Interactions In Gas Phase Research Articles

A numerical investigation into factors affecting gas and aqueous phase plumes in the subsurface

An investigation into the face and transport of volatile organic compounds (VOCs) in the subsurface requires the consideration of contaminant mass in both the aqueous and soil gas phases. As a result of water/gas phase partitioning, contaminated by partitioning from underlying ground water pollution. Conversely, soil gas can be contaminated by partitioning from underlying ground water VOC plumes. This soil gas and aqueous phase interaction has motivated the popularity of soil gas sampling technology as a method of characterizing ground water VOC contamination. A finite-element-based numerical model was developed to accurately simulate the interaction between the soil gas phase and the aqueous phase. This interaction is complicated since the saturation of the aqueous phase varies dramatically across the capillary fringe. The two-phase flow equations for gas and water are used to describe the flow regime, while the advective-dispersive transport of the VOC is considered in both phases. Dissolution and volatilization from a non-mobile non-aqueous phase liquid is included as a volatile organic contaminant source. A deforming mesh allows the model to accurately track the water table movement, and a Eularian-Lagrangian formulation is used to control some of the numerical difficulties associated with the numerical solution of the advection-dispersion equation. An investigation into diffusion of a VOC from below the water table demonstrated that both the frequency and the magnitude of water table fluctuations have a profound influence on the degree of soil gas contamination. Two-dimensional large-scale, long-term simulations were performed to estimate the aqueous and soil gas phase plumes resulting from an immobilized trichloroethylene residual located in the unsaturated zone. The simulation results indicate that these plumes are very sensitive to the vertical position of the contaminant source. In addition, it was determined that seasonal fluctuations in soil gas VOC concentrations are primarily controlled by temperature fluctuations, while ground water VOC concentration fluctuations are primarily a result of infiltration fluctuations.

Gas-phase binding of Li +, Na + and Mg 2+ to formaldehyde, acetaldehyde and their silicon and sulfur analogs. A theoretical study by means of ab initio molecular orbital methods at the G2 level of theory

Ab initio molecular orbital methods at the G2 level of theory have been used to study the structure and gas-phase bonding of Li +, Na + and Mg 2+ to formaldehyde, acetaldehyde, silanone, methylsilanone, thioformaldehyde and thioacetaldehyde. Calculated interaction energies vary as Mg 2+ > Li + > Na +. Absolute values of gas-phase interaction energies computed by the G2, G2(MP2) and G1 methods were comparable in accuracy. The binding energies evaluated by these three methods are in good agreement with experimental data.

A computational analysis of interaction energies in methane and neopentane dimer systems

The gas-phase interaction energies of methane and neopentane dimers are calculated at various intermolecular distances and geometries using several molecular mechanics and semiempirical parameter sets. For comparisons, a set of reference calculations are also performed using the 6–311G (2d, 2 p) basis set with the inclusion of second-order Möller-Plesset energies (MP2) and basis set superposition corrections. These calculations are further used to examine the mechanism by which the AM1 and PM3 methods account for dispersion interactions in molecular systems. While no specific parameter(s) are included in semiempirical energy functions to capture such effects, the results indicate that both methods produce favorable interaction energies at near contact distances for the dimer systems. AM1 energies, however, show much closer agreement with the reference calculations, indicating potential deficiencies in the PM3 parameter set. Although the source of the dispersion energy could be traced to the attractive Gaussians of the core repulsion function in the AM1 Hamiltonian, a similar link could not be established for PM3. In contrast, PM3 dispersion energies apparently stem from a collection of contributions implicitly included during parameter optimization, providing no clear mechanism for correction or adjustment. Based on the analysis presented, an approach is also suggested for improving the AM1 parameter set. © 1997 by John Wiley & Sons, Inc.

Mass transfer-reaction coupling in two-phase multicomponent fluid systems

Abstract The paper considers two-phase multicomponent mass transfer complicated by chemical reactions. The basis of theoretical description is partial differential equations of convective mass transport which are coupled by diffusional interactions in gas (vapor) phase and by boh diffusional and chemical interactions in liquid phase. In addition, the boundary conditions of conjugation are taken into account at the interfaces which are of matrix coupled from as well. A calculation method combining analytical and numerical techniques is proposed for solving the problem under consideration. The method makes it possible to determine local concentration distributions of the components and, on this basis, to obtain all the relevant information about the process. Calculated examples demonstrating the regularities and peculiarities of the two-phase mass transfer-reaction process are given for different flow conditions.

