Showing papers on "Enthalpy published in 1994"

An equation of state for liquid iron and implications for the Earth's core

Abstract: An equation of state is presented for liquid iron based on published ultrasonic, thermal expansion, and enthalpy data at 1 bar and on pulse-heating and shock wave compression and sound speed data up to 10 Mbar. The equation of state parameters, centered at 1 bar and 1811 K (the normal melting point of iron), are density, ρ_0 = 7019 kg/m^3, isentropic bulk modulus, K_(S0) = 109.7 GPa, and the first- and second-pressure derivatives of K_S, K′_(S0) = 4.66 and K″_(S0) = −0.043 GPa^(−1). A parameterization of the Gruneisen parameter γ as a function of density ρ and specific internal energy E is γ = γ_0 + γ′(ρ/ρ_0)^n(E - E_0) where γ_0 = 1.735, γ′ = −0.130 kg/MJ, n = −1.87, and E_0 is the internal energy of the liquid at 1 bar and 1811 K. The model gives the temperature dependence of γ at constant volume as (∂γ/∂T)_(v|1bar,1811K) = −8.4 × 10^(−5) K^(−1). The constant volume specific heat of liquid Fe at core conditions is 4.0–4.5 R. The model gives excellent agreement with measured temperatures of Fe under shock compression. Comparison with a preliminary reference Earth model indicates that the light component of the core does not significantly affect the magnitude of the isentropic bulk modulus of liquid Fe but does decrease its pressure derivative by ∼10%. Pure liquid Fe is 3–6% more dense than the inner core, supporting the presence of several percent of light elements in the inner core.

Water chemistry on surface defect sites : chemidissociation versus physisorption on mgo(001)

Abstract: The following paper presents the results of a theoretical study that probed the chemistry of water at structural defects on the MgO (001) surface. The computational technique used was periodic Hartree–Fock (PHF) theory with density functional based correlation corrections. The adsorption energies for water adsorbed on isolated corner, edge, and surface sites on the MgO surface were compared to the hydroxylation energies for the same sites. As stated in a previous paper, the binding of water to the perfect surface is exothermic by 4.1‐5.6 kcal/mol whereas hydroxylating the perfect surface was endothermic by 24.5 kcal/mol. At step‐edge sites, the process of water adsorption is exothermic and comparable in magnitude to the hydroxylation of these sites. The binding energies associated with water bound to the step‐edge are 6.5–10.5 kcal/mol, and hydroxylation of this site is exothermic by 7.3 kcal/mol. At corner sites we find a strong preference for hydroxylation. The binding of water to a corner is exothermic by 20.7 kcal/mol, and hydroxylation is exothermic by 67.3 kcal/mol. Mulliken populations indicate that the formation of a hydroxylated surface is governed by the stability of the hydroxyl bond where a hydrogen is bonded to a surface oxygen ion. As the coordination number of this oxygen binding site decreases, its ionic character also decreases, and it forms a more stable bond with the incoming hydrogen. This trend is confirmed by the densities of states for these sites. Finally, hydroxylation of the perfect (001) surface was examined as a function of lattice dilation. It was determined that, as the lattice constant increases, hydroxylation becomes more energetically favorable. This may be important in interpreting experimental thin‐film results where the lattice constant of the substrate upon which the MgO film is deposited is slightly larger than that of bulk MgO.

Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units

Abstract: The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, Kx, for the disproportionation reaction among the structural units coexisting in the melts, Si2O5(2Q3)⇔SiO3(Q2)+SiO2(Q4), was calculated (Kx is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln Kx vs l/T relationships the enthalpy (ΔHx) for the disproportionation reaction is in the range of-30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the ΔHx increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li2O·2SiO2 (Li/Si=1), the ΔHx is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M=K, Na, Li), there is a positive, near linear correlation between the ΔHx and the Z/r2 of the metal cation. The slope of the ΔHx vs Z/r2 regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, γi, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form γi=a lnX i+b, where a is between 0.6 and 0.87, and X i is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.

Thermodynamics of ubiquitin unfolding

Abstract: The energetics of ubiquitin unfolding have been studied using differential scanning microcalorimetry. For the first time it has been shown directly that the enthalpy of protein unfolding is a nonlinear function of temperature. Thermodynamic parameters of ubiquitin unfolding were correlated with the structure of the protein. The enthalpy of hydrogen bonding in ubiquitin was calculated and compared to that obtained for other proteins. It appears that the energy of hydrogen bonding correlates with the average length of the hydrogen bond in a given protein structure.

The thermodynamics of solvent exchange.

