Time-resolved gas-phase kinetic, quantum chemical and RRKM studies of reactions of silylene with cyclic ethers

Time-resolved kinetic studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with oxirane, oxetane, and tetrahydrofuran (THF). The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at four or five temperatures in the range 294-605 K. All three reactions showed pressure dependences characteristic of third-body-assisted association reactions with, surprisingly, SiH2 + oxirane showing the least and SiH2 + THF showing the most pressure dependence. The second-order rate constants obtained by extrapolation to the high-pressure limits at each temperature fitted the Arrhenius equations where the error limits are single standard deviations: log(k(oxirane)(infinity)/cm(3) molecule(-1) s(-1)) = (-11.03 +/- 0.07) + (5.70 +/- 0.51) kJ mol(-1)/RT In 10 log(k(oxetane)(infinity)/cm(3) molecule(-1) s(-1)) = (-11.17 +/- 0.11) + (9.04 +/- 0.78) kJ mol(-1)/RT In 10 log(k(THF)(infinity)/cm(3) molecule(-1) s(-1)) = (-10.59 +/- 0.10) + (5.76 +/- 0.65) kJ mol(-1)/RT In 10 Binding-energy values of 77, 97, and 92 kJ mol(-1) have been obtained for the donor-acceptor complexes of SiH2 with oxirane, oxetane, and THF, respectively, by means of quantum chemical (ab initio) calculations carried Out at the G3 level. The use of these values to model the pressure dependences of these reactions, via RRKM theory, provided a good fit only in the case of SiH2 + THF. The lack of fit in the other two cases is attributed to further reaction pathways for the association complexes of SiH2 with oxirane and oxetane. The finding of ethene as a product of the SiH2 + oxirane reaction supports a pathway leading to H2Si=O + C2H4 predicted by the theoretical calculations of Apeloig and Sklenak.

The gas-phase reaction between silylene and 2-butyne: kinetics, isotope studies, pressure dependence studies and quantum chemical calculations

Time-resolved kinetic studies of the reactions of silylene, SiH2, and dideutero-silylene, SiD2, generated by laser. ash photolysis of phenylsilane and phenylsilane-d(3), respectively, have been carried out to obtain rate coefficients for their bimolecular reactions with 2-butyne, CH3C CCH3. The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas at five temperatures in the range 294-612 K. The second-order rate coefficients, obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equations where the error limits are single standard deviations: log(k(H)(infinity)/cm(3) molecule(-1) s(-1)) = (-9.67 +/- 0.04) + (1.71 +/- 0.33) kJ mol(-1)/RTln10 log(k(D)(infinity)/cm(3) molecule(-1) s(-1)) = (-9.65 +/- 0.01) + (1.92 +/- 0.13) kJ mol(-1)/RTln10 Additionally, pressure-dependent rate coefficients for the reaction of SiH2 with 2-butyne in the presence of He (1-100 Torr) were obtained at 301, 429 and 613 K. Quantum chemical (ab initio) calculations of the SiC4H8 reaction system at the G3 level support the formation of 2,3-dimethylsilirene [cyclo-SiH2C(CH3)=C(CH3)-] as the sole end product. However, reversible formation of 2,3-dimethylvinylsilylene [CH3CH=C(CH3)SiH] is also an important process. The calculations also indicate the probable involvement of several other intermediates, and possible products. RRKM calculations are in reasonable agreement with the pressure dependences at an enthalpy value for 2,3-dimethylsilirene fairly close to that suggested by the ab initio calculations. The experimental isotope effects deviate significantly from those predicted by RRKM theory. The differences can be explained by an isotopic scrambling mechanism, involving H - D exchange between the hydrogens of the methyl groups and the D-atoms in the ring in 2,3-dimethylsilirene-1,1-d(2). A detailed mechanism involving several intermediate species, which is consistent with the G3 energy surface, is proposed to account for this.

