Activation barriers in the homolytic cleavage of radicals and ion radicals

As revealed by several experimental examples, radicals and ion radicals may, in contrast with closed-shell molecules, undergo exothermic homolytic cleavages (.A..B --> A: +.B) with substantial activation barriers. A two-state semiclassical model is proposed for explaining the existence of the barrier and estimating its magnitude. It is based on the intersection of the potential energy surfaces characterizing the dissociation of a bonding state, .A..B -->.A. +.B, on one hand, and the approach to bonding distance of a repulsive state, A: +.B --> A therefore B, on the other. After inclusion of the bond cleavage and formation as Morse curves in the normal-mode analysis, a simple activation driving force relationship is obtained, the two main ingredients of the intrinsic barrier being the triplet excitation energy of the A moiety and the pi*--> sigma* excitation energy in .A-B. The model is then tested by quantum chemical calculations, first on a simplified system to evaluate the calculation techniques and then on a real system. A comparison of the model predictions with experiment is finally performed using the rate data recently gathered for the cleavage of 4-cyanophenyl alkyl ether anion radicals, which cover a respectable range of driving forces, showing satisfactory agreement between theoretical predictions and experimental data.     

Experimental investigation and analytical description of a vibro-impact NES coupled to a single-degree-of-freedom linear oscillator harmonically forced

In this paper, the dynamics of a system composed of a harmonically forced single-degree-of-freedom linear oscillator coupled to a vibro-impact nonlinear energy sink (VI-NES) is experimentally investigated. The mass ratio between the VI-NES and the primary system is about \(1\%\). Depending on the external force’s amplitude and frequency, either a strongly modulated response (SMR) or a constant amplitude response (CAR) is observed. In both cases, an irreversible transfer of energy occurs from the linear oscillator toward the VI-NES: process known in the literature as passive targeted energy transfer. Furthermore, the problem is analytically studied by using the method of multiple scales. The obtained slow invariant manifold shows the existence of a stable and of an unstable branch of solutions, as well as of an energy threshold (a saddle-node bifurcation) for the solutions to appear. Subsequently, the fixed points of the problem are calculated. When a stable fixed point is reached, the system is naturally drawn to it and a CAR is established, whereas when no stable point is attained, the system exhibits a SMR regime. Finally, a good correlation between the experimental and the analytical results is presented.     

Diamondoid Nanostructures as sp3‐Carbon‐Based Gas Sensors

Diamondoids, sp³‐hybridized nanometer‐sized diamond‐like hydrocarbons (nanodiamonds), difunctionalized with hydroxy and primary phosphine oxide groups, enable the assembly of the first sp³‐C‐based chemical sensors by vapor deposition. Both pristine nanodiamonds and palladium nanolayered composites can be used to detect toxic NO₂ and NH₃ gases. This carbon‐based gas sensor technology allows reversible NO₂ detection down to 50 ppb and NH₃ detection at 25–100 ppm concentration with fast response and recovery processes at 100 °C. Reversible gas adsorption and detection is compatible with 50 % humidity conditions. Semiconducting p‐type sensing properties are achieved from devices based on primary phosphine–diamantanol, in which high specific area (ca. 140 m² g^?1) and channel nanoporosity derive from H‐bonding.     

Direct Oxidative Coupling of N‐Acetyl Indoles and Phenols for the Synthesis of Benzofuroindolines Related to Phalarine

Inspired by the biogenetic synthesis of benzofuroindoline‐containing natural products, we designed an oxidative coupling between phenol and N‐acetyl indoles. This straightforward and direct radical process, mediated by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone and FeCl₃ allowed the regioselective synthesis of benzofuro[3,2‐b]indolines, whose structure is found in the natural product phalarine.     

Ternary complexes in gels from agarose and from chemically‐modified agarose

Thermodynamic data and mechanical measurements are shown for gels prepared in aqueous binary solvents (water/DMSO, water/DMF, water/methyl formamide and water/formamide). When electrostatic interactions, as opposed to hydrogen bonding, can be established with the cosolvent (DMSO, DMF, methyl formamide) we come to the conclusion that ternary complexes are formed (agarose/water/cosolvent). In the case of chemically‐modified agarose (OH groups replaced by OCH₃ groups) we suggest that these cosolvents are directly involved in the formation of the gel.     

