Mechanistic elucidation of ketenimine-butynoate cycloaddition reaction: role of biradical intermediates in isotopomeric purity of benzyl (1,1a,6,6a-C4)-6-methyl anthranilic ester

Thermal Diels-Alder reaction of vinyl ketenimines with ¹³C-labeled 2-butynoate, efficiently prepared from ¹³C₄-acetoacetate via the enol triflate, rendered at 160 °C the corresponding methylanthranilic ester isotopomerically pure. However, when the reaction was run at 100 °C, 7% of isotope exchange between C^6a and C⁵ was observed. This result is explained through a base-like catalyzed isomerization of the alkyne to the corresponding allene. Combined DFT and coupled-cluster computations show the feasibility of this mechanistic proposal. Better agreement between DFT and coupled-cluster activation and reaction energies is observed for the meta-GGA MPWB1K functional as compared to previous B3LYP results.     

Fluorescence micro(spectro)scopy as a tool to study catalytic materials in action

Following its widespread use in biomedical research, fluorescence microscopy has recently been introduced in the catalysis field to study chemocatalytic processes with a high spatiotemporal resolution, a unique sensitivity down to the single molecule level and this under in situ conditions. This tutorial review is structured around the length scales that are currently accessible in fluorescence microscopy and discusses the different conceptual approaches that have been developed to study molecular concentration and dynamics like diffusion and catalytic conversion at these micron and sub-micron levels.     

Reduced mobility of di-propylene glycol methylether in its aqueous mixtures by quasielastic neutron scattering

The hydrogen (H-) bonding interplay between water and other organic molecules is important both in nature and in a wide range of technological applications. Structural relaxation and, thus, diffusion in aqueous mixtures are generally dependent on both the strength and the structure of the H-bonds. To investigate diffusion in H-bonding mixtures, we present a quasielastic neutron scattering study of di-propylene glycol methylether (2PGME) mixed with H(2)O (or D(2)O) over the concentration range 0-90 wt.% water. We observe a nonmonotonic behavior of the dynamics with a maximum in average relaxation time for the mixture with 30 wt.% water, which is more than a factor 2 larger compared to that of either of the pure constituents. This is a result in qualitative agreement with previous calorimetric studies and the behavior of aqueous mixtures of simple mono-alcohols. More surprisingly, we notice that the dynamics of the 2PGME molecules in the mixture is slowed down by more than a factor 3 at 30 wt.% water but that the water dynamics indicates an almost monotonous behavior. Furthermore, in the low momentum transfer (Q) range of the 2PGME, where the intermediate scattering function I(Q,t) is considerably stretched in time (i.e., the stretching parameter β ? 1), it is evident for the 2PGME-D(2)O samples that the Q-dependence of the inverse average relaxation time, <τ>(-1), is greater than 2. This implies that the relaxation dynamics is partly homogenously stretched, i.e., the relaxation of each relaxing unit is somewhat intrinsically stretched in time.     

The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4− in Superoxide Dismutation

The reaction of OH^? with O₃ eventually leads to the formation of ^.OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982 , 16, 676–681), it was suggested that the first step occurred by O transfer: OH^?+O₃→HO₂^?+O₂ and that ^.OH was generated in the subsequent reaction(s) of HO₂^? with O₃ (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one‐step O transfer such as that mentioned above would require the release of O₂ in its excited singlet state (¹O₂, O₂(¹Δ_g)); this state lies 95.5 kJ mol^?1 above the triplet ground state (³O₂, O₂(³Σ_g^?)). The low experimental rate constant of 70 M ^?1 s^?1 is not incompatible with such a reaction. However, according to our calculations, the reaction of OH^? with O₃ to form an adduct (OH^?+O₃→HO₄^?; ΔG=3.5 kJ mol^?1) is a much better candidate for the rate‐determining step as compared with the significantly more endergonic O transfer (ΔG=26.7 kJ mol^?1). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO₄^? have been probed: HO₄^?→HO₂^?+¹O₂ is spin allowed, but markedly endergonic (ΔG=23.2 kJ mol^?1). HO₄^?→HO₂^?+³O₂ is spin forbidden (ΔG=?73.3 kJ mol^?1). The decay into radicals, HO₄^?→HO₂^.+O₂^.?, is spin allowed and less endergonic (ΔG=14.8 kJ mol^?1) than HO₄^?→HO₂^?+¹O₂. It is thus HO₄^?→HO₂^.+O₂^.? by which HO₄^? decays. It is noted that a large contribution of the reverse of this reaction, HO₂^.+O₂^.?→HO₄^?, followed by HO₄^?→HO₂^?+³O₂, now explains why the measured rate of the bimolecular decay of HO₂^. and O₂^.? into HO₂^?+O₂ (k=1×10⁸ M ^?1 s^?1) is below diffusion controlled. Because k for the process HO₄^?→HO₂^.+O₂^.? is much larger than k for the reverse of OH^?+O₃→HO₄^?, the forward reaction OH^?+O₃→HO₄^? is practically irreversible.     