Induced random fields in the LiHoxY1-xF4 quantum ising magnet in a transverse magnetic field

The LiHoxY1-xF4 magnetic material in a transverse magnetic field Bxx̂ perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in a random disordered system. We show that the Bx-induced magnetization along the x̂ direction, combined with the local random dilution-induced destruction of crystalline symmetries, generates, via the predominant dipolar interactions between Ho3+ ions, random fields along the Ising ẑ direction. This identifies LiHoxY1-xF4 in Bx as a new random field Ising system. The random fields explain the rapid decrease of the critical temperature in the diluted ferromagnetic regime and the smearing of the nonlinear susceptibility at the spin-glass transition with increasing Bx and render the Bx-induced quantum criticality in LiHoxY1-xF4 likely inaccessible.

Dielectric relaxation around the nematic-isotropic transition of liquid crystalline polymers

This study is concerned with a series of acrylate based side-chain liquid crystalline (LC) polymers. Previous studies have shown that these LC polymers have a preference for parallel or perpendicular alignment with respect to the polymer chain which depends on the length of the coupling chain joining the mesogenic unit to the polymer backbone. On the other hand, the dielectric relaxation of these side-chain LC polymers shows a strong relaxation associated to the mesogenic unit dynamics. For samples with parallel alignment, it was found that the dielectric relaxation of the nematic is weaker and broader than the relaxation of the isotropic. By contrast, for samples with perpendicular alignment, the isotropic to nematic transition reduces the broadening the relaxation and increases the relaxation strength. These two features are more evident for samples with short coupling units for which the dielectric relaxation observed appears to be strongly coupled with the backbone dynamics.

Spin-canted antiferromagnetic phase transitions in alternating phenoxo- and carboxylato-bridged Mn(III)-salen complexes

Three new MnIII complexes, {[Mn-2(salen)(2)(OCn)](ClO4)}(n) (1), {[Mn-2(salen)(2)(OPh)](ClO4)}(n) (2) and {[Mn-2(salen)(2)(OBz)](ClO4)}(2) (3) (where salen = N,N'-bis(salicylidene)-1,2-diaminoethane dianion, OCn = cinnamate, OPh = phenylacetate and OBz = benzoate), have been synthesized and characterized structurally and magnetically. The crystal structures reveal that all three structures contain syn-anti carboxylatebridged dimeric [Mn-2(salen)(2)(OOCR)](+) cations (OOCR = bridging carboxylate) that are joined together by weak Mn center dot center dot center dot O(phenoxo) interactions to form infinite alternating chain structures in 1 and 2, but the relatively long Mn center dot center dot center dot O(phenoxo) distance [3.621(2)angstrom] in 3 restricts this structure to tetranuclear units. Magnetic studies showed that 1 and 2 exhibited magnetic long-range order at T-N = 4.0 and 4.6 K (T-N = Neel transition temperature), respectively, to give spin-canted antiferromagnetic structures. Antiferromagnetic coupling was also observed in 3 but no peaks were recorded in the field-cooled magnetization (FCM) or zero-field-cooled magnetization (ZFCM) data, indicating that 3 remained paramagnetic down to 2 K. This dominant antiferromagnetic coupling is attributed to the carboxylate bridges. The ferromagnetic coupling expected due to the Mn-O(phenoxo)center dot center dot center dot Mn bridge plays an auxiliary role in the magnetic chain, but is an essential component of the bulk magnetic properties of the material.

Transition from an anti-phase error-correction-mode to a synchronization mode in the mutual hand tracking

Proactive motion in hand tracking and in finger bending, in which the body motion occurs prior to the reference signal, was reported by the preceding researchers when the target signals were shown to the subjects at relatively high speed or high frequencies. These phenomena indicate that the human sensory-motor system tends to choose an anticipatory mode rather than a reactive mode, when the target motion is relatively fast. The present research was undertaken to study what kind of mode appears in the sensory-motor system when two persons were asked to track the hand position of the partner with each other at various mean tracking frequency. The experimental results showed a transition from a mutual error-correction mode to a synchronization mode occurred in the same region of the tracking frequency with that of the transition from a reactive error-correction mode to a proactive anticipatory mode in the mechanical target tracking experiments. Present research indicated that synchronization of body motion occurred only when both of the pair subjects operated in a proactive anticipatory mode. We also presented mathematical models to explain the behavior of the error-correction mode and the synchronization mode.

