is an example of the orthogonal Procrustes problem, which is a matrix optimization problem using the singular value decomposition.

Oxygen versus sulfur: Structure and reactivity of substituted arsine oxides and arsine sulfides.

Although arsenic in its inorganic forms is a well know toxic agent, biotransformations in the environment and in the human body can produce organoarsenic compounds that are generally of much lower toxicity. Foremost among these products is a range of dimethylated arsine oxides and their analogous sulfides, which are crucial to the arsenic detoxification process. We have investigated the formation and interconversion of substituted and unsubstituted arsenicals (R²₂As(=Z)R¹, R² = CH₃, R¹ = CH₂CH₂OH, CH₂COOH; Z = S or O) with density functional theory (DFT)/B3LYP.

Mechanistic insight into the reactivity of oxotransferases by novel asymmetric dioxomolybdenum(VI) model complexes.

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-flurobenzyl)-1,4-diazepane, and 1,4-bis(2-hydroxy-4-chlorobenzyl)-1,4-diazepane (H(2)(L1)-H(2)(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO(2)(L)] 1-6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO(2)(L)] 1-4 have been successfully determined by single-crystal X-ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis-β configuration.

Solvation of carbon nanotubes by aniline calculated with density functional tight binding.

The poor solubility of carbon nanotubes in aromatic solvents is a well known issue. This work is concerned with the fundamentals of the dissolution process of carbon nanotubes. Based on previous studies about adsorption of small aromatics on carbon nanotubes, different arrangements and different numbers of aniline molecules on single walled zigzag and armchair nanotubes are investigated by ab initio density functional based tight binding method.

Identification and thermodynamic treatment of several types of large-amplitude motions.

We present a partially automated method for the thermodynamic treatment of large-amplitude motions. Starting from the molecular geometry and the Hessian matrix, we evaluate anharmonic partition functions for selected vibrational degrees of freedom. Supported anharmonic vibration types are internal rotation and inversion (oscillation in a double-well potential).

Oligovalent link atoms in embedding calculations.

Making reasonable statements about large systems is a notoriously difficult task of quantum chemistry. If such a system consists of a small but electronically important region, the core, which requires a treatment on a high level of theory, and a larger remainder, the bulk, which only acts in a perturbative fashion and thus admits a description on a lower level of theory, then it is appropriate to partition it accordingly and use an embedding scheme to provide for the coupling of the two regions. In many cases of said partitioning it will be necessary to cut covalent bonds so that the emerging free valences have to be saturated by link atoms.