Simulation microscopic chemical processes. IV. Temperature dependence of deactivation and dissociation of bromide in argon

Previous molecular dynamics simulations of the deactivation or dissociation of a bromine molecule at an energy just above the minimum required for dissociation and surrounded by an argon medium in canonical equilibrium has been extended from 1500 K to 160 and 295 K. The aim is to observe the extent to which temperature determines the qualitative as well as quantitative features of the process. Despite an observed sensitivity to the initial state of the medium which required introduction of a Monte Carlo selection procedure the final results indicate that the deactivation—dissociation mechanism is insensitive to temperature changes in the range presently explored. We conclude that a properly constructed independent binary collision theory would have a wide range of applicability.     

Melting and dissociation of ammonia at high pressure and high temperature

Raman spectroscopy and synchrotron x-ray diffraction measurements of ammonia (NH(3)) in laser-heated diamond anvil cells, at pressures up to 60 GPa and temperatures up to 2500 K, reveal that the melting line exhibits a maximum near 37 GPa and intermolecular proton fluctuations substantially increase in the fluid with pressure. We find that NH(3) is chemically unstable at high pressures, partially dissociating into N(2) and H(2). Ab initio calculations performed in this work show that this process is thermodynamically driven. The chemical reactivity dramatically increases at high temperature (in the fluid phase at T > 1700 K) almost independent of pressure. Quenched from these high temperature conditions, NH(3) exhibits structural differences from known solid phases. We argue that chemical reactivity of NH(3) competes with the theoretically predicted dynamic dissociation and ionization.     

Positron annihilation in argon intercalated n-alkanes at high pressure

Positron annihilation lifetime spectroscopy was used to observe the effects of argon intercalation in some solid long-chain alkanes at high pressure. The ortho-Ps lifetime rises with argon pressure, which means increase of free volumes in the alkane structure. The range of pressures in which the rotator phase exists increases, comparing to pure alkane. In n-heptadecane, n-nonadecane, and possibly n-heneicosane, a stepwise change of ortho-Ps lifetime and intensity at ≈12 MPa is observed, suggesting the transition to a new kind of the rotator phase. The transition rate is low, final lifetime value is ≈3.3 ns. Despite a large size of free volumes corresponding to such a lifetime, their compressibility is found negligible up to the pressure of 90 MPa. At low pressures the compressibility of free volumes in the rotator phase is negative.     

Thermal conductivity of solid argon at high pressure and high temperature: a molecular dynamics study

The thermal conductivity of solid argon at high-pressure (up to 50 GPa) and high-temperature (up to 2000 K) has been calculated by equilibrium molecular dynamics simulations using the Green-Kubo formalism and an exponential-6 interatomic potential. A simple empirical expression is given for its pressure and temperature dependence. The results are compared with predictions based on kinetic theory. The relative change of the thermal conductivity lambda with density rho is found to be consistent with a partial differential ln lambda/ partial differential ln rho slope of approximately 6 in a wide range of pressures and temperatures, in good agreement with predictions based on kinetic theory.     

The viscosity of argon at very high pressure, up to the melting line

The viscosity of argon has been measured as a function of pressure at 223.15 K, 301.15 K and 323.15 K by means of a vibrating wire viscometer. The measurement of the 223.15 K isotherm has been carried out right up to the melting pressure (7790 bar).     

Gamma radiation-initiated polymerization of styrene at high pressure. III. Emulsion polymerization with anionic emulsifiers

Values calculated for the activation volume for chain propagation, ΔV, for the polymerization of styrene in emulsions under a variety of conditions agree closely with that previously obtained in pure styrene (ΔV = ?18.6 cm³ mol^?1). The rate of initiation of emulsion polymerization by radicals produced in the water phase was independent of pressure; therefore ΔV is zero. This differs from initiation in pure styrene which is slightly retarded by pressure (ΔV = 2.0 cm³ mol^?1). The activation energy for the reaction in emulsion, as in pure monomer, decreases slightly with pressure. Chain transfer to monomer occurs to a much greater extent in emulsions than in pure monomer under similar temperature and pressure conditions. Values for the dependence of the polymerization rate on the initiator (i.e., the irradiation dose rate) and emulsifier concentration are consistent with Smith–Ewart, Case II kinetics.     