Gas-phase interaction of Me3Si+ ions withcis- andtrans-cyclohexanediols, their methyl ethers, and acetates

The effect of stereochemistry on the mechanism of gas-phase fragmentation of [M+SiMe3]+ ions was studied usingcis- andtrans-1,2- and -1,4-cyclohexanediols, their methyl ethers, and acetates as model compounds. The higher stability of the [M+SiMe3]+ions is characteristic of cis-isomers of all the compounds examined, which is associated with chelation in the case ofcis-cyclohexanediols andcis-methoxycyclohexanols and with the higher reactivity oftrans-isomers due to anchimeric assistance of the methoxy and acetoxy groups. Dehydration is characteristic of the [M+SiMe3]+ ions formed from cyclohexanediols; both hydrogen atoms of the hydroxyl groups take part in the process, thus providing direct evidence of the chelation.

Structures and Energies of Hydrogen-Bonded DNA Base Pairs. A Nonempirical Study with Inclusion of Electron Correlation

Hydrogen bonding of DNA bases was investigated by reliable nonempirical ab initio calculations. Gradient optimization was carried out on 30 DNA base pairs using the Hartree−Fock (HF) approximation and the 6-31G** basis set of atomic orbitals. The optimizations were performed within Cs symmetry. However, the harmonic vibrational analysis indicates that 13 of the studied base pairs are intrinsically nonplanar. Interaction energies of base pairs were then evaluated at the planar optimized geometries with inclusion of the electron correlation energy using the second-order Møller−Plesset (MP2) method. The stabilization energies of the studied base pairs range from −24 to −9 kcal/mol, and the calculated gas phase interaction enthalpies agree well (within 2 kcal/mol) with the available experimental values. The binding energies and molecular structures of the base pairs are not determined solely by the hydrogen bonds, but they are also strongly influenced by the polarity of the monomers and by a wide variety of secondary long-range electrostatic interactions that also involve the hydrogen atoms bonded to ring carbon atoms. The stabilization of the base pairs is dominated by the Hartree−Fock interaction energy. This result confirms that the stability of the base pairs originates in the electrostatic interactions. For weakly bonded base pairs, the correlation interaction energy amounts to as much as 30−40% of the stabilization. For some other base pairs, however, repulsive correlation interaction energy was found. The latter fact is explained as a result of a reduction of the electrostatic attraction upon inclusion of the electron correlation. The empirical London dispersion energy does not reproduce the correlation interaction energy. For the sake of comparison, results of a first gradient optimization for a DNA base pair at a correlated level (CC base pair, MP2/6-31G** level) are reported. In addition, the ability of the economical density functional theory (DFT) method to reproduce the ab initio data was investigated. The DFT method with present functionals is not suitable to consistently study the whole range of the DNA base interactions. However, it gives good estimates of interaction energies at the reference HF/6-31G** geometries.

Mass resolved low-energy ion backscattering spectrometry at target-to-projectile mass ratios near unity

The basic features of mass resolved ion (back-)scattering spectrometry (MARISS) at target-to-projectile mass ratios μ near unity have been explored using a quadrupole based secondary ion mass spectrometer with a simple (biased) energy analyzer. Sputter cleaned targets of Mg, Al, Si, Ti, Nb and Mo were bombarded at near-normal incidence with isotopically pure beams of Ne +, Ar + and Kr +, at impact energies E i between 1.5 and 10 keV. Positively charged scattered ions as well as secondary ions, with final energies E f and charge states n = 1 and 2, were energy analysed by ramping the target bias. The energy spectra showed narrow binary scattering surface peaks (full width at half maximum 15–28 eV) with shoulders or tails on the low and the high-energy side (due to subsurface scattering and double or multiple scattering, respectively). The peak-to-“background” ratio was significantly different for the projectile-target combinations studied (as high as 40–80 for Ne Al,Si and Kr Nb ; only 2–4 for Ne Mg and Ar Ti ). The fractional sca E f E i ranged from as low as 1.9 × 10 −3 to 3.7 × 10 −2 (1.08 ≤ μ ≤ 1.4). Mass analysis was indispensable for removing interference with secondary i All binary scattering events appear to be associated with inelastic energy losses Q. Assuming Q = 45 eV for scattering of Ne + from Mg, Al and Si, the true scattering angle θ has been determined as a function of the scattering energy (124 ≤ θ ≤ 139°). For scattering of 3–9 keV Ar + from Ti and 10 keV Kr + from Nb, Q was found to be ∼ 90 and ∼ 250 eV, respectively. Using four different Kr isotopes the characteristic velocity for neutralization of Kr + escaping from Nb (1.5 × 10 6 cm/s) could be determined without knowing the scattering cross section. Relatively high yields of Ne 2+ were recorded in scattering from Mg, Al and Si (up to 10% of the Ne + yield), with no shoulders on the low-energy side of the single scattering peaks ( Q ≈ 120 eV). Ionization of reemitted inert gases by two-particle gas-phase interaction has also been observed.

