Stability energy factor

Before we proceed to discuss energy changes in detail it is first necessary to be clear that two factors determine the stability of a chemical system—stability here meaning not undergoing any chemical change. These two factors are the energy factor and the kinetic factor,... 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

The behaviour, which is not controlled by the topochemical rule but is greatly influenced by non-topochemical factors, is discussed in Section 2 in terms of molecular mobility, stabilization energy by orbital interaction and energy transfer in the crystals. 

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

The stabilization energy of the formed radical is a very important factor (compare the AH values for the addition to olefins and styrenes). 

The same conclusion is not applicable to the NO+ complexes, in which the magnitudes of the formation constants are much more strongly dependent on the ionization potential of the arene donor (see Fig. 10A). Thus the factor of >104 that separates the formation constant of the benzene complex with NO+ from that of the hexamethylbenzene complex corresponds to more than 5 kcal mol-1 of extra stabilization energy in the... 

In general, the thermal stability of metal-containing polymer systems is relatively enchanced compared to that of the bulk polymer. Various factors including size and concentration of the metal ions, and crystal field stabilization energy of the anchored metal complexes influence the stability to different extents. 

One measure of the resonance component of radical stability in polymerizations is the so-called Q value of the monomer, which quantifies the resonance stabilization of the radical (Stevens 1990). However, the experimentally determined value of Q can be influenced by other factors unrelated to resonance. To evaluate the extent to which their measured Q values were consistent with resonance stabilization of the monomer radical, the authors compared isodesmic energies from Eq. (6.14) to measured Q values for R = Me, rBu, PhO, CN, Ph, vinyl, and phenylethynyl. The largest stabilization energy was computed for the R = phenylethynyl case, about 101 kJ mol-1, although at the HF/3-21G level the expected linear correlation between log(2 and stabilization energy was only fair (R2 = 0.86 a better correlation for the non-phenylethynyl substituents had been obtained previously at a higher level of theory). 

When one attempts to extend these empirical correlations once again, now considering rate constants for vinylcyclopropane to cyclopentene rearrangements, a fair linear correlation is obtained (Figure 4). The correlation line has an intercept of 56.7 kcal mol 1 and a slope of 0.690 (R2 = 0.99). The rate constants utilized were corrected for symmetry (a factor of 1/2 for vinylcyclopropane, and of 1/4 for 1,1-dicyclopropylethene) and the radical stabilization energies of-CH2CR=CH2 for R=Me or cyclopropyl were taken to be identical. The rate constants for vinylcyclopropane to cyclopentene rearrangements respond... 

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