A mechanistic investigation of (Me3Si)3SiH oxidation

The interaction of (Me₃Si)₃SiH with O₂ is known to afford (Me₃SiO)₂Si(H)SiMe₃ in which the two oxygen atoms arise from the same oxygen molecule. In order to investigate the mechanism of this unusual reaction, the oxidation rates were measured in the temperature range 30-70 °C by following oxygen uptake in the presence and absence of hydroquinone as inhibitor. The rate constant for the spontaneous reaction of (Me₃Si)₃SiH with O₂ was determined at 70 °C to be ∼3.5 × 10⁻⁵ M⁻¹ s⁻¹. A sequence of the propagation steps is proposed by combining the previous and present experimental findings with some theoretical results obtained at the semiempirical level. These calculations showed that the silylperoxyl radical (Me₃Si)₃SiOO undergoes three consecutive unimolecular steps to give (Me₃SiO)₂Si()SiMe₃. Evidence has been obtained that the rate determining step is the rearrangement of silylperoxyl radical to a dioxirand-like pentacoordinated silyl radical. Our findings are of considerable importance for the understanding of the oxidation of hydrogen-terminated silicon surfaces.     

Networking particles over distance using oligonucleotide-based devices

Microparticles covered with DNA-based computing elements that sense inputs and release oligonucleotides as outputs could be used to construct autonomous networks with increasingly complex functions. We demonstrate cascades of particles with up to three layers and a nonlinear network with an AND gate hub. In order to establish functional networks, particles do not have to be in direct physical contact.     

Insight into the permeation barrier of glued Langmuir-Blodgett bilayers

Permeation measurements have been made for He, CO2, and N2 across single Langmuir-Blodgett (LB) bilayers derived from 1,2,4,5-tetrakis[(N-(undecanoamidoethyl)-N,N-dimethyl ammonium) methyl]benzene tetrabromide (2) and 1,2,4,5-tetrakis[(N-(perfluoroundecanoamidoethyl)-N,N-dimethyl ammonium)methyl]benzene tetrabromide (3) in the absence and in the presence of entrapped poly(acrylic acid) (PAA). In the absence of PAA, single LB bilayers of 3 show a higher permeance for He and N2 but a lower permeation rate of CO2, as compared with analogous LB bilayers made from 2. The relatively low permeation rate of CO2 for the former has been attributed to reduced associative interactions with the fluorocarbon-rich bilayer. The same behavior has also been observed for LB bilayers containing PAA, formed under conditions that yield glued bilayers of 2 and 3 having similar diffusional pathways, as judged by He/N2 selectivities. These results, together with the fact that glued bilayers of 2 (having a thinner PAA layer as compared with those made from 3) exhibit lower He and N2 permeances, provide compelling evidence that the main barrier for gas transport is the combination of surfactant plus PAA and not simply a thin PAA layer that is encased within the surfactant bilayer.     

Sulfanyl chloride induced heterocyclization of N-(pyrazolyl)styrylacetamides

Heterocyclization of N-(pyrazol-3-yl)styrylacetamides with arenesulfenyl chlorides yields the corresponding tetrahydrofuran-2-iminium perchlorates isolable after chromatography as trans-5-aryl-4-(arylthio)tetrahydrofuran-2-ones and 5-aryl-4-(arylthio)-1-(pyrazol-3-yl)pyrrolidin-2-ones, with the latter predominating. N-(Pyrazol-4-yl)styrylacetamides react under analogous conditions to give 5-aryl-4-(arylthio)-1-(pyrazol-4-yl)pyrrolidin-2-ones, 4-aryl-3-(arylthio)-4-chloro-N-(1-methylpyrazol-4-yl)butanamides, and tetrahydrofuran-2-iminium perchlorates which, when chromatographed, produce 5-aryl-4-(arylthio)dihydrofuran-2(3H)-ones and 4-aryl-3-(arylthio)-4-methoxy-N-(1-methyl-1H-pyrazol-4-yl)butanamides.     

Direct calculation of the crystal-melt interfacial free energy via molecular dynamics computer simulation

We review our recent work on the direct calculation of the interfacial free energy, gamma, of the crystal-melt interface via molecular dynamics computer simulation for a number of model systems. The value of gamma as a function of crystal orientation is determined using a thermodynamic integration technique employing moving cleaving walls [Phys. Rev. Lett. 2000, 85, 4751]. The calculation is sufficiently precise to resolve the small anisotropy in gamma, which is crucial in determining the kinetics and morphology of dendritic growth. We report values of gamma for the hard-sphere and Lennard-Jones systems, as well as recent results on the series of inverse-power potentials. For the inverse sixth-, seventh-, and eighth-power systems, we determine gamma for both fcc and bcc crystal structures. For these systems, the bcc-melt gamma is lower than that for fcc crystals by about 25%, consistent with recent experiments and computer simulations on fcc-forming systems that show preferential formation of bcc nuclei in the initial stages of crystallization. In addition, we show that our results give a molecular interpretation of Turnbull's rule, which is the empirical relationship between gamma and the enthalpy of fusion.     

Dynamic and molecular association in premicellar aqueous solutions of dicarboxylate amino acid-based surfactant as studied by 1H NMR

We examined a series of amino acid-based surfactants with two carboxylic groups separated by a spacer of one, two, or three carbon atoms with sodium and calcium counterions in the premicellar concentration region near the CMC. ¹H nuclear magnetic resonance (NMR) spectroscopy and NMR diffusometry techniques were used to study the local environment, association, and translational dynamics of the surfactant's molecules. We measured the self-diffusion coefficients of the micelles, calculated the effective hydrodynamic radii, and determined the temperature region in which the premicelles exist. With an increase in temperature from 295 to 335 K, the premicellar state of the surfactant is replaced by the monomeric state.     

Time-indexed formulations for scheduling chains on a single machine: An application to airborne radars

Airborne radars are widely used to perform a large variety of tasks in an aircraft (searching, tracking, identifying targets, etc.) Such tasks play a crucial role for the aircraft and they are repeated in a “more or less” cyclic fashion. This defines a scheduling problem that impacts a lot on the quality of the radar output and on the overall safety of the aircraft.     

Low-temperature IR and NMR studies of the interaction of group 8 metal dihydrides with alcohols

The reactions of the octahedral dihydrido complexes [MH(2)(PP(3))] [M=Fe, Ru, Os; PP(3)=P(CH(2)CH(2)PPh(2))(3)] with a variety of weak ROH acids have been studied by IR and NMR methods in either CH(2)Cl(2) or THF in the temperature range from 190 to 290 K. This study has allowed the determination of the spectral and thermodynamic properties associated with the formation of dihydrogen bonds (DHB) between the terminal hydrides and the OH group. Both the DHB enthalpy values and the hydride basicity factors (E(j)) have been found to increase in the order Fe < Ru < Os. The proton transfer process, leading to the DHB complexes, and eventually to eta(2)-H(2) products, has been found to depend on the acidic strength of the alcohol as well as the nature of the solvent. Low temperature IR and NMR techniques have been used to trace the complete energy profile of the proton transfer process involving the osmium complex [OsH(2)(PP(3))] with trifluoroethanol.