A microscopic model for calculations of chemical processes in aqueous solutions

A new microscopic model for calculations of chemical processes in aqueous solutions is presented. The model, referred to here as the “surface constrained soft sphere dipoles” (SCSSD) model avoids the problems of the continuum models by explicitly including the solvent molecules. Each solvent molecule is represented as a point dipole attached to the center of a soft sphere. The solvation energy is evaluated by minimizing the solute-solvent energy with respect to the orientations and positions of those dipoles while constraining the surface dipoles to have the orientations and positions of the bulk solvent. The model is demonstrated by calculating the energetic of charge separation in aqueous solution and evaluating the corresponding dielectric constant. The SCSSD model can be used for quantitative studies of ionic reactions in solutions. This is demonstrated by calculation of the potential surface for the dissociation of formic acid in aqueous solution.     

Spectrophysical properties of the plasma of a high frequency low power discharge in argon at atmospherical pressure

Some Spectrophysical characteristics (spatial distribution of radiation, shape of spectral lines, temperature of particles, electron density and continuum intensity) of a high frequency low power discharge in argon, described previously [1] were measured. The obtained results aid in assessing the analytical possibilities of such a discharge.     

The chemical processes accompanying the hydrogenation of fats III. Glyceride isomerization

Summary It has been established that the catalytic hydrogenation of a fat is accompanied by its glyceride isomerization (transesterification), which is particularly pronounced in the initial stage of the process. Transesterification takes place as an intramolecular reaction.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 603–608, September–October, 1972.     

Radiation-induced polymerization at high dose rate. III. Isoprene in bulk

Radiation-induced polymerization of isoprene in bulk state was studied at 25°C in a wide dose rate range. Variations of the rate of polymerization and molecular weight of the products were essentially the same as those observed in the monomers which were capable of both radical and cationic polymerizations. At low dose rate, 7.0-230 rad/sec, radical polymerization took place. At high dose rate, 8.8 × 10³-2.2 × 10⁵ rad/sec, radical and cationic polymerizations took place concurrently. The average molecular weight of the high-dose-rate product was about 850, independent of dose rate. The microstructure of the products at high dose rate consisted mainly of trans- 1,4 units with only about 7% of cis- 1,4 and 10% of 3,4-vinyl units. The residual unsaturation in the high-dose-rate products was 90%. Decreases in cis units and residual unsaturation at high dose rate were accounted for by the change in predominant mechanism of polymerization with dose rate.     

Modeling of thermal processes in high pressure liquid chromatography: I. Low pressure onset of thermal heterogeneity

Heat due to viscous friction is generated in chromatographic columns. When these columns are operated at high flow rates, under a high inlet pressure, this heat causes the formation of significant axial and radial temperature gradients. Consequently, these columns become heterogeneous and several physico-chemical parameters, including the retention factors and the parameters of the mass transfer kinetics of analytes are no longer constant along and across the columns. A robust modeling of the distributions of the physico-chemical parameters allows the analysis of the impact of the heat generated on column performance. We developed a new model of the coupled heat and mass transfers in chromatographic columns, calculated the axial and radial temperature distributions in a column, and derived the distributions of the viscosity and the density of the mobile phase, hence of the axial and radial mobile phase velocities. The coupling of the mass and the heat balances in chromatographic columns was used to model the migration of a compound band under linear conditions. This process yielded the elution band profiles of analytes, hence the column efficiency under two different sets of experimental conditions: (1) the column is operated under natural convection conditions; (2) the column is dipped in a stream of thermostated fluid. The calculated results show that the column efficiency is remarkably lower in the second than in the first case. The inconvenience of maintaining constant the temperature of the column wall (case 2) is that retention factors and mobile phase velocities vary much more significantly across the column than if the column is kept under natural convection conditions (case 1).     

