Hydrogenated amorphous silicon nanostructures: novel structure�Creactivity relationships for cyclization and ring opening in the gas phase

The effects of the reactive center connectivity and internal rotations on the reactivity of hydrogenated silicon nanostructures toward cyclization and ring opening pathways have been investigated. Rate coefficients for 25 cyclization and ring opening reactions for hydrides containing up to eight silicon atoms have been calculated using G3//B3LYP. The overall reactions exhibit two elementary steps. Overcoming the first barrier results in the formation of a hydrogen-bridged cyclic intermediate from a substituted silylene. Passing over the second barrier converts this intermediate into a cyclic silicon hydride. The rate-determining step varied according to the ring size formed and the temperature. Assuming a rate-determining step, values for the single-event Arrhenius pre-exponential factor, $ \tilde{A}$ , and the activation energy, E _a, were calculated from G3//B3LYP rate coefficients corrected for internal rotations, and a group additivity scheme was developed to predict $ \tilde{A}$ and E _a. The values predicted by group additivity are more accurate than structure?Creactivity relationships currently used in the literature, which rely on a representative $ \tilde{A}$ value for each reaction class and the Evans-Polanyi correlation to predict E _a. Internal rotation corrections played a prominent role in cyclization pathways, impacting $ \tilde{A}$ values for larger ring formation reactions more strongly than any variations in the connectivity of the reactive center.     

Translocation versus cyclisation in radicals derived from N-3-alkenyl trichloroacetamides

Under radical reaction conditions, two different and competitive reaction pathways were observed for N-(α-methylbenzyl)trichloroacetamides with a N-3-cyclohexenyl substituent: 1,4-hydrogen translocation and radical addition to a double bond. However, for radicals with an acyclic alkenyl side chain, the direct cyclisation process was exclusively observed. The dichotomy between translocation and direct radical cyclisation in these substrates has been theoretically studied using density functional theory (DFT) methods at the B3LYP/6-31G** computational level.     

Resonant inelastic x-ray scattering at the limit of subfemtosecond natural lifetime

We present measurements of the resonant inelastic x-ray scattering (RIXS) spectra of the CH(3)I molecule in the hard-x-ray region near the iodine L(2) and L(3) absorption edges. We show that dispersive RIXS spectral features that were recognized as a fingerprint of dissociative molecular states can be interpreted in terms of ultrashort natural lifetime of ～200 attoseconds in the case of the iodine L-shell core-hole. Our results demonstrate the capacity of the RIXS technique to reveal subtle dynamical effects in molecules with sensitivity to nuclear rearrangement on a subfemtosecond time scale.     

Scintillation neutron detectors based on solid-state photomultipliers and lightguides

Neutron detectors based on scintillation screens ZnS(Ag)/LiF and solid-state photomultipliers have been developed. Lightguides are used to collect light. The application of a coincidence scheme provides a low dark count and a neutron detection efficiency as high as 70%. A scheme of x-y neutron detector based on wavelength shifting fibers is also proposed. Tests of the proposed versions of detectors in a neutron beam have shown their efficiency.     

Electron localization and radiation chemistry of amides

Alkylamides (such as N,N'-dimethylformamide, N,N'-diethylformamide, and N,N'-dimethylacetamide) are aprotic solvents that are widely used in organic synthesis. These polar molecules have no electron affinity, and it is believed that irradiated liquid and solid amides stabilize excess electrons as cavity-type species analogous to hydrated and ammoniated electrons. In this study, we use isotope substitution and EPR spectroscopy to demonstrate that, in frozen amides, the suspected "cavity electron" is, in fact, a solvent-stabilized monomer anion. Our observations call into question other attributions of such features in the literature, both in low temperature solids and room temperature liquids. We also provide a general scheme describing amide radiolysis, as the related amides are used as metal ion extracting agents in nuclear separations.     

Reactivity of nucleosides with a hydroxyl radical in non-aqueous medium

DNA damage: The reactivity of HO(.) with silylated 2'-deoxyribonucleosides was investigated in acetonitrile by means of a time-resolved technique. The obtained rate constants were in general slightly lower than those reported for the natural nucleosides in water. Analysis of the reaction mixture by UPLC-MS revealed that HO(.) attack occurred at the nucleobase (see scheme).     

Viscoelastic properties of polymers: From molecular models to non-linear behavior

The molecular models of polymer physics (reptation, tube renewal) give a reasonable picture of the diffusion and relaxation of long and flexible chains: the concept of “tube renewal” (constraint release) added to the reptation idea explains the polydispersity effects for multimodal blends as well as for commercial linear polymers. The real issue now is to introduce these concepts in the formalism of non-linear viscoelasticity in order to explain the experimental data, as a first step in the range of moderate rates of deformation, then at very high strains.