Simple Chemical Model Research Articles

Ventilation of Black Sea pycnocline by the Mediterranean plume

We present here results of numerical simulations with reduced gravity model of the Mediterranean plume intruding into the Black Sea. The model has horizontal resolution of 600 m. The scenarios analyzed in the paper aim at quantifying the sensitivity of the plume to the ambient stratification and the fluxes of mass, momentum and buoyancy through the Bosphorus Straits. The simulated plume characteristics are compared against observations. It is found that the mixing of Mediterranean and Black Sea water, as well as the termination depth of the plume, are very sensitive to specific combinations of the governing parameters. The behavior of gravity currents on the shelf and on the continental slope is also studied and the role of topographic control is demonstrated. The relatively large entrainment rate (∼10–12) compared to the one in the Atlantic ocean (∼2–3), shallow penetration and small deflection to the right caused by the earth rotation are explained as a result of the specific combination of governing parameters, topography routing and ambient stratification. A simple two-component chemical model for the interaction between H 2S and O 2 is coupled with the dynamical model in order to investigate the impact of the Bosphorus plume (rich in O 2) on the oxidation of anoxic water.

Active chlorine in the remote marine boundary layer: Modeling anomalous measurements of δ13C in methane

Measurements of δ13C in methane in the marine boundary layer (MBL) of the extratropical Southern Hemisphere imply a kinetic isotope fractionation much larger than would be expected if the hydroxyl radical were the only tropospheric methane sink. We use a simple chemical box model to show that the assumption of a MBL active chlorine (Cl•) sink can explain these anomalous observations provided there is a seasonal cycle in the Cl• sink with a summer‐winter concentration difference ∼ 6 · 10³ cm−3. The required summer maximum and yearly mean Cl• concentrations are plausible, and imply a global Cl• sink strength for methane of < 5 Tg yr−1. Choice of a Cl• sink seasonal cycle linked to the nonsinusoidal dimethyl sulfide seasonal cycle gives the observed fractionation with a smaller yearly mean Cl• concentration than equivalent sinusoidal Cl• cycles.

Local atomic environment of Si suboxides at the SiO2/Si(111) interface determined by angle-scanned photoelectron diffraction.

Local environments of Si suboxides at the interface between a thermally grown SiO2 film and Si(111) were studied by angle-scanned photoelectron diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. The components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The obtained diffraction patterns of the various suboxides exhibit different features. Comparison of these patterns with multiple scattering calculations including a multipole R-factor analysis shows that a simple chemical abrupt interface model describes well the environment of the suboxides and indicates ordered SiO2 close to the interface.

Investigation of the SiO2/Si(111) interface by means of angle-scanned photoelectron diffraction

Abstract Photoelectron diffraction measurements of the Si 2p core level were carried out in order to investigate the interface structure of a SiO 2 film thermally grown on Si(111). The photoemission spectra show chemically shifted components derived from the individual oxidation states, which exhibit different diffraction patterns. These diffraction patterns are compared with those obtained by multiple scattering calculations for a simple chemical abrupt interface structure model. In this model each oxidation state occurs in various bonding configurations. Thus the experimental data are compared with superpositions of simulated patterns for possible model clusters. As a result of a multipole R -factor analysis we obtain a parameter set that reproduces the experimental data well.

A Chemical Model of Homeostasis.

A closed spherical structure which can self-maintain or grow/reproduce depending upon the chemical parameter setting would be a simple chemical model of the homeostatic cell. One such model is presented here. It is based on oleic acid/oleate vesicles which host two competitive reactions which take place within the boundary of the bilayer, that is, one reaction which produces more vesicles and one which destroys them.

Thermodynamic and kinetic study of copper(II) complexes with N-methylene(phenylphosphinic acid) derivatives of cyclen and cyclam