Abstract: A model for solvation in mixed solvents, which was developed for the free energy and preferential interaction [J. A. Schellman (1987), Biopolymers, Vol. 26, pp. 549–559; (1990), Biophysical Chemistry, Vol. 37, pp. 121–140; (1993), Biophysical Chemistry, Vol. 45, pp. 273–279], is extended in this paper to cover the thermal properties: enthalpy, entropy, and heat capacity. An important result is that the enthalpy of solvation H responds directly to the fraction of site occupation. This differs from the free energy Ḡ and preferential interaction Γ32, which are measures of the excess binding above a random distribution of solvent molecules. In other words, the enthalpy is governed by K while Ḡ and Γ32 are governed by (K − 1) where K is the equilibrium constant on a mole fraction scale [Schellman (1987)]. The solvation heat capacity Cp consists of two term: (1) the intrinsic heat capacity of species in solution with no change in composition, and (2) a term that accounts for the change in composition that accompanies solvent exchange. Binding to biological macromolecules is heterogeneous but experiementalists must use binding isotherms that assume the homogeneity of sites. Equations are developed for the interpretation of the experimental parameters (number of sites nexp, equilibrium constant Kexp, and enthalpy, Δhexp), when homogeneous formulas are applied to the heterogeneous case. It is shown that the experimental parameters for the occupation and enthalpy are simple functions of the moments of the distribution of equilibrium constants over the sites. In general, nexp is greater than the true number of sites and Kexp is greater than the average of the equilibrium constants. The free energy and preferential interaction can be fit to a homogenious formula, but the parameters of the curve are not easily represented in terms of the moments of distributions over the sites. The strengths and deficiencies of this type of thermodynamic model are discussed. © 1994 John Wiley & Sons, Inc.

Enthalpy of hydrogen bond formation in a protein-ligand binding reaction.

Abstract: Parallel measurements of the thermodynamics (free-energy, enthalpy, entropy and heat-capacity changes) of ligand binding to FK506 binding protein (FKBP-12) in H2O and D2O have been performed in an effort to probe the energetic contributions of single protein-ligand hydrogen bonds formed in the binding reactions. Changing tyrosine-82 to phenylalanine in FKBP-12 abolishes protein-ligand hydrogen bond interactions in the FKBP-12 complexes with tacrolimus or rapamycin and leads to a large apparent enthalpic stabilization of binding in both H2O and D2O. High-resolution crystallographic analysis reveals that two water molecules bound to the tyrosine-82 hydroxyl group in unliganded FKBP-12 are displaced upon formation of the protein-ligand complexes. A thermodynamic analysis is presented that suggests that the removal of polar atoms from water contributes a highly unfavorable enthalpy change to the formation of C=O...HO hydrogen bonds as they occur in the processes of protein folding and ligand binding. Despite the less favorable enthalpy change, the entropic advantage of displacing two water molecules upon binding leads to a slightly more favorable free-energy change of binding in the reactions with wild-type FKBP-12.

Comparative sorption equilibrium studies of toxic phenols on flyash and impregnated flyash

Abstract: In the present study, the sorption of toxic phenols, which include phenol, o-cresol, m-cresol, p-cresol, o-nitrophenol, m-nitrophenol and p-nitrophenol, has been investigated. The influences of various factors, such as particle size, impregnation of flyash (IFA), pH and temperature on the sorption capacity have been studied. Equilibrium modelling has been carried out using Langmuir and Freundlich isotherm equations and constants have been calculated under different conditions. Thermodynamic studies have also been carried out and values of standard free energy (ΔG o ), enthalpy (ΔH o ) and entropy (ΔS o ) calculated

Estimation of the viscosities of binary metallic melts using Gibbs energies of mixing

Abstract: In the present work, information on the integral molar Gibbs energies of mixing is employed to calculate the viscosities of binary substitutional metallic melts. A correlation has been established between the second derivative of the integral molar Gibbs energy of mixing with respect to composition and the corresponding function for the Gibbs energy of activation for viscosity. The viscosities predicted from available thermodynamic data in the case of a number of binary metallic systems using this correlation show satisfactory agreement with the values reported from experimental measurements. The value of this correlation in predicting the viscosities of complex metallic melts is also examined.