Small strain behavior of polyethylene: In situ SAXS measurements

Stack lamella deformation depends on their orientation with respect to the loading axis, the intrinsic properties of the lamellae, and the mechanical coupling between crystalline and amorphous phases. The aim of this work is to investigate the influence of the stress transmitter (ST) density and the crystallinity X_c on the local deformation. A wide experimental campaign has been undertaken on several polyethylenes with controlled molecular parameters and subjected to different thermal treatments. The ST density has been evaluated by the natural draw ratio and calculated by the Brown's model. The local deformation was measured by SAXS along a tensile test by using the long period stretching of the equatorial lamella stacks. The ratio ε_local/ε_macro was found to be a constant close to 0.5. This surprising low value has highlighted that the equatorial regions could be either the stiffest zone of the spherulite or submitted to a lower stress. It is proposed that the stability of the ratio ε_local/ε_macro is the result of two opposite phenomena: On one hand, the increase of X_c leads to unload the equatorial regions due to partial percolation of the crystalline phase and so decreases the stresses. On the other hand, when increasing X_c, the ST density decreases which causes the decrease in the local equatorial modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1535–1542, 2010     

Photoinduced electron transfer in Zn(II)porphyrin-bridge-Pt(II)acetylide complexes: variation in rate with anchoring group and position of the bridge

The synthesis and photophysical characterization of two sets of zinc porphyrin platinum acetylide complexes are reported. The two sets of molecules differ in the way the bridging phenyl-ethynyl unit is attached to the porphyrin ring. One set is attached via an ethynyl unit on the β position, while the other set is attached via a phenyl unit on the meso position of the porphyrin. These were compared with previously studied complexes where attachment was made via an ethynyl unit on the meso position. Femtosecond transient absorption measurements showed in all systems a rapid quenching of the porphyrin singlet state. Electron transfer is suggested as the quenching mechanism, followed by an even faster recombination to form both the porphyrin ground and triplet excited states. This is supported by the variation in quenching rate and porphyrin triplet yield with solvent polarity, and the observation of an intermediate state in the meso-phenyl linked systems. The different linking motifs between the dyads resulted in significant variations in electron transfer rates.     

Syzygies among reduction operators

We introduce the notion of syzygy for a set of reduction operators and relate it to the notion of syzygy for presentations of algebras. We give a method for constructing a linear basis of the space of syzygies for a set of reduction operators. We interpret these syzygies in terms of the confluence property from rewriting theory. This enables us to optimise the completion procedure for reduction operators based on a criterion for detecting useless reductions. We illustrate this criterion with an example of construction of commutative Gröbner basis.     

Complex interpolation between two weighted Bergman spaces on tubes over symmetric cones

We prove that the complex interpolation space [A_ν^p₀,A_ν^p₁]_θ, 0<θ<1, between two weighted Bergman spaces A_ν^p₀ and A_ν^p₁ on the tube in $\text{C}{}^{\mathrm{n}}$, n?3, over an irreducible symmetric cone of $\text{R}{}^{\mathrm{n}}$ is the weighted Bergman space A_ν^p with 1/p=(1?θ)/p₀+θ/p₁. Here, ν>n/r?1 and 1?p₀<p₁<2+ν/(n/r?1) where r denotes the rank of the cone. We then construct an analytic family of operators and an atomic decomposition of functions, which are related to this interpolation result. To cite this article: D. Békollé et al., C. R. Acad. Sci. Paris, Ser. I 337 (2003).     

H-bond relays in proton-coupled electron transfers. Oxidation of a phenol concerted with proton transport to a distal base through an OH relay

Four molecules comprising a phenol moiety and a distal pyridine base connected by an intermediary H-bonding and an H-bonded alcohol group have been synthesized and their electrochemistry has been investigated by means of cyclic voltammetry. The molecules differ by the substituent at the alcohol functional carbon and by methyl groups on the pyridine. The reaction follows a concerted proton-electron transfer pathway as confirmed by the observation of a significant H/D kinetic isotope effect in all four cases. The standard rate constants characterizing each of the four compounds are analyzed in terms of reorganization energy and pre-exponential factor. Intramolecular and solvent reorganization energies appear as practically constant in the series, in which a previously investigated aminophenol is included, whereas significantly different pre-exponential factors are observed. That the latter, which is a measure of the efficiency of proton tunneling concerted with electron transfer, be substantially smaller with the H-bond relay molecules than with the aminophenol is related to the fact that two protons are moved in the first case instead of one in the second. Within the H-bond relay molecules, the pre-exponential factor varies with the substituent present at the alcohol functional carbon in the order CF(3) > H > CH(3), presumably as the result of a fine tuning of the balance between the H-bond accepting and H-bond donating properties of the central OH group. The kinetic H/D kinetic isotope effect increases accordingly in the same order.