Photodissociation dynamics of C6HxF6-x (x = 1-4) at 193 nm

Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C(6)H(3)F(2) and C(6)H(2)F(3) from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made.     

Study of the non-adiabatic transitions with application to NO

An expression for the non-adiabatic transition probability is derived from the viewpoint of the non-stationary character of the adiabatic approximation. A numerical calculation has been made for the free NO molecule. The non-adiabatic transition probability for the transition (B ² =0)(a ⁴ =9) is estimated to be 10^–6 sec^–1 by using the wave functions proposed by Moser et al.

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Zusammenfassung Für die nicht adiabatische Übergangswahrscheinlichkeit wurde aus dem nicht-stationären Charakter der adiabatischen Näherung ein Ausdruck hergeleitet, der für den Fall des NO-Moleküls numerisch ausgewertet wurde. Dabei ergab sich unter Verwendung der Wellenfunktionen von Moser u. Mitarb. eine Wahrscheinlichkeit für den Übergang (B ² =0) (a ⁴ =9) von der Größenordnung von 10^–6 sec^–1.

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Résumé Une expression pour la probabilité de la transition non adiabatique est obtenue du point de vue du caractère non stationnaire de l'approximation adiabatique. Un calcul numérique a été effectué pour la molécule NO isolée. La probabilité de transition non adiabatique pour la transition (B ² =0)(a ⁴ =9) est évaluée à 10^–6 sec^–1 en utilisant les fonctions d'onde proposées par Moser et al.

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This work was supported by The Faculty Grant of Arizona State University.     

Time‐Resolved Phase Transitions of the Nanocrystalline Cubic to Submicron Monoclinic Phase in Mn2O3‐ZrO2

The nanocrystalline cubic phase of zirconia was found to be thermally stabilized by the addition of 3 to 40 mol % manganese. The nanocrystalline cubic, tetragonal and monoclinic phases of zirconia stabilized with manganese (III)oxide (Mn‐Stabilized Zirconia) were prepared by thermal decomposition of carbonate and hydroxide precursors. Both the crystallization and isothermal phase transitions associated with Mn‐SZ were studied using high temperature x‐ray diffraction and x‐ray diffraction of quenched samples. Cubic Mn‐SZ initially crystallized and progressively transformed to tetragonal, and monoclinic structures above 700°C. The nanocrystalline cubic Mn‐SZ containing 25 mol % Mn was found to have the greatest thermal stability, retaining its cubic form at temperatures as high as 800°C for periods up to 25 hours. Higher than 40 mol %, cubic Mn₂O₃ was found to coexist with cubic Mn‐SZ. The crystallite sizes observed for the cubic, tetragonal and monoclinic Mn‐SZ phases ranged from 50 to 137, 130 to 220, and 195 to 450 Å respectively, indicating, for ZrO₂, that particle size was a primary factor in determining its polymorphs. The classical Avrami equation for nucleation and growth was applied to the observed phase transformations.     

Dipolar Compounds Containing Fluorene and a Heteroaromatic Ring as the Conjugating Bridge for High‐Performance Dye‐Sensitized Solar Cells

A novel series of dipolar organic dyes containing diarylamine as the electron donor, 2‐cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conjugating bridge have been developed and characterized. These metal‐free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fabricated as efficient nanocrystalline TiO₂ dye‐sensitized solar cells (DSSCs). The solar‐energy‐to‐electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88 %, which reached 68–96 % of a standard device of N719 fabricated and measured under the same conditions. With a TiO₂ film thickness of 6 μm, DSSCs based on these dyes had photocurrents surpassing that of the N719‐based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.     

Poly(dimethylsiloxane) Star Polymers Having Nanosized Silica Cores

Summary: Poly(dimethylsiloxane) (PDMS) star polymers having a nanosized silica particle as a core were prepared by reacting silica nanoparticles with monoglycidylether‐terminated poly(dimethylsiloxane). This star polymer was a hybrid material having an extremely high content of silica. The PDMS arms formed an organic domain to separate the silica particles and to prevent particle aggregation. The star polymers exhibited good thermal stability and high activation energy of their degradation reaction, in comparison to the linear PDMS polymer and the PDMS/silica blending materials. This star polymer can be used as a flame retardant for polymeric materials and this preparation technique can be applied to prepare other star polymers.

