Kinetics and mechanism of the dehydration reaction of sarcosine to a bislactame through diacyclperoxide intermediate in strong acidic medium

The influence of substitution on the amine functional group of glycine in the permanganic oxidation of such an α‐amino acid in moderately concentrated sulfuric acid medium has been investigated. Reaction products analysis has revealed that contrary to the usual α‐amino acid oxidation product, which is an aldehyde species, a valuable compound, namely 1,4‐dimethylpiperazine‐2,5‐dione, has been obtained as the main product via a cheap, simple, efficient, and novel method. Sarcosine has been chosen as a substituted derivative of glycine, and the kinetics and mechanism of its permanganic oxidation have been investigated using a spectrophotometric technique. Conclusive evidence has proven delayed autocatalytic activity for Mn(II) in this reaction, analogous to some α‐amino acids. It has been revealed that such activity can show up when a certain concentration ratio of Mn(II) to sarcosine is built up in the medium, which we call the “critical ratio.” The magnitude of the latter ratio depends on the sulfuric acid concentration. Considering the “delayed autocatalytic behavior” of Mn(II) ions, rate equations satisfying observations for both catalytic and noncatalytic routes have been presented. The reaction shows first‐order dependence on permanganate ions and sarcosine concentrations in both catalytic and noncatalytic pathways, and apparent first‐order dependence on Mn²⁺ ions in catalytic pathways. The correspondence of pseudo‐order rate constants of the catalytic and noncatalytic pathways to Arrhenius and Eyring laws has verified “critical ratio” as well as “delayed autocatalytic behavior” concepts. The activation parameters associated with both pathways have been computed and discussed. Mechanisms for both catalytic and noncatalytic routes involving radical intermediates as well as a product having a diketopiperazine skeleton have been reported for the first time. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 689–703, 2009     

Investigation of the effect of nozzle shape on supersonicJhypersonic impactors designed for size discrimination of nanoparticles

In this study the flow field and the nanoparticle collection efficiency of supersonic/hypersonic impactors with different nozzle shapes were studied using a computational modeling approach. The aim of this study was to develop a nozzle design for supersonic]hypersonic impactors with the smallest possible cut-off size d5o and rather sharp collection efficiency curves. The simulation results show that the changes in the angle and width of a converging nozzle do not alter the cut-off size of the impactor; however, using a conical Laval nozzle with an L]Dn ratio less than or equal to 2 reduced d5o. The effect of using a cap as a focuser in the nozzle of a supersonic/hypersonic impactor was also investigated. The results show that adding a cap in front of the nozzle had a noticeable effect on decreasing the cut-off size of the impactor. Both fiat disks and conical caps were examined, and it was observed that the nozzle with the conical cap had a lower cut-off size.     

Multicomponent fluid flow analysis using a new set of conservation equations

In this work hydrodynamics of multicomponent ideal gas mixtures have been studied. Starting from the kinetic equations, the Eulerian approach is used to derive a new set of conservation equations for the multicomponent system where each component may have different velocity and kinetic temperature. The equations are based on the Grad's method of moment derived from the kinetic model in a relaxation time approximation (RTA). Based on this model which contains separate equation sets for each component of the system, a computer code has been developed for numerical computation of compressible flows of binary gas mixture in generalized curvilinear boundary conforming coordinates. Since these equations are similar to the Navier-Stokes equations for the single fluid systems, the same numerical methods are applied to these new equations. The Roe's numerical scheme is used to discretize the convective terms of governing fluid flow equations. The prepared algorithm and the computer code are capable of computing and presenting flow fields of each component of the system separately as well as the average flow field of the multicomponent gas system as a whole. Comparison of the present code results with those of a more common algorithm based on the mixture theory in a supersonic converging-diverging nozzle provides the validation of the present formulation. Afterwards, a more involved nozzle cooling problem with a binary ideal gas (helium-xenon) is chosen to compare the present results with those of the ordinary mixture theory. The present model provides the details of the flow fields of each component separately which is not available otherwise. It is also shown that the separate fluids treatment, such as the present study, is crucial when considering time scales on the order of (or shorter than) the intercollisions relaxation times.     

An experimental study on MWCNT–water nanofluids flow and heat transfer in double-pipe heat exchanger using porous media

Journal of Thermal Analysis and Calorimetry - This paper presents the results of an experimental investigation on the heat transfer characteristics of multi-walled carbon nanotube aqueous...     

Quasi-classical trajectory and direct-dynamics CVT study on the initiation steps of methanol combustion

Direct-dynamics canonical variational transition-state theory (CVT) and quasi-classical trajectory (QCT) calculations have been performed to study the dynamics of the initiation steps in the methanol combustion at high oxygen concentration. The initiation steps in combustion of methanol is hydrogen abstraction from carbon or oxygen in methanol to produce hydroxymethyl radical (CH₂OH) or methoxy radical (CH₃O), respectively, and hydroperoxyl radical (HO₂). A new analytical potential energy function driven from our DFT calculations is constructed to study the dynamics of the title reactions. Reactive cross sections and reaction probabilities at various relative translational energies and initial vibrational and rotational reactant excitation were obtained to calculate the rate constants. The calculated rate constants from CVT and QCT calculations are compared.     

Ab initio and RRKM study of the HCN/HNC elimination channels from vinyl cyanide

Ab initio CCSD and CCSD(T) calculations with the 6-311+G(2d,2p) and the 6-311++G(3df,3pd) basis sets were carried out to characterize the vinyl cyanide (C(3)H(3)N) dissociation channels leading to hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC). Our computations predict three elimination channels giving rise to HCN and another four channels leading to HNC formation. The relative HCN/HNC branching ratios as a function of internal energy of vinyl cyanide were computed using RRKM theory and the kinetic Monte Carlo method. At low internal energies (120 kcal/mol), the total HCN/HNC ratio is about 14, but at 148 kcal/mol (193 nm) this ratio becomes 1.9, in contrast with the value 124 obtained in a previous ab initio/RRKM study at 193 nm (Derecskei-Kovacs, A.; North, S. W. J. Chem. Phys.1999, 110, 2862). Moreover, our theoretical results predict a ratio of rovibrationally excited acetylene over total acetylene of 3.3, in perfect agreement with very recent experimental measurements (Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. J. Chem. Phys.2009, 130, 044307).