Dealing with a local heating effect when measuring catalytic solids in a reactor with Raman spectroscopy

In continuation of our previous work on the applicability of the G(R(infinity)) correction factor for the quantification of Raman spectra of coke during propane dehydrogenation experiments (Phys. Chem. Chem. Phys., 2005, 7, 211), research has been carried out on the potential of this correction factor for the quantification of supported metal oxides during reduction experiments. For this purpose, supported chromium oxide catalysts have been studied by combined in situ Raman and UV-Vis spectroscopy during temperature programmed reduction experiments with hydrogen as reducing agent. The goal was to quantify on-line the amount of Cr(6+) in a reactor based on the measured in situ Raman spectra. During these experiments, a significant temperature effect was observed, which has been investigated in more detail with a thermal imaging technique. The results revealed a temperature 'on the spot' that can exceed 100 degrees C. It implies that Raman spectroscopy can have a considerable effect on the local reaction conditions and explains observed inconsistencies between the in situ UV-Vis and Raman data. In order to minimize this heating effect, reduction of the laser power, mathematical matching of the spectroscopic data, a different cell design and a change in reaction conditions has been evaluated. It is demonstrated that increasing the reactor temperature is the most feasible method to solve the heating problem. Next, it allows the application of in situ Raman spectroscopy in a reliable quantitative way without the need of an internal standard.     

Strong optical nonlinearities of self-assembled polymorphic microstructures of phenylethynyl functionalized fluorenones

The polymorphs of a phenylethynyl functionalized fluorenone derivative, and their controlled self-assembly for microstructures with different morphologies have been studied. These polymorphic microcrystals exhibit very distinctive NLO properties, which are highly correlated to their electronic and supramolecular structures.     

A New Ir‐NHC Catalyst for Signal Amplification by Reversible Exchange in D2O

NMR signal amplification by reversible exchange (SABRE) has been observed for pyridine, methyl nicotinate, N‐methylnicotinamide, and nicotinamide in D₂O with the new catalyst [Ir(Cl)(IDEG)(COD)] (IDEG=1,3‐bis(3,4,5‐tris(diethyleneglycol)benzyl)imidazole‐2‐ylidene). During the activation and hyperpolarization steps, exclusively D₂O was used, resulting in the first fully biocompatible SABRE system. Hyperpolarized ¹H substrate signals were observed at 42.5 MHz upon pressurizing the solution with parahydrogen at close to the Earth's magnetic field, at concentrations yielding barely detectable thermal signals. Moreover, 42‐, 26‐, 22‐, and 9‐fold enhancements were observed for nicotinamide, pyridine, methyl nicotinate, and N‐methylnicotinamide, respectively, in conventional 300 MHz studies. This research opens up new opportunities in a field in which SABRE has hitherto primarily been conducted in CD₃OD. This system uses simple hardware, leaves the substrate unaltered, and shows that SABRE is potentially suitable for clinical purposes.     

Co-crystal Prediction by Artificial Neural Networks**

A significant amount of attention has been given to the design and synthesis of co-crystals by both industry and academia because of its potential to change a molecule's physicochemical properties. Yet, difficulties arise when searching for adequate combinations of molecules (or coformers) to form co-crystals, hampering the efficient exploration of the target's solid-state landscape. This paper reports on the application of a data-driven co-crystal prediction method based on two types of artificial neural network models and co-crystal data present in the Cambridge Structural Database. The models accept pairs of coformers and predict whether a co-crystal is likely to form. By combining the output of multiple models of both types, our approach shows to have excellent performance on the proposed co-crystal training and validation sets, and has an estimated accuracy of 80 % for molecules for which previous co-crystallization data is unavailable.     

Chirality and conformational changes in 4-phenylphenanthrenes and 1-phenylbenzo[c]phenanthrene derivatives

NMR data of several 4-phenylphenanthrenes (15, 16) have revealed that the crowding in these compounds does not lead to chirality at temperatures as low as ?90°. The easy rotation of the phenyl substituent observed by NMR implies that notwithstanding the phenanthrene moiety in average behaves as a planar part the phenyl group does not experience steric hindrance.The analysis of temperature-dependent NMR spectra of several derivatives of 1-phenylbenzo[c]phenanthrenes (17-20) indicated that in these compounds exchange processes do occur. By calculations of the free energies of activation from the NMR data two processes could be distinguished: rotation of the phenyl substituent at one side of the helical benzo[c]phenanthrene moiety, for which ΔG^XXX_rot , is ca. 13.0$\text{kcal}\text{mol}$ or slightly larger when bulky substituents are present at C₂, and racemisation by a rotation of the phenyl group around the opposite end of the benzo[c]phenanthrene skeleton with simultaneous inversion of the helical conformation. For this process ΔG^XXX_rac is ca. 16$\text{kcal}\text{mol}$. The results have been compared with comparable data of related compounds like 1.8-diphenylnaphthalene, hexahelicene, and 4-methylbenzo[c] phenanthrenes, and gave evidence for the remarkably small, space-demanding properties of the phenyl substituent in these compounds.     