Equilibria in the Cu 2+–H 4L 1 and Cu 2+–H 4L 2 systems, where H 4L 1 is 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetrayl-tetramethylene-tetrakis(phenylphosphinic acid) and H 4L 2 is 1,4,8,11-tetraaza-cyclotetradecane-1,4,8,11-tetrayl-tetramethylene-tetrakis(phenylphosphinic acid), were investigated by glass electrode potentiometry at 25°C ( I=0.1 mol dm −3 KNO 3). A simple chemical model with the metal:ligand molar ratio 1:1 was found in the systems. The presence of main species, [CuL 1] 2− (log β=20.37(4)) and [CuL 2] 2− (log β=17.19(2)), was also confirmed by MALDI-TOF/MS. The dissociation kinetics of the complexes was followed by spectrophotometry and a mechanism of the dissociation was proposed. Activation parameters (activation enthalpy and entropy) of the dissociation were estimated. For the Cu 2+–H 4L 1 system, the complex dissociation starts after protonation of the phosphinic pendant arms and its mechanism is similar to the decomplexation of [Cu(cyclen)] 2+. The Cu 2+ complex with H 4L 2 is kinetically much less stable. The proton transfer from the phosphinic pendant arm to the azacycle plays a significant role in the reaction mechanism of both the complexes.

Dipole Moments of Partially Bound Lewis Acid−Base Adducts

Stark effect measurements have been performed on six Lewis acid−base complexes containing 32SO3 and 11BF3. The following dipole moments have been obtained: HC14N−SO3 (4.4172 ± 0.0031 D); CH3C14N−SO3 (6.065 ± 0.018 D); HC15N−BF3 (4.1350 ± 0.0073 D); H315N−BF3 (5.9027 ± 0.0093 D (A state), 5.917 ± 0.010 (E state)); (CH3)315N−BF3 (6.0157 ± 0.0076 D); and (CH3)315N−B(CH3)3 (4.5591 ± 0.0097 D). Across a series of complexes with a common acid, the induced dipole moment increases sharply as the dative bond shortens. Contributions to the total molecular dipole moment arising from distortion, polarization, and charge transfer have been estimated for these and a number of related complexes, using the block-localized wave function energy decomposition analysis of Mo, Gao, and Peyerimhoff. Mulliken and natural population analyses are presented, as are electron density difference maps for HCN−SO3, (CH3)3N−BF3, and (CH3)3N−SO3. Theoretical values for the degree of charge transfer are compared with experimental estimates based on nuclear hyperfine parameters, and the validity of a simple chemical model involving charge transfer and bond moments is examined. Ab initio calculations of the induced dipole moment of HCN−SO3 and H3N−SO3 are given as a function of N−S bond length and compared with the experimentally observed values for a series of SO3 complexes. The results suggest that the induced moments of the series collectively approximate the induced dipole moment function for individual members of the series. Similar results are obtained using previously published dipole moment functions for HCN−BF3 and H3N−BF3.

A simple chemical example of hierarchical thermodynamic interactions: the protonation equilibria of inorganic polyprotic acids

A general method for formulating complex thermodynamic systems in terms of hierarchical interactions has been developed, and has been applied in a previous analysis to hemoglobin oxygen binding data. Polyprotic acids can be considered a simple chemical model of thermodynamic interaction between ligand binding events. To further illustrate the hierarchical interaction approach it is applied to the analysis of the thermodynamic interactions between proton binding events in inorganic polyprotic acids. p K values for arsenate, carbonate, chromate, phosphate, phosphite, selenite, sulfide and sulfite were recast into hierarchical interaction terms. The intrinsic K d, h for protonation ranged from 8.8×10 −13 (M) for phosphate to 1.3×10 −6 (M) for chromate. Pairwise interactions ( K d, hh ) between protonation events ranged from 1.3×10 4 for phosphite to 9.4×10 5 for carbonate. Third order interactions ( K d, hhh ) were 0.91 and 0.51 for arsenate and phosphate, respectively, values relatively close to the no interaction value of 1. A principle feature of systems described by hierarchical interactions is that higher order interactions, representing more complex interactions, are less likely to be significant than lower order interactions, and this is further illustrated by these observations from polyprotic acids. The set of significant hierarchical interaction values can be used to predict values for as yet unobserved events, and projected p K values are made for all the polyprotic acids included in this study. Finally, application of this method to the protonation equilibria of water demonstrates a profound pairwise interaction between protonation events ( K d, hh =1.3×10 17), which is attributed to oxygen's small size and lack of polarizability.

