Gudrun Scholz

Novel Sol-Gel Synthesis of Acidic MgF2-x(OH)(x) Materials

Novel magnesium fluorides have been prepared by a new fluorolytic sol-gel synthesis for fluoride materials based on aqueous HF. By changing the amount of water at constant stoichiometric amount of HF, it is possible to tune the surface acidity of the resulting partly hydroxylated magnesium fluorides. These materials possess medium-strength Lewis acid sites and, by increasing the amount of water, Brønsted acid sites as well. Magnesium hydroxyl groups normally have a basic nature and only with this new synthetic route is it possible to create Brønsted acidic magnesium hydroxyl groups. XRD, MAS NMR, TEM, thermal analysis, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials. XPS measurements, FTIR with probe molecules, and the determination of N(2)/Ar adsorption-desorption isotherms have been carried out to investigate the surface properties. Furthermore, activity data have indicated that the tuning of the acidic properties makes these materials versatile catalysts for different classes of reactions, such as the synthesis of (all-rac)-[alpha]-tocopherol through the condensation of 2,3,6-trimethylhydroquinone (TMHQ) with isophytol (IP).

Variation of sol–gel synthesis parameters and their consequence for the surface area and structure of magnesium fluoride

High surface area MgF2 was prepared by the sol–gel technique from magnesium alkoxides. The influence of different ratios of Mg(OCH3)2 to HF, of different magnesium alkoxides, catalysts, solvents as well as different aging times was investigated. XRD, MAS-NMR, FT-IR, thermal analysis and elemental analysis were applied to study the structure, composition and thermal behaviour of the bulk materials. The determination of the surface properties was accomplished with N2 and Ar adsorption–desorption isotherms.

Local structural orders in nanostructured Al2O3 prepared by high-energy ball milling

Nanostructured Al2O3 powders were prepared by high-energy ball milling of corundum. Both the solid state nuclear magnetic resonance spectra of the Al3+ ions and the solid state electron paramagnetic resonance spectra of incorporated Fe3+ ions are governed by noticeable spectral changes dependent on the duration of the mechanical treatment. The quadrupolar parameters of the 27Al nuclei and the zero-field splitting parameters of the Fe3+ ions as well as their statistical distributions were determined as functions of the milling time. Structural changes of the Al2O3 matrix were also followed by powder x-ray diffraction and transmission electron microscopy measurements. Direct relations between the structural disorder as obtained by x-ray data and the spin Hamiltonian parameters of both ions could be established. These results suggest that the milled powders consist of nanocrystalline grains embedded in amorphous grain boundaries even for the longest milling time. The grains can be described in terms of ordered AlO6-octahedra as in the starting crystalline material exhibiting a slight rhombic distortion. The grain boundaries look like random arrangements of these octahedral units. The specific behaviour of the environment of the Fe3+ paramagnetic probe points out that such a point defect acts as an activation centre of the amorphization process.

Synthesis, structure determination, and formation of a theobromine:oxalic acid 2:1 cocrystal†

The structure and the formation pathway of a new theobromine:oxalic acid (2:1) cocrystal are presented. The cocrystal was synthesised mechanochemically and its structure was solved based on the powder X-ray data. The mechanochemical synthesis of this model compound was studied in situ using synchrotron XRD. Based on the XRD data details of the formation mechanism were obtained. The formation can be described as a self-accelerated (‘liquid like’) process from a highly activated species.

Electron paramagnetic resonance investigations of α-Al2O3 powders doped with Fe3+ ions: experiments and simulations