An SEM image of poly(dimethylsiloxane) star polymers having nanosized silica particles as a core.     

Prediction of human cytochrome P450 2B6‐substrate interactions using hierarchical support vector regression approach

The human cytochrome P450 2B6 can metabolize a number of clinical drugs. Inhibition of CYP2B6 by coadministered multiple drugs may lead to drug–drug interactions and undesired drug toxicity. The aim of this investigation is to develop an in silico model to predict the interactions between P450 2B6 and novel inhibitors using a novel hierarchical support vector regression (HSVR) approach, which simultaneously takes into account the coverage of applicability domain (AD) and the level of predictivity. Thirty‐seven molecules were deliberately selected and rigorously scrutinized from the literature data, of which 26 and 11 molecules were treated as the training set and the test set to generate the models and to validate the generated models, respectively. The generated HSVR model gave rise to an r² value of 0.97 for observed versus predicted pK_m values for the training set, a q² value of 0.93 by the 10‐fold cross‐validation, and an r² value of 0.82 for the test set. Additionally, the predicted results show that the HSVR model outperformed the individual local models, the global model, and the consensus model. Thus, this HSVR model provides an accurate tool for the prediction of human cytochrome P450 2B6‐substrate interactions and can be utilized as a primary filter to eliminate the potential selective inhibitor of CYP2B6. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009     

Excimer Formation in a Confined Space: Photophysics of Ladderphanes with Tetraarylethylene Linkers

Communication between chromophores is vital for both natural and non‐natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene‐ and polycyclobutene‐based ladderphanes are ideal model compounds in which all tetraarylethylene (TAE) linkers are aligned coherently. The spans for each of the monomeric units in these ladderphanes are 4.5–5.5 Å. Monomers do not exhibit emission, because bond rotation in TAE can quench the excited‐state energy. However, polymers emit at 493 nm (Φ=0.015) with large Stokes shift under ambient conditions and exhibit dual emission at 450 and 493 nm at 150 K. When the temperature is lowered, the emission intensity at 450 nm increases, whereas that at 493 nm decreases. At 100 K, both monomers and polymers emit only at 450 nm. This shorter‐wavelength emission arises from the intrinsic emission of TAE chromophore, and the emission at 493 nm could be attributed to the excimer emission in the confined space of ladderphanes. The fast kinetics suggest diffusion‐controlled formation of the excimer.     

Rotaxanes Synthesized Through Sodium‐Ion‐Templated Clipping of Macrocycles Around Nonconjugated Amide and Urea Functionalities

A single urea or amide functionality in a dumbbell‐shaped guest can be “clipped” by a macrocycle generated from a diamine and a dialdehyde through the templating effect of a Na⁺ ion (see scheme). The resulting imine‐containing rotaxanes can then be reduced to allow isolation of stable amine‐based rotaxanes.     

Simplified Derivatization Method for Triclosan Determination in Personal Care Products by Gas Chromatography‐Mass Spectrometry

An efficient and eco‐friendly injection‐port tert‐butyldimethylsilylated (TBDMS) derivatization and gas chromatography‐mass spectrometry (GC‐MS) was developed to determine triclosan (TCS, an antibacterial agent) in the samples of toothpaste, liquid hand‐soap and facial cleansing cream. This on‐line derivatization coupled ultrasonic extraction provides sensitivity, fast and reproducible results for TCS analysis. The accuracy and precision of the method was evaluated. The developed method was successfully applied to determine the content of TCS in selected personal care products.     

N‐Heterocyclic Carbene Copper(I) Rotaxanes Mediate Sequential Click Ligations with All Reagents Premixed

We have prepared NHC‐Cu^I complexes with a rotaxane structure and used them as sterically sensitive catalysts for one‐pot sequential copper‐catalyzed azide/alkyne cycloadditions in solutions containing all of the coupling partners premixed in unprotected form. Most notably, a photolabile and sterically encumbered complex first catalyzed the coupling of a less bulky azide/alkyne pair; after removing the protective macrocyclic component from the rotaxane structure, through irradiation with light, the exposed dumbbell‐shaped NHC‐Cu^I complex catalyzed the second click reaction of a bulkier azide/alkyne pair. Using this approach, we obtained predominantly, from a single sealed pot, a bis‐triazole product (84 %) from a mixture of two sterically distinct azides and a diyne.