Thermochemical stability

Thermochemical stabilization (in kcal/mol) based on difference between A/ and summation of standard bond energies (benzene RE = 45.8 kcal/mol)."... 

Table 12.7. Thermochemical Stabilization Energies for Some Substituted Radicals...

Boltzmann Equation. The equation which relates the composition of equilibrium mixtures to relative thermochemical stabilities and temperature. 

Thermodynamically-Controlled Reaction. A reaction the product ratio for which is determined solely by the relative thermochemical stabilities of the different products (product formation must be reversible, or separate low-energy pathways interconnecting the products must exist). 

Fig. 7.68 Thermochemical stability diagram for the system Fe-S-O at I 000 K showing the relative corrosion potentials of the atmospheres in conventional coal combustion (A), fluidised bed combustion (B), conventional coal gasification (C) and coal gasificiation using nuclear heat (D) (after Gray and Starr )...

Fig. 7.69 Thermochemical stability diagram for the 310 stainless-steel-S-O system at 750°C (from Gray and Starr after Natesan and Chopra ")...

Gangue minerals and salinity give constraints on the pH range. The thermochemical stability field of adularia, sericite and kaolinite depends on temperature, ionic strength, pH and potassium ion concentration of the aqueous phase. The potassium ion concentration is estimated from the empirical relation of Na+/K+ obtained from analyses of geothermal waters (White, 1965 Ellis, 1969 Fournier and Truesdell, 1973), experimental data on rock-water interactions (e.g., Mottl and Holland, 1978) and assuming that salinity of inclusion fluids is equal to ffZNa+ -h m + in which m is molal concentration. From these data potassium ion concentration was assumed to be 0.1 and 0.2 mol/kg H2O for 200°C and 250°C. 

Studies have shown that carbene reactivity toward a wide variety of substrates is dramatically affected by the nature and multiplicity of the electronic state. - Similarly, the structure, electronic state, thermochemical stability, and reaction kinetics of both singlet and triplet carbenes can be significantly affected by the R-substituents. If R provides steric hindrance, the carbene center can be shielded to slow down inter-molecular reactions (kinetic stabilization). Additionally, bulky and/or geometrically... 

Similarly, < (170) of 34 is 8 ppm lower than that of 28. Accordingly, although the 1,3-diene system of 33 does not possess the nature and thermochemical stability of ordinary conjugated 1,3-dienes, substituent effects are transmitted at least as efficiently through this system as they are transmitted through the aromatic system of 34. 

The active layer must provide the required activity, selectivity and thermochemical stability properties. Different active phases can be adopted depending on the operating constraints and the fuel type. In the following we will mainly focus on CH4 (i.e. the main constituent of natural gas) as the reference fuel for GT applications. In this respect, the combustion catalysts that have been most extensively investigated for configurations based on lean combustion concepts are PdO-based systems and metal-substituted hexaaluminates. 

Coming now to conjugation, results like those due to Malrieu et al. play an important role in the discussion of polyunsaturated systems. They offer the theoretical background for the well-known thermochemical stabilization of a chemical bond due to conjugation. This stabilization is not reflected in the aa qk + F part of... 

This response is surely significant, even for small departures from electroneutrality. A fragment K increases its thermochemical stability when it gains electronic charge from its environment A (K) becomes larger. 

The comparison between dissociation and intrinsic bond energies is intriguing, to say the least. It suggests (Fig. 12.1) that those bonds that contribute more to thermochemical stability are the ones that break up more easily. This is certainly a point requiring clarification ... 

On the other hand, looking at the enthalpies AHf, the same comparisons show that one gauche interaction reduces the thermochemical stability by only 1.0 kcal/mol, meaning that part of the loss in binding energy is recovered by Fg. These are rough numbers, of course they will be rehned later on. Let us first examine the physical content of Fg. 

Example 14.5 The Dimethylnaphthalene Isomers. For the isomer with the substituents in the 1,8 position, our calculation yields A = 2748.6 kcal/mol for the planar form, in error with respect to its experimental counterpart, 2745.0 kcal/mol. The thermochemical stability is overestimated by 3.6 kcal/mol, thus suggesting a possible loss of conjugation in this molecule, which, in fact, is known to suffer considerable distortion from the normal naphthalene geometry [278]. Indeed, a calculation following the lines described above for biphenyl and non-planar 4,5-dimethylphenan-threne agrees with experiment [129]. In contrast, 2,6-dimethylnaphthalene can safely be assumed to retain the planar geometry of naphthalene. The result deduced for this form, A = 2751.3, agrees weU with the experimental value, 2751.0 kcal/mol. 

This chapter provides examples of the use of molecular modeling to quantify thermochemical stabilities of what might normally be considered unstable molecules. 

One of the major advantages of molecular modeling over experiment is its generality. Thermochemical stability or even existance is not a necessary criterion for investigation by molecular modeling as it is for experiment. This leads to the intriguing and very real possibility that modeling can be used to explore how to stabilize unstable molecules, and so make them ammenable to scrutiny by experiment. The examples provided in this chapter illustrate some possibilities. 

While the desired goal, to reverse the thermochemical stabilities of benzene and Dewar benzene, has not been achieved, the calculations have clearly shown their value as a viable alternative to experiment to rapidly explore the limits of what is possible. 

Kinetically-Controlled Reaction. Refers to a chemical reaction which has not gone all the way to completion, and the ratio of products is not related to their thermochemical stabilities, but rather inversely to the heights of the energy barriers separating reactants to products. 

Since they generally have more symmetry than cis isomers, trans isomers in most cases have higher melting points and lower solubilities in inert solvents. The cis isomer usually has a higher heat of combustion, which indicates a lower thermochemical stability. Other noticeably different properties are densities, acid strengths, boiling points, and various types of spectra, but the differences are too involved to be discussed here. 

Dewar et al.83 showed that the energy of linear polyenes is an additive quantity of bond energy increments. This enabled Dewar to quantify the thermochemical stability or instability of a cyclic molecule relative to a reference molecule. The reference molecule R for a conjugated cyclic molecule... 

Our aromaticity-determining reactions for the three heterocycles are endothermic by 70, 40, and 48 kJ mol-1, to be compared with the exothermicity for the corresponding carbocycle, indene, of nearly 9 kJ mol-1. Indole, benzofuran, and benzothiophene are truly aromatic species in terms of thermochemical stabilization. 

RNA molecules form secondary structure by folding their polynucleotide chains via hydrogen bond formations between AU pairs and GC pairs. The thermochemical stability of forming such hydrogen bonds provides useful criterion for deducing the cloverleaf secondary structure of tRNAs that is, tRNA molecules are folded into DFI... 

Raemy and Schweizer (1983) assert the following thermochemical stabilities ... 

Specifically, D-glucose < maltose < maltotriose < amylose < starch < amylo-pectin < cellulose (Greenwood, 1967). Trends indicated are that thermochemical stability increases with the DP, branching, and 1,4-fi bonding. Chemical bonds other than 1,4-a and 1,4-(3 introduce heat and acid instability. Either of these two bonds is less easily depolymerized when the sixth pyranose carbon is oxidized to the carboxyl group rather than esterified for this reason, low-methoxyl pectin is more stable than high-methoxyl pectin. 

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