The thermal dissociation of diatomic molecules at high temperatures

On the basis of a solution of gas-kinetic equations describing the population of molecules (cut-off harmonic oscillators) at various oscillatory levels, the process of thermal dissociation has been analyzed. It has been shown that thermal dissociation, in addition to disturbing the Boltzmann distribution, leads to a reduction in the vibrational temperature compared with the translational one. This affects to a considerable extent the rate of thermal dissociation, and also the process of vibrational relaxation. The question of the applicability of the well-known relation of statistical thermodynamics connecting the rate constants of forward and back reactions has also been analyzed. The results obtained agree qualitatively with the experimental data.     

Photophysical processes in quinoxaline vapor at low pressure

Excitation and pressure dependence of fluorescence and phosphorescence quantum yields has been reinvestigated in detail for quinoxaline in the static vapor phase at pressure range from 10(-3) to 10(-1) Torr. It is shown that the ratio of the nonradiative rate from T(1)(pi, pi*) to the rate of the S(1)(n, pi*) approximately -->T(1)(pi, pi*) intersystem crossing decreases with increasing the excitation energy in the S(0)-->S(1) excitation region. The phosphorescence quantum yield measured as a function of the excitation energy at low pressure shows an abrupt decrease on going the excitation from S(0)-->S(1) to S(0)-->S(2), indicating the slow vibrational energy redistribution between the S(1) levels optically populated and those populated through the internal conversion from S(2) to S(1).     

Modulated structure and molecular dissociation of solid chlorine at high pressures

Among diatomic molecular halogen solids, high pressure structures of solid chlorine (Cl(2)) remain elusive and least studied. We here report first-principles structural search on solid Cl(2) at high pressures through our developed particle-swarm optimization algorithm. We successfully reproduced the known molecular Cmca phase (phase I) at low pressure and found that it remains stable up to a high pressure 142 GPa. At 150 GPa, our structural searches identified several energetically competitive, structurally similar, and modulated structures. Analysis of the structural results and their similarity with those in solid Br(2) and I(2), it was suggested that solid Cl(2) adopts an incommensurate modulated structure with a modulation wave close to 2∕7 in a narrow pressure range 142-157 GPa. Eventually, our simulations at >157 GPa were able to predict the molecular dissociation of solid Cl(2) into monatomic phases having body centered orthorhombic (bco) and face-centered cubic (fcc) structures, respectively. One unique monatomic structural feature of solid Cl(2) is the absence of intermediate body centered tetragonal (bct) structure during the bco → fcc transition, which however has been observed or theoretically predicted in solid Br(2) and I(2). Electron-phonon coupling calculations revealed that solid Cl(2) becomes superconductors within bco and fcc phases possessing a highest superconducting temperature of 13.03 K at 380 GPa. We further probed the molecular Cmca → incommensurate phase transition mechanism and found that the softening of the A(g) vibrational (rotational) Raman mode in the Cmca phase might be the driving force to initiate the transition.     

Metastable decay of argon clusters after photoionisation at high excess energies

Recently Märk and collaborators [1, 2] reported the metastable emission of large fractions from argon and neon cluster ions after electron impact ionisation at high excess energies. The decay was interpreted as the result of an intra-cluster excitation of a metastable state by one of the electrons involved in the ionisation process. Here we report the first direct observation of such a correlated two electron process during photoionisation of argon clusters using synchroton radiation and the TPEPICO technique. We observe at least two distinct maxima of the metastable TPEPICO spectrum at around 27 eV and 28.5 eV, the former being consistent with the previously reported energetic threshold for electron impact ionisation [1, 2].     