Surface modification of oxidic nanoparticles using 3-methacryloxypropyltrimethoxysilane

Tin oxide, antimony-doped tin oxide (ATO), and silica nanosized particles in aqueous dispersion were reacted with various amounts of 3-methacryloxypropyltrimethoxysilane (MPS). The kinetics were followed by 29Si NMR and the products were analyzed by FTIR and 29Si NMR. The kinetic experiments on ATO and silica revealed that the hydrolysis is the rate-determining step in these reactions. The reaction of MPS with the particles is favored over the homocondensation of MPS. Quantitative analysis using FTIR revealed that the amount of MPS grafted onto the tin oxide and silica particles is limited to the amount needed to fill one monolayer. For ATO the maximum amount of grafted MPS was only 50-70% of the amount that is needed for a closed monolayer. The MPS molecules are for the most part oriented parallel to the oxide surface, and a hydrogen bond between the MPS-carbonyl and the oxide is formed.     

Chirality and conformational changes in 4,5-diaryltriphenylenes

Line-shape analysis of temperature dependent NMR spectra of several substituted 4,5-diphenyl-triphenylenes has been performed to determine the free energy of activation for rotation (ΔG_rot^*) of the phenyl groups. The rotational barrier (ΔG_rot^*) depends on the presence and position of substituents on the phenyl groups; it is the largest in compounds with ortho-substituents. The independent determined free energy of activation of racemization (ΔG_rac^*) is about equal to ΔG_rot^* in 4-phenyl-5-(3,5-dimethylphenyl)triphenylene, but in 4,5-bis-(3,5- dimethylphenyl) triphenylene ΔG_rac^* is much larger than ΔG_rot^*. It is concluded that the racemization does not occur via a process in which the phenyl groups remain parallel but via a molecular movement in which the phenyl groups turn around each other like cog wheels.     

UV Curable Hard Transparent Hybrid Coating Materials on Polycarbonate Prepared by the Sol-Gel Method

UV curable, hard, transparent inorganic/organic composites with covalent links between the inorganic and the organic networks were prepared by the sol-gel method. These hybrid coating materials were synthesized using a commercially available, acrylate end-capped polyester or polyurethane oligomeric resin (EBC80, EBC284), hexanedioldiacrylate (HDDA) as a reactive solvent, 3-(trimethoxysilyl)propoxymethacrylate (TMSPM) as a coupling agent between the organic and inorganic phase, and a metal alkoxide, tetraethylorthosilicate (TEOS). The materials were applied on primer or oxygen plasma pretreated polycarbonate sheets and UV cured, followed by a thermal treatment to give a transparent coating with a good adhesion and abrasion resistance. The high transmission and the thermogravimetric behavior indicate the presence of a nanoscale hybrid composite, as is confirmed by SAXS and TEM measurements. In a Taber Abrasion Test, uncoated polycarbonate sheets exhibit a 48% decrease in light transmittance at 600 nm after 300 wear cycles, whereas the EBC80 hybrid coating system containing 9 wt% SiO₂ (EBC80/25Si) shows only 28% decrease in light transmittance. A maximal improvement of the abrasion resistance is achieved when 23 wt% SiO₂ is incorporated (EBC80/60Si, EBC284/60Si) with only 20% decrease in light transmittance. The abrasion resistance of glass is not yet encountered. For optimal results, it is essential that the rate of condensation of the silanol groups is sufficiently high to form a dense three-dimensional network.     

Through-Space Polar-π Interactions in 2,6-Diarylthiophenols

Molecular recognition between polar groups and aromatic molecules is fundamentally important to rational drug design. Although it has been well established that many polar functionalities interact with electron-rich aromatic residues through energetically favorable polar-π interactions, there is a limited understanding of the association between thiols and aromatic systems. Herein we report physical-organic chemistry studies on 2,6-diarylthiophenols that possess the central thiophenol ring and two flanking aromatic rings with tunable electronic properties caused by substituents at distant para position. Hammett analysis revealed that pK_a values and proton affinities correlate well with Hammett sigma values of substituents. Additional energy decomposition analysis supported the conclusion that both through-space SH-π interactions and S⁻-π interactions contribute to intramolecular stabilization of 2,6-diarylthiophenols.