Mass transfer-reaction coupling in two-phase multicomponent fluid systems

The paper considers two-phase multicomponent mass transfer complicated by chemical reactions. The basis of theoretical description is partial differential equations of convective mass transport which are coupled by diffusional interactions in gas (vapor) phase and by boh diffusional and chemical interactions in liquid phase. In addition, the boundary conditions of conjugation are taken into account at the interfaces which are of matrix coupled from as well. A calculation method combining analytical and numerical techniques is proposed for solving the problem under consideration. The method makes it possible to determine local concentration distributions of the components and, on this basis, to obtain all the relevant information about the process. Calculated examples demonstrating the regularities and peculiarities of the two-phase mass transfer-reaction process are given for different flow conditions.

Theoretical investigation of the molecular structure of the pi kappa DNA base pair.

The structure of the nonclassical pi kappa base pair (7-methyl-oxoformycin B. . .2,4-diaminopyrimidine) was studied at the ab initio Hartree-Fock (HF) and MP2 levels using the 6-31G* and 6-31G** basis sets. The pi kappa base pair is bound by three parallel hydrogen bonds with the donor-acceptor-donor recognition pattern. Recently, these bases were proposed as an extension of the genetic alphabet from four to six letters (Piccirilli et al, Nature 343,33 (1990)). By the HF/6-31G* method with full geometry optimization we calculated the 12 degree propeller twist for the minimum energy structure of this complex. The linearity of hydrogen bonds is preserved in the twisted structure by virtue of the pyramidal arrangement of the kappa-base amino groups. The rings of both the pi and kappa molecules remain nearly planar. This nonplanar structure of the pi kappa base pair is only 0.1 kcal/mol more stable than the planar (Cs) conformation. The HF/6-31G* level gas-phase interaction energy of pi kappa (-13.5 kcal/mol) calculated by us turned out to be nearly the same as the interaction energy obtained previously for the adenine-thymine base pair (-13.4 kcal/mol) at the same computational level. The inclusion of p-polarization functions on hydrogens, electron correlation effects (MP2/6-31G** level), and the correction for the basis set superposition error (BSSE) increase this energy to -14.0 kcal/mol.

Adsorption of CO 2 and SO 2 on activated carbons with a wide range of micropore size distribution

The adsorption isotherms of N 2 at 77K, CO 2 at 251, 273 and 298K, and SO 2 at 262 and 273K have been determined on a series of physically activated carbons with a wide range of micropore size distributions. Since the series includes carbons with very high burn-off, it shows the problems involved in the characterization of microporsity in superactivated carbons. On the other hand, the results show that the carbon surface-adsorbate interactions for SO 2 at low relative pressures are weaker than for N 2 and CO 2, as a result of the strong adsorptive-adsorptive interactions in the bulk gas phase.

Amino/Aromatic Interactions in Proteins: Is the Evidence Stacked Against Hydrogen Bonding?

We investigate the suggestion that aromatic rings can act as hydrogen-bond acceptors in proteins, by an analysis of 55 non-homologous high-resolution protein chain structures. Approximately 10% of interactions between sp2 hybridized nitrogen atoms, from either side-chains or main-chains, and phenylalanine or tyrosine rings have the nitrogen atom positioned above the ring. In these instances, however, the sp2 nitrogen atoms tend to form stacked interactions with the aromatic rings, these geometries outnumbering amino/aromatic hydrogen bonds by around 2·5:1. The statistically expected distribution, in contrast, would have only a few stacked structures and many more with larger interplanar angles, corresponding to amino/aromatic hydrogen bonds. Thus, although we do find some of these unconventional hydrogen bonds, they are clearly disfavoured relative to stacked geometries.In stacked geometries, the nitrogen-bearing groups are observed to fulfil their hydrogen-bonding potential by forming conventional, energetically stronger, hydrogen bonds with other groups in protein or solvent. This may explain the favourability of stacking. Thus, although ab initio calculations of the gas phase interaction energies for three model systems generally favour the amino/aromatic hydrogen-bonded over the stacked geometries, the differences are small enough to be outweighed easily by the additional conventional hydrogen bonding in stacked structures.