Critical analysis of viscosity data of thermal argon plasmas at atmospheric pressure

Reliable values of the viscosity in thermal argon plasmas are most important for our understanding of the momentum transfer and for realistic modeling of various plasma applications. Despite numerous attempts to determine reliable viscosity values over the last three decades, discrepancies still exist among the data reported by different authors. In this paper, a critical analysis is undertaken of calculated and experimental data of the argon viscosity based on recent publications. Our recalculation of viscosities in thermal argon plasmas are performed by using Lennard-Jones, Morse, Aziz, and exponential repulsive potentials for Ar-Ar atom interactions in different temperature ranges from 300 to 20,000 K. The contributions of elastic collisions of e-Ar, e-Ar⁺, and Ar⁺-Ar, as well as charge exchange of Ar⁺-Ar, to the viscosity become important with increasing temperature and degree of ionization in argon plasmas. Based on a critical analysis and recalculations, improved values of the argon viscosity are recommended, covering temperatures from 300 to 20,000 K. Polynomial expressions have been developed for calculating argon viscosities, which will be useful for numerical work and other applications of thermal argon plasmas at atmospheric pressure.     

Comparison of hexamethyldisiloxane dissociation processes in plasma

Different hexamethyldisiloxane (HMDSO) dissociation processes are investigated by means of absorption spectroscopy and mass spectrometry. All of these processes are expected to occur in plasma containing Ar-HMDSO gas mixture. We successively study interactions of the HMDSO molecule with electrons (energy ranges from 15 to 70 eV), with Ar((3)P(2)) metastable species (internal energy 11.55 eV) and with VUV photon (7.3 to 10.79 eV). The studies of HMDSO interactions with Ar((3)P(2)) and VUV photon provide new results concerning the dissociation pathways and the collision cross-sections. In the case of Ar((3)P(2)), the dissociation mechanisms result mainly in Si-C or Si-O bond breaking, producing SiMe(2,1) radicals. Less efficient mechanisms involve also Si-C and Si-O bond breaking producing Me, Si(2)Me(5)O, or SiMe(3), on one hand, and, on the other hand, Si-C and C-H bond breaking producing Si(2)Me(4)OH. In the case of photon interaction, the dissociation process is more selective and mainly produces Si(2)OMe(5) pentadisiloxane and methyl radicals due to Si-C bond breaking. Si-O bond breaking produces also SiMe(3) in a lower concentration. Dissociation cross-section values of HMDSO ranging from σ = 45 × 10(-20) m(2) to 180 × 10(-20) m(2) and from σ = 0.7 × 10(-22) m(2) to 18.3 × 10(-22) m(2), correspond to a global dissociation mechanism by Ar((3)P(2)) collision and to a selective dissociation mechanism (producing Si(2)OMe(5) and Me) by VUV photon interaction, respectively. All results are compared and discussed.     

Modeling and experimental study of a transferred arc stabilized with argon and flowing in a controlled-atmosphere chamber filled with argon at atmospheric pressure

For a transferred arc with a flat anode working at atmospheric pressure in an argon atmosphere, the influence of the gas injector design close to the cathode tip has been systematically studied for arc currents below 300 A, gas flowrates between 5 and 60 slm, and anode-cathode distances between 10 and 46 mm. Two types of injector configurations hare been studied: a cylindrical one with its wall parallel to the cathode axis and a conical one with the same cone angle as that of the cathode tip. The arc temperature was measured using flit, absolute intensity of ArI and ArII lines. Beside the roltagc and arc current, the losses at the cathode and at the anode were continuously recorded. An elliptic model was used to calculate the flow velocity, the temperature, and the current density close to the cathode and in the arc column. This model was either laminar or turbulent (K - ), with the empirical constants being functions of the Reynolds nunther of turbulence. A cathode sheath with nonequilibrium conditions was used to obtain accurate cathode boundary conditions. Experiments and modeling hart shown the benefits of using conical injectors which constrict drasfically the plasma_ flow and enhance the gas velocity and the current density, thus increasing the heat flux to the anode. With the cylindrical injector, recirculations close to the cathode lip modify deeply its heating and reduce the plasma jet constriction: velocities and temperatures are lower when the recirculation velocity is higher. This results in lower heat fluxes to the anode compared to the conical injector.