Structural characterization,thermal investigation,and liquid crystalline behavior of 4-[(4-chlorobenzyl)oxy]-3,4′-dichloroazobenzene

Using the Williamson method, a new dye 4-[(4-chlorobenzyl)oxy]-3,4′-dichloroazobenzene (CODA) with liquid crystalline properties was synthesized. The structure and the thermal behavior of CODA were investigated by means of nuclear magnetic resonance, X-ray diffraction, differential scanning calorimetry, and light polarized optical microscopy techniques. The thermophysical processes were monitored by heating–cooling cycles, but the formation of liquid crystal phases were exhibited only for small values of the cooling rates. For the first heating–cooling cycle, the melting and the solidification processes, thus the characteristic temperatures, are shifted to higher values when compared to the following cycles.     

Simultaneous measurements of As, Mo, Sb, V and W using a ferrihydrite diffusive gradients in thin films (DGT) device

The ferrihydrite-backed DGT (diffusive gradients in thin films), recently developed for arsenic and phosphate measurements was, for the first time, characterized with respect to molybdate, antimonate, vanadate and tungstate determination. Arsenate was included in the characterization to allow comparison with literature data and thus provide quality control of the measurements. In addition to laboratory experiments, field measurements were carried out in a natural stream in northern Sweden affected by mine drainage. It was shown that ferrihydrite-DGT is suitable for simultaneous determination of labile arsenic, molybdate, antimonate, vanadate and tungstate over a wide pH range. Diffusion coefficients were estimated using two different methods; diffusion cell and direct uptake to DGT devices in synthetic solutions. Estimations of the coefficients using the direct uptake method were performed between pH 4 and 8. The results from the two methods agreed well irrespective of pH, except for molybdate and antimonate that showed decreased values at pH 8. Adsorption of the analytes to ferrihydrite gel-discs was rapid at all pH values. However, there was a tendency toward lower adsorption affinity for antimonate compared to the other anions. 100% recovery of accumulated analytes was achieved through complete dissolution of the ferrihydrite adsorbent using 1.4 molL(-1) HNO(3) with 0.1 molL(-1) HF. From field sampling it was concluded that the opportunities for accurate antimonate and molybdate determination decrease at pH≥8.7. DGT-labile concentrations were generally lower than dissolved concentrations. Relatively lower DGT concentrations, compared to dissolved (<0.45 μm), were observed under a period when ferric oxide precipitations were detected on the DGT protective filter.     

A single-channel microparticle sieve based on Brownian ratchets

We present a novel device for the separation of microparticles in a single channel, which is made of inversely asymmetric Brownian ratchets. It enables separation into two different fractions with an adjustable threshold and can be modeled with good agreement. This device serves as proof of concept for an extremely compact class of microsieves.     

3 d metal ions in highly unusual eight-coordination: the phosphate-capped dodecapalladate(II) nanocube

Trapped in a noble cube: A novel family of noble metalates has been discovered in which a 3d metal ion M (M = Mn(II), Fe(III), Co(II), Cu(II), Zn(II)) is encapsulated by a 12 palladium-oxo cage {Pd(12)O(32)}, which is capped by eight phosphate groups. Such discrete nanocubes were further investigated by EPR spectroscopy, electrochemistry, and in homogeneous hydrogenation catalysis.