Sven L. M. Schroeder

Nucleation of organic crystals - A molecular perspective

The outcome of synthetic procedures for crystalline organic materials strongly depends on the first steps along the molecular self-assembly pathway, a process we know as crystal nucleation. New experimental techniques and computational methodologies have spurred significant interest in understanding the detailed molecular mechanisms by which nuclei form and develop into macroscopic crystals. Although classical nucleation theory (CNT) has served well in describing the kinetics of the processes involved, new proposed nucleation mechanisms are additionally concerned with the evolution of structure and the competing nature of crystallization in polymorphic systems. In this Review, we explore the extent to which CNT and nucleation rate measurements can yield molecular-scale information on this process and summarize current knowledge relating to molecular self-assembly in nucleating systems.

Correlating catalytic activity of Ag-Au nanoparticles with 3D compositional variations.

Significant elemental segregation is shown to exist within individual hollow silver-gold (Ag-Au) bimetallic nanoparticles obtained from the galvanic reaction between Ag particles and AuCl4(-). Three-dimensional compositional mapping using energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope (STEM) reveals that nanoparticle surface segregation inverts from Au-rich to Ag-rich as Au content increases. Maximum Au surface coverage was observed for nanoparticles with approximately 25 atom % Au, which correlates to the optimal catalytic performance in a three-component coupling reaction among cyclohexane carboxyaldehyde, piperidine, and phenylacetylene.

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).

The imitation game - A computational chemical approach to recognizing life

When is an artificial cell alive? A Turing test–like method may provide the answer.

Oxygen chemisorption on Au(1 1 0)-(1 × 2) I. Thermal desorption measurements

Oxygen chemisorption on Au(110)-(1 x 2) was induced by electron bombardment (500 eV) of physisorbed oxygen molecules at 28 K. TDS reveals the successive occupation of two desorption states around 550 K (β 2 ) and 490 K (β 1 ). The β 2 state desorbs at low coverages (Θ < 0.35 ML) with a second-order kinetics, indicating associative O 2 desorption, and with an activation energy of 140 kJ/mol. At higher coverages (0.35 ML < Θ < 1.0 ML), desorption is dominated by an autocatalytic behaviour resulting in a high-temperature shift and a narrow peak width. The TD spectra of the submonolayer range were simulated on the basis of a kinetic model that includes formation of oxygen islands and desorption from a phase equilibrium between two-dimensional condensed and gas phases. As the coverage exceeds 1.0 ML, another desorption state (β 1 ) is observed, which we associate with gold oxide.

Towards a 'universal curve' for total electron-yield XAS

Depth distribution functions of KLL Auger electrons emitted from 40 solid elements (10 < Z < 57) have been calculated by Monte-Carlo simulations of electron trajectories. A simple analytical expression reproduces all results quite accurately in terms of the total electron range, which is a material- and energy-dependent parameter. It is shown that this formula allows quick estimates of the depth information contained in total electron-yield (TEY) X-ray absorption spectra at photon energies in the keV range.

Molecular propane adsorption dynamics on Pt(110)−(1 × 2)

Abstract The molecular adsorption probability of propane on clean and propane-covered Pt(110)-(l × 2) was measured as a function of incident translational energy, E i , incident polar angle, θ i , propane surface coverage, θ cov , and azimuthal angle at a surface temperature of 95 K. The dependence of the adsorption probability on E i , and θ i at all propane coverages reflects significant participation of parallel momentum in the adsorption process due to corrugation in the propane/Pt(110)-(1 × 2) interaction. When the tangential velocity component of the molecular beam is directed along the close-packed atomic rows (the [110] direction), the initial adsorption probability increases with increasing θ i ; however, the initial adsorption probability decreases with θ i , when the crystal azimuthal angle is rotated 90°, indicating parallel momentum exchange dominates the adsorption process on this azimuth. Adsorbed propane facilitates trapping; S 0 increases linearly with propane coverage up to 0.55 saturation coverage. As θ cov increases, corrugation in the adsorbed layer dominates the adsorption process for all angles of incidence, and the azimuthal dependence of the adsorption probability becomes negligible except at the most glancing angles of incidence. A sudden change in the coverage dependence of the adsorption probability is observed at surface coverages above 0.55 saturation coverage.

Identification of Protonation State by XPS, Solid-State NMR, and DFT: Characterization of the Nature of a New Theophylline Complex by Experimental and Computational Methods

Recent studies suggested that X-ray photoelectron spectroscopy (XPS) sensitively determines the protonation state of nitrogen functional groups in the solid state, providing a means for distinguishing between co-crystals and salts of organic compounds. Here we describe how a new theophylline complex with 5-sulfosalicylic acid dihydrate was established as a salt by XPS prior to assignment with conventional methods. The presence of a C=NH(+) (N9) N1s peak in XPS allows assignment as a salt, while this peak is clearly absent for a theophylline co-crystal. The large low frequency shift for N9 observed by (15)N solid-state nuclear magnetic resonance spectroscopy (ssNMR) and corresponding density functional theory (DFT) calculations confirm that protonation has occurred. The crystal structure and further analytical studies confirm the conclusions reached with XPS and ssNMR. This study demonstrates XPS as an alternative technique for determining whether proton transfer has occurred in acid-base complexes.

Salt or Co-Crystal? Determination of Protonation State by X-Ray Photoelectron Spectroscopy (XPS)

Combined (15)N ssNMR and X-ray photoelectron spectroscopy (XPS) investigations for theophylline, a theophylline co-crystal, and a theophyllinium salt demonstrate that XPS allows direct observation of the degree of proton transfer, and thus identification of whether a salt or a co-crystal has been formed. The presence of a strongly binding-energy-shifted N 1s XPS peak with protonation indicates a salt (C==NH(+)), while this peak is unmistakably absent in the co-crystal. XPS should be considered as an alternative and complementary technique to single crystal X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy (ssNMR).

Fluorine modified chromium oxide and its impact on heterogeneously catalyzed fluorination reactions

Abstract A series of chromium oxofluorides was prepared by thermal reaction between (NH 4 ) 3 CrF 6 and Cr 2 O 3 . No CrF 3 -phases were detected in the reaction products either by XRD or by XANES. The shift in the peak intensities of the powder diffraction pattern shows that the structure of Cr 2 O 3 (Eskolaite) was modified with increasing fluorine content. Fluorine was found to replace oxygen in the structure of Cr 2 O 3 , resulting in an increasing distortion of the Cr 2 O 3 -lattice. Acidic sites of the samples were determined by analyzing characteristic IR-bands of pyridine adsorbed on the solid surface using FTIR-photoacoustic spectroscopy. The halogen exchange activity of the samples was studied in (i) the dismutation of CCl 2 F 2 and (ii) the fluorination of CH 2 ClCF 3 with HF. All chromium oxofluorides exhibited a significantly higher catalytic activity than both, pure CrF 3 and Cr 2 O 3 . The reasons for this enhanced catalytic activity of the oxofluoride phases are discussed.