Chiral N-isobutyryl-cysteine protected gold nanoparticles: preparation, size selection, and optical activity in the UV-vis and infrared

We have prepared gold nanoparticles covered with N-isobutyryl-l-cysteine and N-isobutyryl-d-cysteine, respectively. These particles with a mean particle size smaller than 2 nm are highly soluble in water and are amenable to chiroptical techniques such as vibrational circular dichroism (VCD) and circular dichroism (CD) spectroscopy. Density functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the experimental VCD spectrum and the calculated VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addition to the sulfur. The particles could furthermore be separated according to their charge and size into well-defined compounds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compounds showed basically identical absorption spectra as analogous gold particles protected with l-glutathione. This shows that these particles have identical core sizes (10-12, 15 and 18 gold atoms, respectively) and indicates that the number and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some separated compounds show strong optical activity with opposite sign when covered with the d- and l-enantiomer, respectively, of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.     

The early stages of three Palaearctic species of saproxylic hoverflies (Syrphidae, Diptera)

The third stage larva and puparium of the cerioidine hoverflies, Ceriana vespiformis (Latreille, 1804) and Sphiximorpha subsessilis (Illiger in Rossi, 1807) and the milesine, Spilomyia digitata (Rondani, 1865) (Diptera, Syrphidae) are described from specimens collected in the field. Ceriana vespiformis and S. digitata larvae were obtained from wet, decaying roots and tree holes of Fraxinus angustifolius L., respectively, in Spain and S. subsessilis from an exudation of sap on an Abies alba Miller tree in France.     

A comparative study of sound generation by laminar,combusting and non-combusting jet flows

Sound production by two-dimensional, laminar jet flows with and without combustion is studied numerically and theoretically. The compressible Navier–Stokes, energy and progress variable equations are solved by resolving both the near field and the acoustics. The combusting jet flows are compared to non-combusting jets of the same jet Mach number, with the non-combusting, non-isothermal jets having the same steady temperature difference as the combusting jets. This infers that the magnitude of entropic and density disturbances is similar in some of the combusting and non-combusting cases. The flows are perturbed by a sinusoidal inlet velocity fluctuation at different Strouhal numbers. The computational domain is resolved to the far field in all cases, allowing direct examination of the sound radiated and its sources. Lighthill’s acoustic analogy is then solved numerically using Green’s functions. The radiated sound calculated using Lighthill’s equation is in good agreement with that from the simulations for all cases, validating the numerical solution of Lighthill’s equation. The contribution of the source terms in Dowling’s reformulation of Lighthill’s equation is then investigated. It is shown that the source term relating to changes in the momentum of density inhomogeneities is the dominant source term for all non-reacting, non-isothermal cases. Further, this source term has similar magnitude in the combusting cases and is one of the several source terms that have similar magnitude to the source term involving fluctuations in the heat release rate.     

Multiferroics by rational design: implementing ferroelectricity in molecule-based magnets

Multiferroic materials: A novel class of multiferroics based on organic-inorganic hybrid materials is synthesized. The ferromagnetic properties originate from the coordination network, whereas the ferroelectricity results from the polar organic ions (see picture, M=magnetization and P=polarization).     

Turnover numbers, turnover frequencies, and overpotential in molecular catalysis of electrochemical reactions. Cyclic voltammetry and preparative-scale electrolysis

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.     

Seeking Hidden Magnetic Phenomena by Theoretical Means in a Thiooxoverdazyl Adduct