The reaction between silylene and ammonia: some gas-phase kinetic and quantum chemical studies

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by 193 nm laser flash photolysis of silacyclopent-3-ene, have been carried out in the presence of ammonia, NH3. Second order kinetics were observed. The reaction was studied in the gas phase at 10 Torr total pressure in SF6 bath gas at each of the three temperatures, 299, 340 and 400 K. The second order rate constants (laser pulse energy of 60 mJ/pulse) fitted the Arrhenius equation: log(k/cm3 molecule-1 s-1) = (-10.37 ± 0.17) + (0.36 ± 1.12 kJ mol-1)/RTln10 Experiments at other pressures showed that these rate constants were unaffected by pressure in the range 10-100 Torr, but showed small decreases in value at 3 and 1 Torr. There was also a weak intensity dependence, with rate constants decreasing at laser pulse energies of 30 mJ/pulse. Ab initio calculations at the G3 level of theory, show that SiH2 + NH3 should form an initial adduct (donor-acceptor complex), but that energy barriers are too great for further reaction of the adduct. This implies that SiH2 + NH3 should be a pressure dependent association reaction. The experimental data are inconsistent with this and we conclude that SiH2 decays are better explained by reaction of SiH2 with the amino radical, NH2, formed by photodissociation of NH3 at 193 nm. The mechanism of this previously unstudied reaction is discussed.

The Hiccup: a dynamical coupling process during the autumn transition in the Northern Hemisphere – similarities and differences to sudden stratospheric warmings

Sudden stratospheric warmings (SSWs) are the most prominent vertical coupling process in the middle atmosphere, which occur during winter and are caused by the interaction of planetary waves (PWs) with the zonal mean flow. Vertical coupling has also been identified during the equinox transitions, and is similarly associated with PWs. We argue that there is a characteristic aspect of the autumn transition in northern high latitudes, which we call the “hiccup”, and which acts like a “mini SSW”, i.e. like a small minor warming. We study the average characteristics of the hiccup based on a superimposed epoch analysis using a nudged version of the Canadian Middle Atmosphere Model, representing 30 years of historical data. Hiccups can be identified in about half the years studied. The mesospheric zonal wind results are compared to radar observations over Andenes (69N,16E) for the years 2000–2013. A comparison of the average characteristics of hiccups and SSWs shows both similarities and differences between the two vertical coupling processes.

On identity reformation and local transition: a case from Poland

Purpose – The purpose of this paper is to explore and understand the process of successful introduction of total quality management (TQM) in Poland and the way in which it impacted on identity of Polish managers. Design/methodology/approach – The study is based on a combination of ethnographic research and repertory grid interviews. Findings – The process of TQM introduction and implementation is examined through the application of translation as a model incorporating cultural and socio-economical dimensions in addition to individual and organizational levels that shaped the development of TQM in Poland. It then draws on the idea of fantasy as theorized in Lacanian psychoanalysis in order to incorporate the unconscious element of translation process which is missing from Latour’s theorization and which forms an important aspect of adoption of new technology and the emergence of a new post-transition generation of managers in Poland. The paper argues that a complex combination of contextual factors, amongst them the notion of fantasy shaped the process of translation of TQM to Poland, the identity formation of Polish managers and to the emergence of a new post-transition generation of managers in Poland. Originality/value – This paper contributes to the literature on the post-command transition by illustrating this process through the fantasy of total quality management explored in a specific socio-cultural and geographical context and by combining the idea of Latour’s translation with Lacanian fantasy.

Phase transitions of cassava starch dispersions prepared with glycerol solutions

The aim of this work was to study the glass transition, the glass transition of the maximally freeze-concentrated fractions, the ice melting and the gelatinization phenomenon in dispersions of starch prepared using glycerol- water solutions. The starch concentration was maintained constant at 50 g cassava starch/100 g starch dispersions, but the concentration of the glycerol solutions was variable (C-g= 20, 40, 60, 80 and 100 mass/mass%). The phase transitions of these dispersions were studied by calorimetric methods, using a conventional differential scanning calorimeter (DSC) and a more sensitive equipment (micro-DSC). Apparently, in the glycerol diluted solutions (20 and 40%), the glycerol molecules interacted strongly with the glucose molecules of starch. While in the more concentrated glycerol domains (C-g> 40%), the behaviour was controlled by migration of water molecules from the starch granules, due to a hypertonic character of glycerol, which affected all phase transitions.

