Stabilities of 3* isomers

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

Heat of combustion can be used to measure relative stability of isomers. 

At this point, we have completed the presentation of the key equations which will be crucial to the development of a predictive theory of molecular structure. These equations will form the basis for determining the relative stability of isomers, the relative stabilization of a cationic, radical or anionic center by substituents, etc. On the other hand, the differential expressions (9) to (12) will form the basis for determining how substitution affects the relative stability of isomers, the relative stabilization of cationic, radical and anionic centers, etc. It is then obvious that a working knowledge of Eqs. (1) to (6) presupposes a great familiarity with the key quantities involved in these equations, namely, orbital energies and interaction matrix elements. 

It is obvious from the definition of standard enthalpy of formation that these quantities do not represent the absolute enthalpic stability of compounds. They merely reflect their enthalpic stability relative to that of the chemical elements in standard reference states (to which AfH° = 0 has been arbitrarily assigned). It is thus unreasonable to state that a given substance is more stable than another just because it has a lower standard enthalpy of formation. We can only use AfH° values to make such direct comparisons when we are assessing the relative stability of isomers. 

Figure 4 HPLC Profile of the Progress of the Oxidative Folding of Reduced co-Conotoxin MVIIC and Stability of Isomer 21 in the Presence of Redox Reagents in 50 mM NH4OAc Buffer Containing 2M (NH4)2S04at pH 7.7 (a, c) and in 50 mM NH4OAc Buffer at pH 7.7 (b, d) at 5 °C Aliquots were Analyzed at the Time Shown in Each Paneli891a b...

An MO study of CO insertion into the Me—Ptn bond has considered a number of pathways, and the factors involved have been weighed. Trans influence arguments and the facility of the supporting ligands to migrate between different structures are considered. The relative stabilities of isomers and the potential barrier for isomerizations are investigated the Y-shaped complex is unstable and isomerizes to a T-shape with no barrier. The relative stability of T-shaped complexes is explained by the trans influence effect.603... 

Alder reactions to give the endo adduct as in (18) ->(19) rather than the exo one as in (18) ->(20)] in terms of qualitative perturbation theory. Similar methods were used by Hoffmann and his co-workers in discussion of favourable and unfavourable conformations of conjugated systems,93 and of relative stabilities of isomers.94 All these treatments are based, implicitly or explicitly, on extended Huckel theory,... 

The interest in this area stems from attempts to assess the relative stability of various possible structures of carbonium ions. Therefore absolute values for the heats of formation are not necessarily required. CNDO calculations can thus be used equally well to determine the relative stability of isomers. Such calculations have been performed by Wiberg 45> and are illustrated in Table 17. 

The ab initio reaction energy that is most commonly calculated is simply the difference in ZPE-corrected energies, AE 1, which is the reaction enthalpy change at 0 K (Eq. 5.181). This provides an easily-obtained indication of whether a reaction is likely to be exothermic or endothermic, or of the relative stabilities of isomers. Table 5.9 illustrates this procedure. The results are only semiquantitatively correct, and the HF/6-31G method is not necessarily better here than the HF/3-21G for such direct (simple subtraction) energies. In fact, it has been documented by... 

Bond strengths can be used to estimate the relative stability of isomers that have different bonds. The isomer that has the larger total bond energy is more stable. One of the following isomers is more stable than the other. The less stable one is rapidly converted to the more stable one, so it cannot be isolated. On the basis of bond dissociation energies, which of these two isomers is more stable ... 

Yet another application of the frontier orbitals lies in the field of the relative stability of isomers and conformers. Let us consider propene. 

Figure 2 The order of thermodynamic stability of valence isomers of diazirine (relative stability of isomers) at the MP3/6-31 //3-21G level (89CPH157) and relative energies (kcal mol-1).

In all these applications of the grand potential, the energy which appears in Equation (4.5) is the electronic energy. The nuclear-nuclear repulsion energy does not appear explicitly, since it is supposedly constant. Also, the proof for the PMH is valid for small departures from equilibrium only. Therefore it cannot be assumed that the relative stabilities of isomers can be linked to the relative sizes of the HOMO-LUMO gap. 

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