Thermodynamic continuity between glassy and normal water

Abstract: The enthalpy, the heat capacity, and Gibbs free energy of glassy water and of metastable water at 153 K have been examined by measuring the heat evolved on its crystallization to cubic ice. Measurements are made both isothermally and for slow heating, since the total heat evolved on crystallization decreases with the temperature. It is shown that a small fraction of metastable water persists up to 180 K when heated at 30 K min -1 and that the excess enthalpy of water at 153 K is 1.2±0.1 kJ mol -1 . Free energy considerations suggest that a thermodynamic continuity is possible only if the residual entropy of glassy water at 0 K is ≤5.3 J K -1 mol -1 , or the excess residual entropy over that of hexagonal ice is ≤1.9 J K -1 mol -1

FTIR analysis of hydrogen bonding in amorphous linear aromatic polyurethanes. I. Influence of temperature

Abstract: FTIR spectral studies of an amorphous linear aromatic polyurethane at various temperatures were performed. Hydrogen bonding was studied in the N-H stretching (3347 cm -1 ) and the bending (1535 cm -1 ) regions, using the band decomposition technique. The variations with temperature are used to calculate the ratio of the absorptivity coefficients for the H-bonded to the «free» N-H vibrations. This ratio is found to be independent of temperature. The enthalpy and the entropy of hydrogen bond dissociation are also obtained as 9.6 kJ mol -1 and 44.8 J mol -1 K -1 , respectively

Comparison of the addition of ch3., ch2oh., and ch2cn. radicals to substituted alkenes : a theoretical study of the reaction mechanism

Abstract: High-level ab initio calculations at the QCISD/6-311G∗∗ + ZPVE level have been carried out to study the addition reactions of CH∗, CHOH∗, and CHCN∗ radicals to the substituted alkenes CH=CHX (X = H, NH, F, CI, CHO, and CN) and the results analyzed with the aid of the curve-crossing model. We find that the reactivity of CH∗ is primarily governed by enthalpy effects, whereas both enthalpy and polar effects are important for the reactions of CHOH∗ and CHCN∗ There is no general barrier height-enthalpy correlation for the latter two radicals because of the presence in some cases of polar effects that stabilize the transition states without a corresponding stabilization of the products. The polar effects are not sufficient, however, to significantly shift the location of the transition states, so a general structure-enthalpy correlation is observed.

Thermodynamic properties of Pd40Ni40P20 in the glassy, liquid, and crystalline states

Abstract: Bulk specimens of the easy glass‐forming alloy Pd40Ni40P20 have been undercooled consistently into the glassy state at cooling rates as low as 10 K/min applying the melt‐fluxing technique in boron trioxide. Due to this low cooling rate, heat capacity measurements could be performed in a commercial heat‐flow differential calorimeter, covering for the first time the entire undercooling regime of a liquid metal from the melting temperature down to the glass transition temperature. Based on the measured specific heat data of the undercooled liquid and the crystalline state, the differences in the thermodynamic functions enthalpy, entropy, and Gibbs free energy are determined in dependence on temperature. The entropy balance yields a value of T0=500±5 K for the ideal glass transition temperature of this metallic system. The experimental values are compared to the corresponding thermodynamic functions, derived from commonly applied Gibbs free energy approximations for the undercooled liquid.

Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes

Abstract: Article discussing research on the calculation of methane C-H oxidative addition trajectory and a comparison to experiment and methane activation by high-valent complexes.

Thermodynamic assessment of the copper-oxygen system

Abstract: The Cu-O system shows complete miscibility between the metallic liquid and the oxide liquid above ∼1623 K and a miscibility gap below that temperature. Because of the practical importance of the system, a wealth of experimental data exists, both on the phase diagram and on the thermodynamic properties. These data have been reviewed, and a consistent set of thermodynamic model parameters has been optimized. An ionic two-sublattice model was used to describe the liquid phase and was found to represent accurately the experimental data.

Thermodynamics of Micellization of Aerosol OT in Binary Mixtures of Water, Formamide, Ethylene Glycol, and Dioxane

Abstract: The results of critical micelle concentration of aerosol OT in pure and binary mixtures of water, formamide, ethylene glycol, and dioxane determined by the methods of conductance, surface tension, spectrophotometry, and calorimetry are presented. The cmc values by the different methods are in agreement with one another and have shown positive deviations with maxima in all the binary solvent mixtures at characteristic compositions. In this behavior, mixtures containing water are distinctly different than those without water. The water containing mixtures have shown both negative and positive enthalpies of micellization whereas those without water are all associated with positive enthalpy. The entropies of micellization are all positive, and they nicely compensate the enthalpies of the process

The relation between plant growth and respiration: A thermodynamic model

Abstract: A thermodynamic model describing the relation between plant growth and respiration rates is derived from mass-and enthalpy-balance equations. The specific growth rate and the substrate carbon conversion efficiency are described as functions of the metabolic heat rate, the rate of CO2 production, the mean oxidation state of the substrate carbon produced by photosynthesis, and enthalpy changes for conversion of photosynthate to biomass and CO2. The relation of this new model to previous models based only on mass-balance equations is explored. Metabolic heat rate is shown to be a useful additional measure of respiration rates in plant tissues because it leads to a more explicit description of energy relations. Preliminary data on three Zea mays (L.) cultivars are reported. The model suggests new rationales for plant selection, breeding and genetic engineering that could lead to development of plants with more desirable growth rates.