Metastable Phase Diagram of Nanocrystalline ZrO(2)-Sc(2)O(3) Solid Solutions

We have investigated the crystal structures and phase transitions of nanocrystalline ZrO(2)-1 to -13 mol % Sc(2)O(3) by synchrotron X-ray powder diffraction and Raman spectroscopy. ZrO(2)-Sc(2)O(3) nanopowders were synthesized by using a stoichiometric nitrate-lysine get-combustion route. Calcination processes at 650 and at 850 degrees C yielded nanocrystalline materials with average crystallite sizes of (10 +/- 1) and (25 +/- 2) nm, respectively. Only metastable tetragonal forms and the cubic phase were identified, whereas the stable monoclinic and rhombohedral phases were not detected in the compositional range analyzed in this work. Differently from the results of investigations reported in the literature for ZrO(2)-Sc(2)O(3) materials with large crystallite sizes, this study demonstrates that, if the crystallite sizes are small enough (in the nanometric range), the metastable t ``-form of the tetragonal phase is retained. We have also determined the t`-t `` and t ``-cubic compositional boundaries at room temperature and analyzed these transitions at high temperature. Finally, using these results, we built up a metastable phase diagram for nanocrystalline compositionally homogeneous ZrO(2)-Sc(2)O(3) solid solutions that strongly differs from that previously determined from compositionally homogeneous ZrO(2)-Sc(2)O(3), Solid solutions with much larger crystallite sizes.

NMR Chemical Shielding and Spin-Spin Coupling Constants of Liquid NH(3): A Systematic Investigation using the Sequential QM/MM Method

The NMR spin coupling parameters, (1)J(N,H) and (2)J(H,H), and the chemical shielding, sigma((15)N), of liquid ammonia are studied from a combined and sequential QM/MM methodology. Monte Carlo simulations are performed to generate statistically uncorrelated configurations that are submitted to density functional theory calculations. Two different Lennard-Jones potentials are used in the liquid simulations. Electronic polarization is included in these two potentials via an iterative procedure with and without geometry relaxation, and the influence on the calculated properties are analyzed. B3LYP/aug-cc-pVTZ-J calculations were used to compute the V(N,H) constants in the interval of -67.8 to -63.9 Hz, depending on the theoretical model used. These can be compared with the experimental results of -61.6 Hz. For the (2)J(H,H) coupling the theoretical results vary between -10.6 to -13.01 Hz. The indirect experimental result derived from partially deuterated liquid is -11.1 Hz. Inclusion of explicit hydrogen bonded molecules gives a small but important contribution. The vapor-to-liquid shifts are also considered. This shift is calculated to be negligible for (1)J(N,H) in agreement with experiment. This is rationalized as a cancellation of the geometry relaxation and pure solvent effects. For the chemical shielding, U(15 N) Calculations at the B3LYP/aug-pcS-3 show that the vapor-to-liquid chemical shift requires the explicit use of solvent molecules. Considering only one ammonia molecule in an electrostatic embedding gives a wrong sign for the chemical shift that is corrected only with the use of explicit additional molecules. The best result calculated for the vapor to liquid chemical shift Delta sigma((15)N) is -25.2 ppm, in good agreement with the experimental value of -22.6 ppm.

The effect of confinement on the temperature dependence of the excitonic transition energy in GaAs/Al(x)Ga(1-x)As quantum wells

We determined by means of photoluminescence measurements the dependence on temperature of the transition energy of excitons in GaAs/Al(x)Ga(1-x)As quantum wells with different alloy concentrations (with different barrier heights). Using a fitting procedure, we determined the parameters which describe the behavior of the excitonic transition energy as a function of temperature according to three different theoretical models. We verified that the temperature dependence of the excitonic transition energy does not only depend on the GaAs material but also depends on the barrier material, i.e. on the alloy composition. The effect of confinement on the temperature dependence of the excitonic transition is discussed.