Combustion Stability Assessment for Utility Pulverized Coal-Fired Boilers under Low Loads

Based on the influence of chemical equivalence ratio on the combustion stability of utility pulverized coal-fired boilers and the control theory about system stability, a combustion stability index, CSI, which refers to the maximum reduction ratio of the fuel mass flow rate that can be overcome by the stable combustion process under a constant air mass flow rate, was proposed to assess quantitatively the combustion stability in the boilers. MLO, the Minimum Load of Operation with stable combustion not supported by firing oil, and MCQ, the Minimum Coal Quality, which gives the lowest heat values of coals with different volatile matter contents for stable operation of boilers, are defined on the basis of CSI. In order to predict MLO and MCQ, a simple chemical reaction system model has been modified by means of the concept of lean flammability of gaseous fuels. A three-dimensional combustion simulation code integrated with the modified model was used to study the stability of combustion process in a 200MWe pulverized coal fired utility boiler. The predictions of MLO and MCQ agreed confidently with operational experiences.

Does gut function limit hummingbird food intake?

Many nectar-feeding bird species decrease food intake when sugar concentration in food is increased. This feeding response can be explained by two alternative hypotheses: compensatory feeding and physiological constraint. The compensatory feeding hypothesis predicts that if birds vary intake to maintain a constant energy intake to match energy expenditures, then they should increase intake when expenditures are increased. Broad-tailed hummingbirds were presented with sucrose solutions at four concentrations (292, 584, 876, and 1,168 mmol L(-1)) and exposed to two environmental temperatures (10 degrees and 22 degrees C). Birds decreased volumetric food intake in response to sugar concentration. However, when they were exposed to a relatively sudden drop in environmental temperature and, hence, to an acute increase in thermoregulatory energy expenditures, they did not increase their rate of energy consumption and lost mass. These results support the existence of a physiological constraint on feeding intake. A simple chemical reactor model based on intestinal morphology and in vitro measurements of sucrose hydrolysis predicted observed intake rates closely. This model suggests that intestinal sucrose hydrolysis rates were near maximal and, thus, may have imposed limits to sugar assimilation. Although sugar assimilation was high (95%), the proportions of excreted sucrose, glucose, and fructose found in excreta differed significantly. The monosaccharides glucose and fructose were about eight and three times more abundant than sucrose, respectively. Broad-tailed hummingbirds are small high-altitude endotherms that face unpredictable weather and the energetic expense of premigratory fattening. Digestive processes have the potential to impose severe challenges to their energy budgets.

Probing the Evolution of Early‐Type Cluster Galaxies through Chemical Enrichment

A simple chemical enrichment model for cluster early-type galaxies is described in which the mechanisms considered in the evolutionary model are infall of primordial gas, outflows and a possible variation in the star formation efficiency. We find that - within the framework of our models - only outflows can generate a suitable range of metallicities. The chemical enrichment tracks can be combined with the latest population synthesis models to simulate clusters over a wide redshift range, for a set of toy models. The color-magnitude relation of local clusters is used as a constraint, fixing the correlation between absolute luminosity and ejected fraction of gas from outflows. It is found that the correlations between color or mass-to-light ratios and absolute luminosity are degenerate with respect to most of the input parameters. However, a significant change between monolithic and hierarchical models is predicted for redshifts $z\simgt 1$. The comparison between predicted and observed mass-to-light ratios yield an approximate linear bias between total and stellar masses: $M_{\rm Tot}\propto M_{\rm St}^{1.15\pm 0.08}$ in early-type galaxies. If we assume that outflows constitute the driving mechanism for the colors observed in cluster early type galaxies, the metallicity of the intracluster medium (ICM) can be linked to outflows. The color-magnitude constraint requires faint $M_V\sim -16$ galaxies to eject 85% of their gas, which means that most of the metals in the ICM may have originated in these dwarf galaxies.

Bifurcation problems with high codimensions

Many biological systems can be modeled by autonomous ordinary differential equations. An important question is how the model behaves at a nonhyperbolic singularity locally. The possible behaviour can be described by the versai deformation of the singularity. In this paper, a classification of the Takens-Bogdanov singularities is presented, and some property of two singularities of higher codimension is proved. Lastly, we illustrate the investigation of singularities on a simple chemical model. The model is of biological interest and it turns out to have a complex dynamic behaviour.