How Does the Electronegativity of the Substituent Dictate the Strength of the Gauche Effect

Abstract: The distinct conformational preferences observed for the pentofuranosyl moieties in various 3'-substituted 2',3'-dideoxythymidine derivatives are closely related to the strength of the 3'-gauche effect, which is directly dictated by the electronegativity of the 3'-substituent. The efficiency of the 3'-gauche effect can now be quantitatively calculated with the help of simple linear calibration curves that correlate the gauche effect enthalpy AH^^") and the group electronegativity (x) of the 3'-substituent.

Thermodynamics of Inhibitor Binding to the Catalytic Site of Glucoamylase from Aspergillus niger Determined by Displacement Titration Calorimetry

Abstract: The binding of different inhibitors to glucoamylase G2 from Aspergillus niger and its temperature and pH dependencies have been studied by titration calorimetry. The enzyme binds the inhibitors 1-deoxynojirimycin and the pseudo-tetrasaccharide acarbose with association constants of 3 x 10(4) and 9 x 10(11) M-1, respectively, at 27 degrees C. The binding free energy for both ligands is remarkably temperature-invariant in the interval from 9 to 54 degrees C as the result of large compensating changes in enthalpy and entropy. Acarbose and 1-deoxynojirimycin bound with slightly different free energy-pH profiles, with optima at 5.5 and 5.5-7.0, respectively. Variations in delta H degrees and T delta S degrees as a function of pH were substantially larger than variations in delta G degrees in a partly compensatory manner. Two titratable groups at or near subsite 1 of the catalytic site were found to change their pKa slightly upon binding. The hydrogenated forms of acarbose, D-gluco- and L-ido-dihydroacarbose, bind with greatly reduced association constants of 3 x 10(7) and 2 x 10(5) M-1, respectively, and the pseudo-disaccharide methyl acarviosinide, lacking the two glucose units at the reducing end compared to acarbose, has a binding constant of 8 x 10(6) M-1; these values all result from losses in both enthalpy and entropy compared to acarbose. Three thio analogues of the substrate maltose, methyl alpha- and beta-4-thiomaltoside and methyl alpha-4,5'-dithiomaltoside, bind with affinities from 3 x 10(3) to 6 x 10(4) M-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimation of Heats of Formation of Organic Compounds by Additivity Methods

Abstract: In the present manuscript describing traditional, macroscopic thermochemical properties, the authors` language will be that of molecular structure. Enthalpies (or heats of formation) are the subject of this article, and since the most important practical application of enthalpies is to explore reactivities and/or equilibria, they take a kineticist`s perspective in answering the question, what is a ``sufficiently accurate`` prediction of an enthalpy of formation. In a general reaction, A + B {yields} C + D, a shift in {Delta}{sub r}H (enthalpy of reaction) of 1 kcal/mol will generally result in a change in the equilibrium constant, K{sub eq}, of exp({minus}500/T) where T is the temperature in Kelvins. At room temperature, this means a factor of over 5 in K{sub eq}; the difference between, say, 90% and 64% reaction yield. Or, in terms of the time required for reaction completion, it could also mean an increase of a factor of 5. This factor of 5 is the same whether the total enthalpy of reaction is 5 kcal/mol or 500 kcal/mol. Thus, while theoreticians have struggled to attain the stage where they can with pride calculate enthalpy quantities with 2--4 kcal/mol uncertainty, they are not solving the practical problems at hand.

A thermodynamic model for structure‐H hydrates

Abstract: A statistical thermodynamics model based on the original work of van der Waals and Platteeuw is presented for structure-H hydrates. The model is an extension of the hydrate prediction method generalized by Parrish and Prausnitz for structure-I and II hydrates. Four structure-H-forming systems, methane + adamantane, methane + neohexane, methane + isopentane, and methane + methylcyclohexane, were considered. Optimized Kihara core parameter are presented for each of the large hydrocarbon guest molecules. The optimized reference chemical potential difference and reference enthalpy difference for structure-H hydrates are also presented. The results show good agreement with the experimentally determined phase-equilibria conditions. A sensitivity analysis is presented for the parameters in the model, and their relative order of influence on the accurate evaluation of the equilibrium pressure is determined.