Stability correlations

Two schemes have been proposed which systematize the available stability constant data. In addition to data contained in the compilations given at the end of this chapter, more qualitative evidence, based for example on the results of displacement reactions, has been included in arriving at the generalizations. Historically, the first scheme is that due to Chatt and Ahrland who pointed out that electron acceptors may be placed in one of three classes. Class-a metals, the most numerous, form more stable complexes with ligands in which the coordinating atom is a first-row element (N, O, F) than with those of an analogous ligand in which the donor is a second-row element (P, S, Cl). Class-b has the relative stabilities reversed. It is not difficult to extend the stability relationships to include heavier donor atoms. Class-a behaviour is, then, typified by a stability order 

Class-b behaviour is rather more complicated and is typified by relative stability constants in the order 

In addition, there is a third class of electron acceptor for which the stability constants do not display either class-a or -b behaviour uniquely. The class-a/class-b classification of some metal ions is given in Table 5.2 (normal valence states are assumed). 

Although not included within the above classification, there are some other useful gradations which have been noted and are conveniently included at this point. For a given ligand, corresponding stability constants of complexes of bivalent ions of the first transition series are usually in the natural order (sometimes called the Irving-Williams order)  

Copper(II) does not coordinate a fifth and sixth ligand particularly strongly, and this order is incorrect for stability constants relating to CuLs and CuL complexes. Complexes of chelating ligands also tend not to follow this order. 

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Petri S, Dueck A, Lehmann G et al (2011) Increased siRNA duplex stability correlates with reduced off-target and elevated on-target effects. RNA 17 737-749... 

Effect of Water Activity. A preliminary study was done to determine the a at which encapsulated orange peel oil was the most stable to oxidation. Figure 1 summarizes the results of this study. The formation of the limonene oxidation product, limonene oxide, was the slowest for the powder adjusted over Mg(NO3)2 (a 0.536). While the levels of oxidation product do not follow in exact order of a, it is evident that better storage stability correlates with a higher a of the powder. This relationship was not anticipated. Literature on lipid oxidation (2, 2) indicates that there is an optimum a for product... 

Based on analysis of over 800 coins, there appears to have been a destabilization of the Tyrian shekel in about 44 B.C.E. and a re-stabilization in about 13 B.C.E. The re-stabilization coincides with the appearance of the control mark KP (or KAP) and which continues throughout the remaining duration of these coins. The destabilization correlates closely with the instability of the Roman Empire prior to the rise of Augustus Caesar, and the re-stabilization correlates to the re-stabilization of Roman mints and with interactions between Augustus, Herod, and the Temple authorities in Jerusalem. 

Different acetylenic diethers have been prepared, either from glyoxal (method B) or from dioxane (method A) their stability correlates well, in a... 

The absence of electronwithdrawing properties and small steric volume of hydrogen substituents are the reasons for the higher stability of cation 36. For 1-substituted analogs of F-allyl and F-methallyl cations (existing as a mixture of cis- and trans-isomers, except 43, X=H for which only czs-isomer was found in solution), stability correlates well with electronegativity of the substituent [63] ... 

The greater the degree of resonance stabilization, the less reactive is RCOG. Thus, NH2 has the greatest degree and RCONH2 is the least reactive, while X has the smallest degree and RCOX is the most reactive. It so happens that the order of resonance stabilization correlates directly with the order of basicity e.g. —N of amide is the best resonance-... 

An experimental study of diacetylene and HF in solid argon suggested both sorts of complexes (a and b in Fig. 6.6) were present and that they are of comparable stability. Correlated (MP2) calculations with a 6-31 -I- -l-G(d,p) basis set found the perpendicular complex (a in Fig. 6.6), wherein FH approaches one of the two triple bonds of diacetylene, is more stable than is complex b wherein C—H acts as proton donor. The electronic contributions to the binding energies of complexes a and b are calculated to be —3.8 and —2.6 kcal/rnol, respectively. However, these values are surely inflated by the failure to correct them for BSSE. One can conclude that the triple bond is a better proton acceptor than the alkynic C—H is a donor, at least when paired with the rather strong acid HF. The preference for this sort of geometry is confirmed by gas-phase measurements, and are valid also when HF is replaced by HCl . The importance of using a satisfactory level of theory for such complexes is reinforced by comparison with earlier SCF-level calculations which predicted a structure like b to be most stable. 

R. Bruce Martin (Inorg. Chim. Acta 1998, 283,30-36) points out the limitations of HS AB theory, showing that metal ion stabilities correlate better with electron affinities than with hardness and softness. Still, this is a good qualitative general rule. 

Experimental data" " show that the emulsion stability correlates well with the lifetime of separate thin emulsion films or of drops coalescing with their homophase. To simplify the treatment we will consider here the lifetime of a single drop pressed against its homophase under the action of gravity. To define fhe lifetime (or drainage time) x, we assume that in the initial and final moments the film... 

The major emphasis of this discussion centered on the importance of surface and subsurface soil characteristics in influencing deep pesticide leaching. Some factors, such as the depth to groundwater and the amount of incipient rainfall or irrigation, are clearly important factors that do affect the probability of pesticide residues reaching groundwater. Similarly, properties of the pesticide itself (especially its inherent mobility and chemical/biological stability) correlate closely with pollution potential, but their evaluation is outside the scope of this review. 

Nuclear stability correlates with the ratio of neutrons to protons in the isotope. For example, for light atoms a neutron proton ratio of 1 characterizes a stable atom. 

The stability of radical-cation salts varies from rapid decomposition in air (e.g., naphthalene salts) to stability for several months under ambient conditions (e.g., perylene and decacyclene salts). The stability correlates with the oxidation potential As a rule of thumb, salts of extendend aromatic systems are more stable than those of smaller ones, and salts of the ideal 2 1 composition are more stable than those of other compositions. 

This case has additional convenience in the possibility of using colored anion (commonly picrate), which is useful for monitoring distribution but is inert with respect to complexation. This technique allowed the determination of a large body of association constants, thus providing a basis for further structure-stability correlations, and Cram s picrate method became a de facto standard. (One experienced in metal ion extraction may note that this approach evidently parallels the well-known distribution method for determining chelate stability constants [34. )... 

Sjoblom and his research team have probed the causes of emulsion stability in a series of North Sea crude oils and observed that emulsion stability correlates with asphaltene, wax, and resin/asphaltene ratio [11]. In their early work, they theorized that asphaltenes may aggregate through stacking interactions mediated by aromatic 7r-bond overlap to form lamellar structures that stabilize the emulsions. Moreover, they further observed... 

Ciesielski, A., Krygowski, T. M., Cyranski, M. K., Dobrowolski, M. A., Balaban, A. T. (2009). Are thermodynamic and kinetic stabilities correlated A topological index of reactivity toward electrophiles used as a criterion of aromaticity of polycyclic benze-... 

Because of the different sizes of specimens, different stability correlation ratios are used, which can be found in Asphalt Institute MS-2. 

As a continuation of the tests discussed above, the permeability of the treated cores are measured periodically over the course of several weeks to several months. In order to develop a gel stability correlation, beaker samples can be also monitored over the same time frame to identify syneresis or gel strength degradation. 

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