Blue Compact Galaxies and the Primordial 4Helium Abundance

Blue compact galaxies (BCG) are ideal objects in which to derive the primordial 4He abundance because they are chemically young and have not had a significant stellar He contribution. We discuss a self-consistent method which makes use of all the brightest He I emission lines in the optical range and solves consistently for the electron density of the He II zone. We pay particular attention to electron collision and radiative transfer as well as underlying stellar absorption effects which may make the He I emission lines deviate from their recombination values. Using a large homogeneous sample of 45 low-metallicity H II regions in BCGs, and extrapolating the Y-O/H and Y-N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.2443±0.0015, in excellent agreement with the weighted mean value Yp = 0.2452±0.0015 obtained from the detailed analysis of the two most metal-deficient BCGs known, I Zw 18 and SBS 0335-052. The derived slope dY/dZ = 2.4±1.0 is in agreement with the value derived for the Milky Way and with simple chemical evolution models with homogeneous outflows. Adopting Yp = 0.2452±0.0015 leads to a baryon-to-photon ratio of (4.7+1.0-0.8) × 10−10 and to a baryon mass fraction in the Universe ωbh250 = 0-068+0.015-0.012, consistent with the value derived from the primordial D abundance of Burles &Tytler (1998).

Phase formation due to high dose aluminum implantation into silicon carbide

High doses of 350 keV aluminum (Al) ions were implanted into hexagonal silicon carbide (6H–SiC) single crystals at 500 °C. Phase formation was studied by transmission electron microscopy, secondary-ion mass spectrometry, and Auger electron spectrometry. A critical Al concentration of about 10 at. % was found below which the 6H–SiC structure remains stable. The Al atoms occupy preferentially silicon (Si) sites in the SiC lattice. The replaced Si atoms seem to be mobile under the implantation conditions and diffuse out. At higher Al concentrations the SiC matrix is decomposed and precipitates of Si and aluminum carbide (Al4C3) are formed. The Al4C3 precipitates have a perfect epitaxial orientation to the SiC matrix. The phase transformation is accompanied by atomic redistribution and strong volume swelling. The resulting changes in the atomic depth profiles can be accounted for by a simple chemical reaction model.

Characterization of lipophilicity scales using vectors from solvation energy descriptors

Lipophilicity scales were characterized by an approach using vectors provided from solvation energy descriptors (SED) of solutes such as an excess molar refraction, the dipolarity/polarizability, the hydrogen-bond acidity, the basicity, and the McGowan characteristic volume. The five components of the SED vector were obtained from the coefficients of the five SED terms of the linear solvation energy relationship (LSER) equation for the lipophilicity scales. The analogy between two lipophilicity scales was expressed as the angle between the two SED vectors, while the difference in the contribution of the five independent SEDs to these two lipophilicity scales was quantified by the difference of the unit vectors of the SED vectors. These approaches were applied to several lipophilicity scales measured using microemulsions, micelles, an immobilized artificial membrane column, and an octanol–water system. As a result, the quantitative classification of these scales was successfully carried out, and the difference in the scales was well characterized. In addition, this vector approach was extended to the estimation of the contribution of each constituent of the microemulsions to the lipophilicity scale. Furthermore, some biological parameters such as skin permeability and the distribution between blood and brain could be predicted by the summation of the SED vectors obtained from the chromatographic systems. These results suggest that complex biological systems can be expressed quantitatively by simple chemical models with their SED vectors.

Modelling the fate of non-polar organic chemicals during the melting of an Arctic snowpack

Recent advances in the understanding of the interactions between non-polar organic chemicals and frozen water have revealed the immense importance of the ice-air interface and the possibility of quantitatively treating the adsorption process on the ice surface in terms of chemical-specific interfacial partition coefficients and the snow specific surface area. A simple fugacity-based chemical fate model is used to describe quantitatively the behaviour of selected non-polar organic contaminants during snowpack melting, namely atmosphere-snow exchange, redistribution within the snowpack and draining with meltwater. The simulations are evaluated using field data on snowpack and meltwater concentrations of these chemicals measured in a small watershed in the Canadian Arctic during the melting period of 1993. The model reproduced the observed preferential elution of hexachlorocyclohexanes in the first meltwater fractions, and the relative retention of polychlorinated biphenyls within the snowpack. The model calculations indicate that the specific surface area of the ice crystals and the air-ice and air-water partition coefficients of the chemical are key parameters controlling the extent and timing of chemical loss with the meltwater. For particle-sorptive substances, the fate of particlcs during snowmelt must also be considered. Uncertainty in describing interfacial partitioning and the behaviour of particles during melting presently limits the capabilities of models to simulate and ultimately predict meltwater concentrations of non-polar organic contaminants.

