Sequence stability

The thermochemistiy of the Group 13 trihalide complexes has been extensively studied and several stability sequences have been... 

Figure 1.1 shows that the stability sequence revealed by chemical reactions and chemical synthesis corresponds to thermodynamic stabilities. An explanation requires a theory that will explain both. To get it we apply the theory of atomic spectra [9]. The energy of the 4f electrons in an ion with the configuration [Xe]4P, F(4P), can be written [nU+E Q-p(4f )] where U, a negative quantity, is the energy of each 4f electron in the field of the positively charged xenon core, and Frep(4P) represents the repulsion between the n 4f electrons. In Table 1.1, rep(4P) is expressed as a function of the Racah parameters and E. The... 

The simple alkyl carbocations have already been seen (p. 83) to follow the stability sequence,... 

The destabilising influence of the electron-donating inductive effect of alkyl groups is seen in the observed carbanion stability sequence ... 

Hardly surprisingly, it is the exact reverse of the stability sequence for carbocations (p. 83). 

Poly(methyl methacrylate) provides a level of stabilization even though the solution in CCl is below the 0-temperature. All the copolymers, both random and block, are better stabilizers than PMM, the methacrylate units acting as anchors, with stabilizing sequences of styrene loops, of block copolymers, or mixed loops and tails, of random copolymers, at better than 0-conditions. Higher M.W. polystyrenes give silica dispersions too unstable to measure by our optical method the sediment volumes are between those of poly(methyl methacrylate) solutions and pure solvent. 

T sites were T9 and T10 sites—if thermodynamics controls the structure of Ti-containing MFI zeolite. The stability sequence of T sites was found to be T9 > T10 > T12 > T1 > T6 > T5 > T3. The exact location of Ti ions in TS-1 is still controversial. There are no similar investigations for other Ti silicates. 

Until recently only complexes of low stability were known for AC s even in the case of anionic chelating ligands (especially in water Table 7). Numerous and much more stable chelate complexes of AEC s with multidentate anionic ligands are known however (see Table 7). The complexation constants (3) always follow the stability sequence Ca2+ > Sr2+, Ba2+ and are in general difficult to modify in a progressive, stepwise fashion. It is obviously of interest to be able to control both stability and selectivity of AC and AEC complexes, especially the former. 

It is also possible to make some inferences about the nature of the transition state. Fast association rates imply stepwise removal of the solvation shell of the cation by consecutive replacement of each solvent molecule by a ligand binding site, so as to minimize the loss of binding energy in the transition state. The fact that the association rates differ less than the dissociation rates (which follow the stability sequence) could indicate that the transition state is nearer to the reagents than to the complex. Furthermore the slowness of the association could be explained by the operation of the following effects on the way to the transition state ... 

Energies for various possible arenium ions and regioisomeric benzylic cations were computed at the DFT B3LYP/6-31G(d) level or by AMI for comparison with the experimental results. These findings provided further evidence in support of the stability sequence 1-pyrenyl > 4-pyrenyl > 2-pyrenyl in a-pyrene-substituted carbocations as models for the intermediates arising from BaP-epoxide ring opening (Fig. 10). 

Thus the "stability sequence" is equivalent to increasing the intensity of weathering or to completing the process of equilibration under conditions of high water content, low concentration of alkali and silica ions and oxidation of iron in a soil profile. In fact the weathering process in a soil horizon sequence is much the same for pelitic rocks in all environments the dominant sequence is repeated with minor variations due to local conditions of pH or efficiency of the oxidation process. 

The stability sequence for the O-trialkysilyl groups appears to be tert-butyl(tetramethylene)silyl > triisopropylsilyl > tert-butyldi-methylsilyl > isopropyl(tetramethylene)silyl > diisopropylmethyl-silyl. In all cases, the 3 -isomer (secondary position) is considerably more stable than the 5 -isomer (primary position), and this difference in reactivity permits303,304 the synthesis of a 3 -(trialkylsilyl) ether of the nucleoside from the corresponding 3, 5 -bis(trialkylsilyl) ether, in yields of 50%. 

In general, it was found that low temperatures and the presence of a five-membered iminophosphane ring (e.g. an R2R3P=dioxaphospholane ring, or an R2R3P= oxazaphospholane ring, for A ) tend to favour the formation of an aminophosphorane, in accordance with the stability sequence acyclic < monocyclic < spirocyclic. Similarly, one can deduce, from Scheme 28, that if the XZ bond cannot be broken, Z cannot migrate to the N atom and the only possibility will be the equilibrium b. 

More recent studies show some variations to the above generalities. 13C and 23Na NMR studies indicate that for Na+ the complexation sequence is 18-crown-6 > 15-crown-5 > benzo-lS-crown-5,482 hinting at the need for ligand flexibility conduction data reveal that for 18-crown-6 the stability sequence is K+>Rb+>Cs+>Na+ 483 extraction studies using metal picrates show that for benzo-15-crown-5, Na+ > K+ > Rb+ > Li+ > Cs+ for 1 1 complexes and that for the corresponding 1 2 complexes K+ > Rb+ > Cs+ 484 further extraction work indicated... 

Just why this sequence is observed is a more difficult question to answer. Notice in the following stability sequence that alkyl cations are more stable the more alkyl groups there are on the positive carbon ... 

Stability constant determinations have been made on a range of monothio-jS-diketonato complexes in dioxane-water solution. The results indicate that (b) class metal ions form more stable complexes with monothio-/5-diketones than with /5-diketones. The stability sequence for monothio-/5-diketonato complexes is Cu > Ni > Zn > Cd > Pb.198 201... 

This is the reverse sequence to that found for carbonium ion stabilities sequence (5) is explainable in terms of the electron-releasing properties of the various alkyl groups, the greater the electron release ( + 1 character) of the alkyl group the more destabilised is the negative charge on the carbanion. 

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