Thermochemical tables stability

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

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

You have seen that measurements of heats of reaction, such as heats of combustion, can provide quantitative information concerning the relative stability of constitutional isomers (Section 2.18) and stereoisomers (Section 3.11). The box in Section 2.18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AHf) for many molecules. The following material shows how two different sources of thermochemical information, heats of formation and bond dissociation energies (see Table 4.3), can reveal whether a particular reaction is exothermic or endothermic and by how much. 

The following Tables 2.1 to 2.3 summarize some examples based exclusively on thermochemical reactions on the sorbent surface which lead to the formation of fluorescent reaction products. The derivatives formed frequently remain stable for weeks [6] and the fluorescence can frequently be intensified and/or be stabilized by treatment with viscous liquids (liquid paraffin, Triton X-100, polyethylene glycol etc.). 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

Comparison between FP and TC lattice stabilities Despite the variety of assumptions that have been used, some general trends for die resultant lattice stabilities have been obtained for various crystal structures across the periodic table. The mean values of such (FP) lattice stabilities can therefore be compared with the equivalent values determined by thermochemical (TC) methods. Such a comparison shows the following irrqiortant features (Miodownik 1986, Watson et al. 1986, Saunders et al. 1988, Miodownik 1992) ... 

The data given in Tables 2 and 3 are, of course, related to one another through a thermochemical cycle. AHi0n(g) and AHaq differ only by the heats of solvation (hydration in aqueous solution) of the reactants and product, and therefore these heats of solvation must affect the absolute bond energies in the gas phases in such a manner as to cause an inversion in the order of stability in cases of class (b) behaviour (see below). 

Available rate and thermochemical data allow estimates to be made for bond homolysis rate constants for virtually all covalent bonds presumed or found to be present in coal and model compound reactions. In Table III is compiled a list of coal-related homolysis rate constants, k, and bond homolysis half lives at 400°C, T1/2 (ti/2 = In 2/k is equal to the time required to break one-half of the bonds if homolysis were the only mode of reaction). Most of these values are estimated relative to measured rate constants for bibenzyl dissociation in tetralin, and relative values are expected to be only weakly dependent on solvent. It is evident from Table III, with few exceptions, that only bonds that yield two resonance stabilized radicals upon breaking are likely to undergo significant homolysis under coal liquefaction conditions. 

The markedly negative redox potentials of tris-catecholate and tris-hydroxamate iron complexes (Figure 4) may be ascribed to the high stabilities of the iron(III) complexes and the rather low stabilities of their iron(II) analogues. Table 9 details the relevant data (interconnected by a thermochemical cycle earlier applied to amino acid pentacyanoferrate complexes ), and documents the remarkably higher stabilities of tris-catecholate than of tris-hydroxamate complexes of iron(III). 

An important, but not widely recognized effect in electron transfer chemistry is the phenomenon that the thermochemical stability order of several tautomeric systems can be inverted upon one-electron oxidation. Hitherto, most of the data stem from gas-phase measurements (photoelectron and mass spectrometry data) or from calculations at several levels of theory (Table 2). While the explicit numbers still differ for each method they all agree on the inversion of the stability order for keto/enol, alkine/allene, imine/enamine, nitrile/isonitrile and aldimine/aminocarbene pairs. One-electron reduction, on the other hand, does not necessarily lead to a thermochemical stability inversion, as demonstrated in the case of acetaldehyde 7cthenol [41]. 

Table 2. Inversion of the thermochemical stability order of several tautomeric systems upon one-electron oxidation...

Similarly, the reversal of the thermochemical stability order upon one-electron oxidation has been demonstrated theoretically and experimentally for several heteroatom substituted carbonyl/enol pairs, e.g. esters [52,53] and acids [54,55]. A recent detailed evaluation of the substituent effect by Heinrich, Frenking and Schwarz using ab initio molecular orbital calculations [56] is summarized in Table 3. Both a- and 7t-donors X stabilize the two cationic tautomeric forms, but with Ji-donating groups (X F, OH, NHj) the enol radical cations are much more stable than the corresponding keto ions. On the other hand, with c-donor/rt-withdrawing substituents this thermochemical preference is less pronounced and in the case X BeH the order of relative stabilities of ionic keto/enol pairs is even reverted. 

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