The oxidation of 1,5‐dimethyl‐3‐(2′‐pyridyl)‐6‐thiooxotetrazane (SvdH₃py) by benzoquinone leads to a 1:1 adduct of 1,5‐dimethyl‐3‐(2′‐pyridyl)‐6‐thiooxoverdazyl radical (Svdpy) with hydroquinone (hq). The single‐crystal X‐ray diffraction of this adduct at room temperature (RT) shows that the radicals exhibit a slight curvature that leads to the formation of alternating head‐to‐tail (antiparallel) stacked 1D chains. Moreover, temperature‐dependent X‐ray measurements at 100, 200, and 303 K reveal that the lateral slippages between the radicals of the stacks |δ₁| and |δ₂| vary from 0.64 to 0.78 Å and 0.54 to 0.40 Å between 100 and 303 K. Despite the alternation of the inter‐radical distances and lateral slippages, the magnetic susceptibility data can be fitted with excellent agreement using a regular one‐dimensional antiferromagnetic chain model with J=?5.9 cm^?1. Wavefunction‐based calculations indicate an alternation of the magnetic interaction parameters correlated with the structural analysis at RT. Moreover, they demonstrate that the thermal slippage of the radicals induces a switching of the physical behavior, since the exchange interaction changes from antiferromagnetic (?0.9 cm^?1) at 100 K to ferromagnetic (1.4 cm^?1) at 303 K. The theoretical approach thus reveals a much richer magnetic behavior than the analysis of the magnetic susceptibility data and ultimately questions the relevance of a spin‐coupled picture based on temperature‐independent parameters.     

A Self‐Reporting Photocatalyst for Online Fluorescence Monitoring of High Throughput RAFT Polymerization

Translating controlled/living radical polymerization (CLRP) from batch to the high throughput production of polymer libraries presents several challenges in terms of both polymer synthesis and characterization. Although recently there have been significant advances in the field of low volume, high throughput CLRP, techniques able to simultaneously monitor multiple polymerizations in an “online” manner have not yet been developed. Here, we report our discovery that 5,10,15,20‐tetraphenyl‐21H,23H‐porphine zinc (ZnTPP) is a self‐reporting photocatalyst that can mediate PET‐RAFT polymerization as well as report on monomer conversion via changes in its fluorescence properties. This enables the use of a microplate reader to conduct high throughput “online” monitoring of PET‐RAFT polymerizations performed directly in 384‐well, low volume microtiter plates.     

Sticky dissociative electron transfer to polychloroacetamides. In-cage ion-dipole interaction control through the dipole moment and intramolecular hydrogen bond

The reductive cleavage of chloro- and polychloroacetamides in N,N-dimethylformamide gives new insights into the nature of the in-cage ion radical cluster formed upon dissociative electron transfer. Within the family of compounds investigated, the electrochemical reduction leads to the successive expulsion of chloride ions. At each stage the electron transfer is concerted with the breaking of the C-Cl bond and acts as the rate-determining step. The reduction further leads to the formation of the corresponding carbanion with the injection of a second electron, which is in turn protonated by a weak acid added to the solution. From the joint use of cyclic voltammetric data, the sticky dissociative electron-transfer model and quantum ab initio calculations, the interaction energies within the cluster fragments (*R, Cl-) resulting from the first electron transfer to the parent RCl molecule are obtained. It is shown that the stability of these adducts, which should be viewed as an essentially electrostatic radical-ion pair, is mainly controlled by the intensity of the dipole moment of the remaining radical part and may eventually be strengthened by the formation of an intramolecular hydrogen bond, as is the case with 2-chloroacetamide.     

Electrochemical approach to concerted proton and electron transfers. Reduction of the water-superoxide ion complex

Concerted proton and electron transfers (CPET) currently attract considerable theoretical and experimental attention, notably in view of their likely involvement in many enzymatic reactions. Electrochemistry, through techniques such as cyclic voltammetry, can provide a quite effective access to CPET in terms of diagnosis and quantitative kinetic characterization. The relationships expressing the rate constant of an electrochemical CPET are given. Besides the CPET standard potential, it depends on two main factors. One is the reorganization energy, which appears as the sum of an intramolecular contribution and two solvent reorganization energies corresponding to proton and electron transfers, respectively. The other is the pre-exponential factor that mainly depends on proton tunneling through the activation barrier. Procedures for estimating these various factors as well as the H/D kinetic isotope effect are described. Application of the theory is illustrated by the experimental results obtained for the cyclic voltammetric reduction of the water-superoxide ion complex in dimethylformamide and acetonitrile.