Ammonium-directed olefinic epoxidation: kinetic and mechanistic insights

The ammonium-directed olefinic epoxidations of a range of differentially N-substituted cyclic allylic and homoallylic amines (derived from cyclopentene, cyclohexene, and cycloheptene) have been investigated, and the reaction kinetics have been analyzed. The results of these studies suggest that both the ring size and the identity of the substituents on nitrogen are important in determining both the overall rate and the stereochemical outcome of the epoxidation reaction. In general, secondary amines or tertiary amines with nonsterically demanding substituents on nitrogen are superior to tertiary amines with sterically demanding substituents on nitrogen in their ability to promote the oxidation reaction. Furthermore, in all cases examined, the ability of the (in situ formed) ammonium substituent to direct the stereochemical course of the epoxidation reaction is either comparable or superior to that of the analogous hydroxyl substituent. Much slower rates of ring-opening of the intermediate epoxides are observed in cyclopentene-derived and cycloheptene-derived allylic amines as compared with their cyclohexene-derived allylic and homoallylic amine counterparts, allowing for isolation of these intermediates in both of the former cases.     

Mass transport at electrodes of arbitrary geometry. Reversible charge transfer reactions in square wave voltammetry

The basis for mass transport of the electroactive species in different diffusion fields is examined, pointing out important insights in relation to the value of the surface concentrations for the case of fast electrode processes. Moreover, a general analytical solution for the transient current is given for several geometries, when an arbitrary sequence of potentials pulses is applied to macroelectrodes (planar), spheres, cylinders, discs and bands. The results are particularized for Square Wave Voltammetry. Explicit solutions for the net current and the forward and backward components are given. The effects of frequency, pulse amplitude and electrode size or shape on the peak current are studied. Moreover, the conditions for the attainment of the steady state response are analyzed in the different geometries as well as the characteristics of this response.     

In Situ Formation of a Uniform Distribution of Silver Nanoparticles in PVDF: Kinetics of Formation and Resulting Properties

Hybrid organic/silver nanoparticle thermoplastic films have been prepared using a single step process where the silver nanoparticles were formed during a cure cycle applied to the film. Figure 1 is a TEM demonstrating the ability of our technique to produce silver nanoparticles in the semi-crystalline polymer Poly(vinylidene fluoride) (PVDF). The current laboratory focus is on characterizing kinetics and particularly the diameter growth and inter-particle distance as a function of time and temperature using the techniques of small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Using the Beaucage model, the SAXS data demonstrates that the particle size reaches equilibrium after approximately 70 minutes of curing at 240 °C. This result is also observed in the XRD results where the half width of the diffraction peaks becomes smaller quickly during the initial hour. In a parallel experiment using DSC, the exothermic heat flow increases rapidly during the first hour but does not reach completion until almost 150 minutes.     

Microwave activation of electrochemical processes: enhanced PbO2 electrodeposition, stripping and electrocatalysis

Microwave activation of electrochemical processes has recently been introduced as a new technique for the enhancement and control of processes at electrode|solution (electrolyte) interfaces. This methodology is extended to processes at glassy carbon and boron-doped diamond electrodes. Deposition of both Pb metal and PbO₂ from an aqueous solution of Pb²⁺ (0.1 M HNO₃) are affected by microwave radiation. The formation of PbO₂ on anodically pre-treated boron-doped diamond is demonstrated to change from kinetically sluggish and poorly defined at room temperature to nearly diffusion controlled and well defined in the presence of microwave activation. Calibration of the temperature at the electrode|solution (electrolyte) interface with the Fe^3+/2+ (0.1 M HNO₃) redox system allows the experimentally observed effects to be identified as predominantly thermal in nature and therefore consistent with a localized heating effect at the electrode|solution interface. The microwave-activated deposition of PbO₂ on boron-doped diamond remains facile in the presence of excess oxidizable organic compounds such as ethylene glycol. An increase of the current for the electrocatalytic oxidation of ethylene glycol at PbO₂/boron-doped diamond electrodes in the presence of microwave radiation is observed. Preliminary results suggest that the electrodissolution of solid microparticles of PbO₂ abrasively attached to the surface of a glassy carbon electrode is also enhanced in the presence of microwave radiation. Electronic Publication     

A high-resolution laser Doppler anemometer for three-dimensional turbulent boundary layers

 A new laser Doppler anemometer optimized for high spatial resolution near a wall is described. The instrument uses short focal length optics, a mirror probe in the flow, and side-scatter collection to produce a measuring volume 35 μm in diameter by 66 μm long. Data are presented for a two-dimensional boundary layer, demonstrating the instrument’s ability to measure Reynolds shear stresses as close to the wall as 0.1 mm, or y ⁺≈3. Received: 6 October 1995/Accepted: 9 April 1996     

A kinetic study of the reaction between N,N-dimethyl-p-toluidine and its electrogenerated radical cation in a room temperature ionic liquid.

The reaction between N,N-dimethyl-p-toluidine (DMT) and the radical cation generated through its one-electron oxidation has been studied electrochemically in the room temperature ionic liquid N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [Py14][NTf2]. Kinetic information obtained as linear sweep and cyclic voltammetry collected at 5 microm, 10 microm and 0.3 mm diameter platinum disk electrodes over a range of initial substrate concentrations and scan rates spanning five orders of magnitude was complemented by chronoamperometric measurements designed to probe the rate of diffusion. At the fastest scan rates the homogeneous reactions following the initial electron transfer were effectively out-run, facilitating an assessment of the electrode kinetics using DIGISIM and a validated Nicholson's method. Through digital simulation the voltammetry was then shown to be consistent with a mechanism established for the same reaction in acetonitrile, involving dimerisation of the DMT radicals following an initial and rate-determining proton transfer step. After careful consideration of all parameters, a bimolecular rate constant of (3.4 +/- 1.1) x 10(2) dm3 mol(-1) s(-1) was deduced by fitting the data. This was compared to the equivalent value for acetonitrile and, in light of this, the implications on the viability of ionic liquids for use as alternative mainstream solvents briefly assessed.     

Proton NMR relaxation study of the CsHSO4 solid acid system

At 141 °C the solid acid CsHSO₄ is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin-spin relaxation time T₂ measurements reported here for CsHSO₄ also reveal substantial increases (factors of 20-30) in the vicinity of the transition temperature. In the temperature range just below the transition (70-136 °C), T₂ increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T₂ results. NMR spin-lattice relaxation T₁ measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T₁'s decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T₁ exhibits a minimum in which the correlation time is found to be ∼2 ns.