Electron paramagnetic resonance (EPR) of Fe3+ ions in Al2O3 is studied in powder samples prepared by different routes and/or modified by thermal or mechanical treatments, with different doping levels and grain sizes. The measurements are performed in various frequency bands (S, X, K, Q and W) and with bimodal detection in X-band. Simulations of the spectra are achieved with a code designed for computing EPR powder spectra described by any spin Hamiltonian including second-, fourth-and sixth-order ZFS terms (S ≤ 7/2). The linewidths, intensities and lineshapes are accounted for. The lineshape is Gaussian at low Fe3+ concentration whereas it is Lorentzian for higher concentration. The linewidths are interpreted as the superimposition of three main contributions: intrinsic linewidth, dipolar broadening and broadening due to lattice imperfections. The latter is tentatively interpreted in terms of quadrupolar spin Hamiltonian parameter distributions treated using first-order perturbation theory. Whatever the sample, only the b22 spin Hamiltonian parameter is found to be distributed around a mean zero value which corresponds to rhombic distortions. Angle and bond length distributions are tentatively extracted from the b22 distributions which gives some insight into the local order around the spin probe in relation to the preparation and treatment of the samples.

Evaluation of the formation pathways of cocrystal polymorphs in liquid-assisted syntheses

The synthesis of the polymorphic cocrystal caffeine:anthranilic acid was investigated to obtain a better understanding of the processes leading to the formation of different polymorphic forms. In the case of these cocrystal polymorphs synthesized by liquid-assisted grinding a distinct influence of the dipole moment of the solvent was found. A pre-coordination between the solvent molecules and the caffeine:anthranilic acid cocrystal could be identified in the formation of form II. In the case of form II the solvent can be regarded as a catalyst. The formation pathway of each polymorph was evaluated using synchrotron X-ray diffraction.

Mechanistic insight into formation and changes of nanoparticles in MgF2 sols evidenced by liquid and solid state NMR

The fluorolytic sol-gel reaction of magnesium methoxide with HF in methanol was studied by (19)F, (1)H and (13)C liquid and solid state NMR. In (19)F NMR five different species were identified, three of which belong to magnesium fluoride nanoparticles, i.e. NMR gave access to local structures of solid particles in suspensions. The long-term evolution of (19)F signals was followed and along with (19)F MAS NMR experiments of sols rotating at 13 kHz mechanistic insights into the ageing processes were obtained.

First-order differential equations in chemistry

Many processes and phenomena in chemistry, and generally in sciences, can be described by first-order differential equations. These equations are the most important and most frequently used to describe natural laws. Although the math is the same in all cases, the student may not always easily realize the similarities because the relevant equations appear in different topics and contain different quantities and units. This text was written to present a unified view on various examples; all of them can be mathematically described by first-order differential equations. The following examples are discussed: the Bouguer–Lambert–Beer law in spectroscopy, time constants of sensors, chemical reaction kinetics, radioactive decay, relaxation in nuclear magnetic resonance, and the RC constant of an electrode.

Modelling of multifrequency ESR spectra of Fe3+ ions in crystalline and amorphous materials: A Simplified approach to determine statistical distributions of spin-spin coupling parameters

Focussed on the understanding of the complex Fe3+ electron spin resonance (ESR) spectra in crystalline and amorphous systems, the simulation program ESR-MAKRO-1 was developed, which allows the calculation of powder spectra of paramagnetic centers with 1/2 ≤S ≤7/2. For the reproduction of the ESR spectra of amorphous systems Gaussian distributions of the zero-field-splitting parameters of second and fourth order (D, E, a, F) were implemented into the program. Applications of ESR-MAKRO-l are given both for crystalline powders and for glassy systems. A stepwise analysis of the ESR spectra, allowing an approach to Fe3+ ESR spectra of amorphous matrices and showing the influence ofD, E, a, andF as well as their distributions, is given.

ESR evidence of order/disorder transitions in solids caused by dehydration

ESR spectroscopy was used to study order/disorder phenomena in paramagnetically doped solids. The dehydration process of aluminium fluoride and oxide hydrates and aluminosilicates was examined by ESRin situ measurements as well as using thermoanalytically quenched samples. Two types of amorphous states of solids have been studied: inorganic glasses in the conventional sense and amorphous states of solids which have not passed the liquid state. The complete ESR fine structure pattern of Fe3+ ions is susceptible with respect to order/disorder processes of the actual host matrix. Dehydration processes bear some model character in this field. A quantitative description of the disorder effects on the Fe3+ ESR could be given on the basis of a complete spin Hamiltonian using distributions of the fourth order spin coupling parameters.