Process and equipment simulation of dry silicon etching in the absence of ion bombardment

The dependence of the etch rate uniformity across the wafer on the reactor design and various process parameters was investigated for the reactive ion etching (RIE) of silicon using pure sulphur hexafluoride (SF 6). The experiments were carried out in two different single wafer reactors without ion bombardment to determine the chemical component of the etch rate. The influence of pressure, gas flow, rf power, and loading on the radial course of the etch rate was measured and compared with the respective results of numerical simulation. The commercially available PHOENICS-CVD simulation tool was used for the solution of the fluid dynamic transport equations after some adaptations for its application to etching. A simple chemical model is proposed for the description of gas phase and surface plasma reactions, respectively. The values of the reaction rate parameters were fitted from the experimental data. The simulation represents the main experimental trends and can be applied to process and equipment optimization.

Observations of HCN, HNC, and NH3in Comet Hale‐Bopp

We have observed the J = 1-0 rotational lines of HCN and HNC, and the (J, K) = (1, 1), (2, 2), and (3, 3) inversion lines of NH3 toward comet Hale-Bopp during 1997 April 19-21. We have made a cross-scan toward the coma, and have found the source sizes of HCN and HNC to be 38'' ± 7'' and 50'' ± 18'', respectively. The [HNC]/[HCN] abundance ratios obtained are 0.13 ± 0.03 and 0.36 ± 0.32 at the center of the coma and in the outer part, respectively. Although the source size of HNC might be slightly larger than that of HCN, the distributions of these two species almost agree with each other within the error limits. Since it has been suggested that HCN and HNC are a parent molecule and a product of coma chemistry, respectively, the present results indicate that the production of HNC is occurring in the inner coma region. We have calculated the density distributions of HCN and HNC in the coma of comet Hale-Bopp by assuming a simple chemical model. The calculated HNC/HCN ratio is almost constant in the coma, which is consistent with our observations. In addition, the average excitation temperature is found from the observations of the NH3 lines to be 54 ± 45 K with the 74'' beam, corresponding to a coma size of 8.6 × 104 km.

State of the art in the modelling of SiC sublimation growth

Different computational tools have helped to provide additional information on the sublimation growth of SiC single crystals by the modified-Lely method. The modelling work was motivated by the need of a better control of the local temperature field inside the crucible. Because there is an environment of strong thermal radiation in which the SiC boule growth process occurs, heat transfer must therefore be coupled with gaseous species transport and reactivity. This highly coupled model must take into account all geometric modifications in crucibles which strongly influences the crystal growth process. Local thermochemical equilibrium linked to heat and mass transfer is the model proposed in this paper to give the magnitude of the growth rate and the shape of the crystal. This modelling field is still too young to propose a software package including all modelling aspects and a reliable material database. However, some parts of the modelling work have reached maturity like electromagnetic heating and thermal modelling coupled with simplified chemical models. We show in this paper selected examples in order to demonstrate the types of information which can be routinely available by simulation and how to approach defect formation from a macroscopic point of view. Minor geometric modifications of the holes for pyrometric measurements drastically change the magnitude of thermal gradients in the crucible. Geometric modifications of the crucible change the computed crystal shapes. The calculated results complete the experimental knowledge by a quantification of the local macroscopic fields.

High-pressure experiments and the varying depths of rock metamorphism

Abstract Hutton’s visions of 1785 included burial and metamorphism of rocks and their reincorporation into the continental crust. As a result of changing environmental conditions, mainly of pressure ( P ) and temperature ( T ), rocks change their mineral assemblages during metamorphism. High-pressure experimentation has now provided geoscientists with quantitative data on the behaviour of rock systems under varying PT conditions, so that depths of burial or subduction and pathways of exhumation can be reconstructed, and the metamorphic histories of rocks can be used to gauge the internal dynamics of our globe during geological time. An important prerequisite is the knowledge of the states of chemical equilibrium as a function of P, T and rock composition. Several simple chemical model systems are chosen here to demonstrate characteristic mineralogical and petrological behaviour during metamorphism and to identify some important and unusual minerals that typify high- or ultrahigh-pressure metamorphism taking place at depths of up to 150 km, e.g. the dense forms of carbon (diamond) and of SiO 2 (coesite). Low-pressure metamorphism within the normal (∼35 km) crust of the Earth, on the other hand, is typified by less dense minerals such as tridymite or cordierite. In this way, deep and very deep subduction of former rocks of the Earth’s crust was recognized. This required new geodynamic concepts involving the participation of crustal materials in the overall secular convection processes within the Earth’s mantle.