Hydrogen-bonding interactions in gas-phase polyether/ammonium ion complexes

Hydrogen bonds are among the most important interactions involved in selective complexation in host—guest chemistry. In this study a variety of hydrogen-bonded crown ether/ammonium ion complexes are generated in the gas phase by association reactions between an amine substrate and a polyether, one of which is initially protonated, and stabilized by many collisions in the chemical ionization source of a triple quadrupole mass spectrometer or in a quadrupole ion trap. The nature of the hydrogen-bonding interactions of the ion complexes are evaluated by comparison of their collision-activated dissociation spectra. After collisional activation, those complexes that are weakly bound dissociate to form intact protonated polyether molecules and/or ammonium ions by simple cleavages of the hydrogen-bond association interactions. In contrast, those complexes strongly bound by multiple hydrogen bonds dissociate not only to the protonated polyether and/or ammonium ions but also by extensive covalent bond cleavage of the protonated ether skeleton. This latter type of dissociation behavior suggests that the polyether/ammonium ion complexes may be sufficiently strongly bound that surpassing the high barrier to decomposition results in formation of internally excited polyether molecules that may then undergo subsequent fragmentation by skeletal cleavages. Moreover, complexes involving multiple hydrogen bonds may have slower dissociation kinetics, allowing competition from fast dissociation processes that have substantial energy barriers.

Structure and magnetic properties of Er/sub 2/Fe/sub 17/C/sub x/ compounds

The interstitial carbides of Er/sub 2/Fe/sub 17/C/sub x/ with x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 have been synthesized by arc melting and by the Gas-Phase Interaction (GPI) method. The unit-cell volume of the compounds is found to increase with increasing carbon concentration. The dependence of Curie temperatures, spin-reorientation temperatures and the magnetic coupling on the carbon content has been investigated. The spin phase diagram of the series is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of the gas phase chemistry in the silicon doping of GaAs grown by metalorganic vapor phase epitaxy using tertiarybutylarsine as the group V source

Gas phase decomposition studies have been combined with metalorganic vapor phase epitaxy (MOVPE) growth and doping experiments to investigate the mechanism behind the Si doping of GaAs grown using tertiarybutylarsine (t-C 4H 9AsH 2 or TBAs) as the Group V source. The use of TBAs leads to an increase in the Si doping efficiency from both SiH 4 and Si 2H 6 sources and this enhancement was proposed to originate from the homogeneous generation of Si-bearing intermediates, (t-C 4H 9)SiH 3 (TBSi) and SiH 3AsH 2, from reactions between SiH 4 and the pyrolysis products of TBAs. We have investigated the formation and role of these intermediates in this Si doping chemistry. Results of our decomposition study show that TBSi pyrolyzes in the gas phase at relatively low temperatures to form SiH 4. Consequently, the use of TBSias a Si dopant source results in a Si doping efficiency and temperature dependence that is identical to SiH 4. TBSi is therefore not a likely route to increased Si incorporation. Our studies of the co-decomposition of TBAs and SiH 4 or Si 2H 6 show that SiH 3AsH 2 is a co-pyrolysis product of TBAs and Si 2H 6, but not of TBAs and SiH 4. Furthermore, there is no indication of gas phase interaction between TBAs and SiH 4. The results of this study suggest that homogeneous reactions are not as important in this doping chemistry as had previously been considered and that surface processes may instead be responsible for the increase in Si doping observed with TBAs.

Influence of oxygen partial pressure on the adhesion of metallic films evaporated on PET

The influence of a controlled oxygen pressure on the metallization (by Al, Ag and Cu) of poly(ethylene terephthalate) (PET) has been investigated. The structure of the interfaces and of the bulk metallic films has been characterized using TEM. The laminate peel energy strength has been measured with a standard peel test. The most significant results concern the A1/PET laminates: (i) wet and dry adhesion are greatly improved if an oxygen partial pressure higher than 10 −5 mbar is introduced during evaporation of the film, and (ii) depending on the oxygen partial pressure, either metallic Al films with various grain size or amorphous aluminium oxide films are prepared. No significant improvement in adhesion is observed with an oxygen partial pressure (10 −6 to 10 −4 mbar) during the evaporation of Ag or Cu. These results are discussed in terms of the mechanism of chemical interaction at the metal-polymer interface. As chemical reactions with oxides or hydroxides are likely to occur with aluminium, the gas phase interaction would contribute to the oxidation process. Ag and Cu being much less reactive, another mechanism involving metal diffusion in the free volume of the polymer dominates and the presence of oxygen has almost no effect.

