Stability thermodynamic calculations

Comparison of calculated and experimental data. Since the homologous series of adducts of the type Et3Al—E(Tms)3 (E = P 28, As 29, Sb 15, Bi 14) has been structurally characterized by single crystal X-ray diffraction, their structures and thermodynamic stabilities were calculated to allow a comparison between experimental and theoretical data (Table 7). 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Stability constants of metal complexes with inorganic ligands are generally well known [30], Inorganic speciation of metals can therefore easily be evaluated by thermodynamic calculations if the composition of the solution is known [29,31],... 

It has been shown by thermodynamic calculations (89TH1) that, under equal structural conditions, the ratio of the tautomeric equilibrium constants for the reversible addition reaction of the SH group and that for the OH group should be 10 in favor of the sulfur addition product. A similar result (>10 ) was estimated (90T6545) from a comparison of the stability of the 1,3-thiazolidine ring with that of the 1,3-oxazolidine ring. 

The overprediction of binding energies for simple molecules can have consequences for ab initio thermodynamic calculations involving these species. In Chapter 7 we examined the stability of bulk oxide materials using this approach. In order to quantitatively predict the conditions at which bulk phases were in equilibrium, the chemical potential of 02 had to be calculated using Eq. (7.6), which includes a contribution from the total energy of 02. One way to correct for the imprecision of DFT for total energies of this kind is to use experimental data when it is available. When this approach is used, it is important that it is clearly stated. 

Nesbitt et al. (1981) performed a detailed analysis of the thermodynamic and kinetic stability of perovskite. Thermodynamic calculations and data for natural groundwaters and hydrothermal... 

Theoretical thermodynamic calculations of the conditions under which Cd(OH)2 should form were also carried out, based on the solubility product of Cd(OH)2, the stability constants of the Cd-NTA system, and the ion product of water at different temperatures. The values of Rc computed from these calculations agreed with those measured experimentally. 

Fahey presents the products of (17) as uncomplexed formaldehyde and HCo(CO)3 rather than a bound-formaldehyde species (43). Free formaldehyde is a thermodynamically unfavorable product from H2 and CO (8), and significant stabilization may be expected as the result of coordination in a metal complex. However, thermodynamic calculations are presented which indicate that small equilibrium concentrations of formaldehyde could be present under the conditions of these cobalt-catalyzed reactions (43). Although small amounts of uncoordinated formaldehyde are indeed expected as a result of the following endothermic (36, 37) equilibrium ... 

At this point (the same pH as that of Wollast, t al., 1968) sepiolite begins to precipitate. In experiments maintaining pH at values above 9, montmorillonoids and talc were formed. Chemical analysis of the precipitates reveal a greater proportion of magnesium as the pH of the experiment is increased. Recalling the information on amorphous silica solubility, a two-fold increase in solubility of SiC>2 occurs between pH 8 and 10.5 (Krauskopf, 1959)—and thus at higher pH it could be expected that relatively less silicious phases would precipitate where the masses of Mg and Si are fixed. Final concentrations of Mg-Si in solution were not determined by Siffert and therefore thermodynamic calculations of mineral stabilities cannot be made. 

Although the semi-empirical approach outlined above is of some value in assessing potential explosibility hazards, much more fundamental work has been done subsequently to institute a more quantitative basis for such assessment. A combination of thermodynamical calculations with laboratory thermal stability and impact-sensitivity determinations has allowed a system to be developed which indicates the relative potential of a given compound or reaction system for sudden energy release, and the relative magnitude of the latter [3]. A similar treatment, specifically for compounds... 

Numerous papers on CDs complexation of azo dyes concern stability, thermodynamics and kinetics of complexation and co-conformation The studies are followed frequently by theoretical calculations. 

The above discussion clearly revealed that the covalently bound oxalic acid dinitrate ester is the most promising candidate as a high oxidizer and potential replacement for AP in this series. In order to evaluate its thermodynamic and kinetic stability we calculated the decomposition into C02 and N02. The reaction enthalpy of AH = 56.5 kcal mol1 clearly indicates that oxalic acid dinitrate ester is (as expected) thermodynamically unstable with respect to its decomposition into C02 and N02. 

At pressures below 30 kbar FeO is not in equilibrium with respect to nonstoichiometric wustite. However, in many experimental works and thermodynamic calculations the nonstoichiometry of wustite is not taken into account, which leads to confusion and incompatibility of the results. As a result, values may differ by 2 to 3 kcal, as we showed earlier (Mel nik, 1972b). Such large discrepancies preclude precise calculation of the parameters of wustite stability. 

In all ab initio calculations so far the transition state is early in the reaction coordinate, the C —O distance is greater than 1.9 A and the molecular geometry of ethene is only slightly distorted the dihedral angle H —C — C —H is calculated to be more than 173 . It is therefore not surprising that thermodynamic stabilities (MM250 calculations, see Table 1) of diastereomeric epoxides correlate badly with observed diastereoselectivities of their formation. 

A thermodynamic calculation of the stability of K2CO3 and CaS04 at the actual temperatures ... 

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