Six-fold oxygen-coordinated triplet (S = 1) palladium(II) moieties templated by tris(bipyridine)ruthenium(II) ions

Tris(bipyridine)ruthenium(II) is used as a templating agent to insert palladium(II) into three-dimensional oxalate-based networks. The templated-assembly of [Ru(bpy)(3)][Pd(2)(ox)(3)] (Pd(2)) and [Ru(bpy)(3)][PdMn(ox)(3)] (PdMn) is described. The latter compound is structurally characterized by powder X-ray diffraction and X-ray absorption spectroscopy. These techniques reveal an unusual 6-fold oxygen environment around the Pd(II) atoms with two short (2.02 Angstrom) and four long (2.17 Angstrom) Pd-O distances. As stated by magnetometry, this environment is associated with a triplet ground state (S = 1) of the palladium(II) ion: when the temperature is decreased, the chiMT product shows a monotonous decrease from 5.54 cm(3) K mol(-1) at 300 K, a value which is slightly lower than the one expected for independent paramagnetic Pd(II) (S = 1, g = 2) and Mn(II) (S = 5/2, g = 2) ions. This thermal variation is due to antiferromagnetic exchange interactions between the two spin bearers. Nevertheless, no long-range magnetic order is detected down to 2 K. These results are confirmed by an analysis of the [MII(C(2)O(4))(3)](4-) (M = Ni, Pd, Pt) complex and of a [Pd(II){mu-(C(2)O(4))Mn(II)(OH(2))(4)}(3)](2+) tetranuclear model using density functional theory.     

Adiabatic and non-adiabatic concerted proton-electron transfers. Temperature effects in the oxidation of intramolecularly hydrogen-bonded phenols

The one-electron electrochemical and homogeneous oxidations of two closely similar aminophenols that undergo a concerted proton-electron transfer reaction, in which the phenolic proton is transferred to the nitrogen atom in concert with electron transfer, are taken as examples to test procedures that allow the separate determination of the degree of adiabaticity and the reorganization energy of the reaction. The Marcus (or Marcus-Hush-Levich) formalism is applicable in both cases, but not necessarily in its adiabatic version. Linearization of the activation-driving force laws simplifies the treatment of the kinetic data, notably allowing the use of Arrhenius plots to treat the temperature dependence of the rate constant. A correct estimation of the adiabaticity and reorganization energy requires the determination of the variation of the driving force with temperature. Application of these procedures led to the conclusion that, unlike previous reports, the homogeneous reaction is non-adiabatic, with a transmission coefficient of the order of 0.005, and that the self-exchange reorganization energy is about 1 eV lower than previously estimated. With such systems, the intramolecular reorganization energy, although sizable, is in fact rather modest, being only slightly larger than that for the outer-sphere electron transfer that produced the cation radical. The electrochemical reaction is, in contrast, adiabatic, as revealed by the temperature dependence of its standard rate constant obtained from cyclic voltammetric experiments. This difference in behavior is deemed to derive from the effect of the strong electric field within which the electrochemical reaction takes place, stabilizing a zwitterionic form of the reactant (in which the proton has been transferred from oxygen to nitrogen). Taking this difference in adiabaticity into account, the magnitudes of the reorganization energies of the two reactions appear to be quite compatible with one another, as revealed by an analysis of the solvent and intramolecular contributions in both cases.     

Heavy-quark energy loss in pQCD and SYM plasmas

We consider heavy-quark energy loss and p _⊥ -broadening in a strongly-coupled N=4 Super Yang–Mills (SYM) plasma, and the problem of finite-extent matter is addressed. When expressed in terms of the appropriate saturation momentum, one finds identical parametric forms for the energy loss in pQCD and SYM theory, while p _⊥ -broadening is radiation dominated in SYM theory and multiple scattering dominated in pQCD. C.M. is supported by the European Commission under the FP6 program, contract No. MOIF-CT-2006-039860.     

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.     

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.     

Origin of activation barriers in the dimerization of neutral radicals: a "nonperfect synchronization" effect?

Dimerizations of delocalized neutral radicals may be endowed with quite significant activation barriers. The origin of these barriers is discussed in terms of a model that emphasizes the role of localization of the unpaired radical upon bond formation. Several examples are given in which the model is compared with the results of quantum chemical calculations including the coupling of allyl radicals and of benzyl radicals at various possible carbon sites. The dimerization behavior of radicals in the NADH family is also examined. The connection between the reasons that underlay the existence of the activation barrier and the principle of "nonperfect synchronization" is discussed. The dimerization of conjugated radicals indeed offers a precious example that can be used to decipher the reasons behind these behaviors, being devoid of the ambiguities arising from the simultaneous involvement of ionic and covalent states, significant solvent reorganization, and the contribution of extensive proton tunneling, in the mostly discussed case of proton transfer at carbon.     

Influence of surface chemistry on the SAXS response of polymer-based activated carbons

Small-angle X-ray scattering (SAXS) measurements using contrast variation are reported for activated carbons prepared from poly(ethyleneterephthalate). The carbon surfaces are functionalized to different degrees by cold and hot nitric acid treatment. The latter treatment reduces the surface area by 75%, but the pore size distribution in the micropore range is hardly affected. Seven liquids, n-hexane, i-octane, i-propanol, cyclohexane, toluene, alpha-pinene, and nitrobenzene, in addition to water vapor, were used as contrast modifiers. Although the values of the specific surface area S(X) deduced from these measurements are relatively insensitive to the solvent, the detailed SAXS spectra reveal interactions occurring on different spatial scales that depend on the surface chemistry of the carbon and on the physicochemical properties of the solvent. In the most heavily oxidized sample, the amphiphilic molecule i-propanol stabilizes the surface structure, whereas nonpolar molecules make the rough surface smoother. In the untreated and the lightly functionalized carbons, water vapor at 50% relative humidity condenses only partially in the micropores at room temperature, whereas in the heavily treated sample condensation in the micropores is practically complete.