The study of the process of xenon difluoride synthesis in plasma containing fluorine

The gas phase interaction of xenon with fluorine in high-voltage smouldering discharge plasma in fluorine-xenon mixture and in gas stream conditions was investigated. The reaction products were captured in the special condensers at T⋍196 K and at variable distances from discharge zone. In every experiment XeF 2 containing a small amount of XeF 4 (from 10 −2 to 1.00 mass.%) was the mean product of the interaction of fluorine with xenon. A content of XeF 4 in XeF 2 was examined from iodometry method and mass-spectrometer analysis. At the given constant P xe, P F2 and an useful electric discharge power the observed XeF 2 formation rate was found to be determined by the electric discharge set output on F-atoms also by the time of gas mixture transport to the condensation zone. The value of fluorine atoms flow intensity at the exit end of the discharge tube, N ° F , was calculated from the concentration of O 2F-radicals which had been at a great extent formed on the reaction: O 2 + F + M ← O 2F· + M The dependencies of the observed rate of XeF 2 formation on N ° F have a direct character up to N° F⋍ 5·10 −6 mol/s. A character of these dependencies at N° F > 5·10 −6 mol/s is determined by the gas mixture transport time. The obtained values of fluorine atoms utilization coefficient calculated as ratio of molar flow of fluorine atoms to that of formed XeF 2 molecules show that fluorine molecules take part in the process of XeF 2 formation or it has a chain mechanism.

An investigation of surface phenomena associated with oscillating oxidation of CO on Pt

The oscillatory oxidation of CO on Pt under atmospheric pressure is modeled on uniform and nonuniform surfaces. The multiplicity of the steady states and the temporal and spatial dynamic behavior are analyzed. Synchronization by temperature, gas phase and CO surface migration are investigated. The latter is proposed as the basic mechanism, but gas phase interaction is required to synchronize surfaces with dimensions of the order of 1 mm or ensembles of crystallites with nonuniform properties. For a nonuniform surface temperature oscillations of the order of several degrees are not sufficient for synchronization. Wave propagation was also studied and velocities of the order of 0.1 m/s seem realistic.

Viscositites and pair potential energy function for the argon–methane gas-phase interaction

The viscosities of gaseous argon–methane mixtures have been measured in the temperature range 118–393 K using a capillary flow technique. The results are accurate to ±0.8%, and are used to calculate interaction viscosities and binary diffusion coefficients considered accurate to ±1.9%. The calculated properties are compared with experimental and calculated values of other workers. A spherically averaged intermolecular pair potential energy function is generated by inversion of the interaction viscosities, and compared with other isotropic and anisotropic potential functions for the Ar–CH4 interaction. Second virial coefficients calculated from the potential function are shown to be in good agreement with experiment.

Application of Laser Interferometry to the Study of Droplet' Gas Phase Interaction and Behavior in Liquid Spray Combustion Systems

Abstract The application or optical scattering techniques is described for the measurement of droplet properties in liquid sprays. Of the various approaches, phase Doppler interferometry demonstrates the highest promise for obtaining the information necessary to understand the formation and emission of toxic combustion byproducts in liquid-fueled spray reactions. The technique deduces drop size from the spatial variation in the imaged fringe patlern due to drop radius of curvature. Combined with the measurement of velocity via laser anemometry, the capability of sizing allows for the determination of continuous phase velocities as well as droplet size and velocity correlations. Such a capability is necessary to develop an understanding of the physical processes occurring within liquid combustion systems associated with incineration. Three applications are presented which are relevant to the formation of combustion byproducts: the effect of swirl on the dispersion of droplets, an assessment of spray symmet...

Helium atom differential cross sections for scattering from single adsorbed CO molecules on a Pt(111) surface

Differential cross sections have been measured for the scattering of helium atoms from isolated CO molecules on a Pt(111) surface. The cross sections reveal an oscillatory structure as a function of scattering angle extending to large momentum transfer on both sides of the specular peak. Using a hard hemisphere model to approximate the interaction potential, the data can be well reproduced by a hard core of radius about 2.5 Å. This result is compared to the charge density profile of the adsorbed molecule and also to the gas phase interaction potential, and in both cases good agreement with the experimentally predicted classical turning points is found.