Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO2 Nanoparticles

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V₁ and V₂, interconnected by a rigid terphenylene bridge. In their parent dication states, V₁²⁺ and V₂²⁺ can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side‐to‐side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO₂ nanoparticle, viologen V₂²⁺ is reduced at significantly more negative potentials (?0.3 V) than in flexible analogues that contain aliphatic bridges between V₁ and V₂. These overpotentials are analysed in terms of electron‐transfer rates and a donor–bridge–acceptor (D–B–A) formalism, in which D is the doubly reduced viologen, V₁⁰, adjacent to the TiO₂ surface (TiO₂–V₁⁰), B is the terphenylene bridge and A is viologen V₂²⁺. We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V₂²⁺ by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution‐based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO₂ surface.     

Chemical Reaction Monitoring using Zero-Field Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers

We demonstrate that heterogeneous/biphasic chemical reactions can be monitored with high spectroscopic resolution using zero-field nuclear magnetic resonance spectroscopy. This is possible because magnetic susceptibility broadening is negligible at ultralow magnetic fields. We show the two-step hydrogenation of dimethyl acetylenedicarboxylate with para-enriched hydrogen gas in conventional glass NMR tubes, as well as in a titanium tube. The low frequency zero-field NMR signals ensure that there is no significant signal attenuation arising from shielding by the electrically conductive sample container. This method paves the way for in situ monitoring of reactions in complex heterogeneous multiphase systems and in reactors made of conductive materials while maintaining resolution and chemical specificity.     

Penetration of methane–oxygen flames through flat obstacles with several openings

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Semiexperimental equilibrium structure of 1-methylisatin from gas-phase electron diffraction data and structural changes in isatin due to 1-methyl and 5-fluoro substituents as predicted by coupled cluster computations

ABSTRACT

The molecular structure of 1-methylisatin (1) has been studied by gas-phase electron diffraction (GED) and quantum chemical computation up to the coupled cluster (CCSD(T)) level of theory. The semiexperimental equilibrium structure (C_s point group symmetry) has been determined from the GED data taking into account anharmonic vibrational corrections calculated in curvilinear coordinates with the B2PLYP force field. To observe fine structural effects due to the presence of different kinds of substituents, the accurate molecular structures of isatin (2) and 5-fluoroisatin (3) were also computed at the CCSD(T) level. The (O=)C–C(=O) carbon–carbon bonds of the pyrroline moiety in 1–3 are found to be remarkably longer than the typical single C–C bond. The electron donating methyl group causes a decrease of the C?N?C angle and an elongation of the N?C bond lengths in the pyrroline ring by 0.7° and up to 0.008?Å, respectively, whereas the electron withdrawing fluorine atom increases the ipso CCC angle by 2.5° in comparison to that in unsubstituted isatin.     

Synthesis of 5-methoxylated 3-pyrrolin-2-ones via the rearrangement of chlorinated pyrrolidin-2-ones

The reaction of N-substituted 4-methyl-2-pyrrolidinones or 4-diethoxyphosphoryl analogues, carrying at least two chlorine atoms between the C(3) and C(6) carbons, with alkaline methoxide in methanol afforded the corresponding 5-methoxylated 3-pyrrolin-2-ones, useful adducts for the preparation of agrochemicals or medicinal compounds.     

Weakly nonoscillatory schemes for scalar conservation laws

A new class of Godunov-type numerical methods (called here weakly nonoscillatory or WNO) for solving nonlinear scalar conservation laws in one space dimension is introduced. This new class generalizes the classical nonoscillatory schemes. In particular, it contains modified versions of Min-Mod and UNO. Under certain conditions, convergence and error estimates for WNO methods are proved.

     

Concise Total Synthesis of Dehaloperophoramidine

Perophoramidine, dehaloperophoramidine, and communesin F are structurally related alkaloids having intriguing polycyclic structures. A strategy for the synthesis of dehaloperophoramidine has been developed. In this synthesis all skeletal atoms and all functional groups required to reach the target molecule are incorporated early in the sequence. This approach led to the discovery of two novel substrate‐specific domino processes, one encompassing four steps and the other comprising five steps, thus resulting in an eight‐step synthesis of dehaloperophoramidine.