Low thermodynamic stability

Oparin and his school in the Biochemical Institute in Moscow worked for many years with coacervate systems. For Oparin, the origin of life was the moment of formation of the first cell. Coacervates are today of only historical importance because of their low thermodynamic stability, they are considered dubious and too unstable. 

Cycloadditions selectively afford the adducts on the 6,6-ring junctions [65], and the products occasionally undergo a facile retro-Diels-Alder reaction as a consequence of the low thermodynamic stability of the adduct. Very stable Diels-Alder cycloadducts have, however, been prepared by use of different substituted o-quinodimethanes, probably because of stabilization by aromatization of the resulting adducts [66],... 

The largest of all isolated Era moieties characterized by an X-ray structure is S20, which contains the unique ring structure shown in Figure 33.83>88>90>93 Unexpectedly, for an even-membered homocycle this S20 contains two long bonds at 210 pm that are flanked by short bonds (202 pm) and smaller enclosed torsion angles of 66° (see before S7, S9, Sn and S13). The long bond may also be responsible for the low-thermodynamic stability of dissolved S20. 

The relatively low thermodynamic stability of complexes of hemicarcerands or other container-type hosts is a direct consequence of structural aspects of the walls that make up the inner surface of such compounds. These walls are lined by aromatic subunits while free electron pairs of heteroatoms such as those of the ether oxygen atoms are preferentially oriented to the outside. Complexes are therefore enthalpically stabilized only by weak dispersive interactions. In the case of positively charged guests cation-re interactions can contribute to binding enthalpy as in a self-assembled calixarene-derived capsule [9], but directed interactions such as hydrogen-bonding interactions are usually absent. 

The Mulliken arguments may be extended to charge-transfer crystals. The first solid charge-transfer crystals studied were colored, but had low thermodynamic stability relative to the separate neutral donor and acceptor crystals. However, if the donor D had low ID and if the acceptor had high Aa ... 

Beside XeOs and Xe04, some xenon oxyfluorides are also thermally unstable compounds. They may detonate, particularly at higher temperatures. Other derivatives of the xenon and krypton fluorides involving ligands less electronegative than fluorine should also be assmned to be of low thermodynamic stability. Many derivatives such as perchlorates and trifluoroacetates are known to be explosive. 

The likelihood of dechelation depends on both the thermodynamic and the kinetic stability. A complex with a low thermodynamic stability will be dechelated easily when there is a change in pH. A low kinetic stability implies that Ca(II) or other endogeneous cations can easily free Gd(III) from its chelates. Clinical CAs are thermodynamically stable with a log/feq of 17-26 (Table 10.2), and the kinetic stability of cyclic CAs is higher than that of acyclic CAs. Among the clinical CAs, Gd-DOTA has the highest thermodynamic and kinetic stability. 

The intrinsic low thermodynamic stability of most formyl derivatives of transition metals (with respect to the decarbonylation product) is confirmed by the observation that when these compounds are prepared by an indirect route, they normally undergo decarbonylation. For example, a formyl derivative of iron(O) was synthesized by reacting [Fe(CO)4] " with acetic-formic anhydride [reaction (c)] however the resulting product [reaction (d)], decarbonylated slowly (ti/2 12 days at 25°C) ... 

Despite this low thermodynamic stability the permanent existence of a single secondary amide peptide bond in cis conformation per 1000 amino acid residues is the minimal population that has to be considered for unfolded polypeptide chains. This cis peptide bond fluctuates across the polypeptide chain in relation to the sequence-specific propensity of a secondary amide peptide bond to adopt the cis conformation. As could be found in folded proteins, nonprolyl cis peptides are frequently located in the fS-region of a q>/y/ plot [22]. It was hypothesized that cis peptide bonds represent high-energy structures able to store potential energy for increasing chemical reactivity [23]. Interconversion rates for the reversible CTI of secondary amide peptide bonds typically lead to half times of about 1 s for dipeptides, which decreases about 4-fold when the peptide bond is positioned in the middle of a longer peptide chain. 

Silicenium ions received some theoretical attention, but to a smaller extent than silylenes. Interest centered on their thermodynamic stabilities relative to the analogous carbenium ions and on substituent effects. These are important questions, because the low thermodynamic stability of silicenium ions was believed by many workers to be one of the major factors frustrating their experimental preparation258. 

Structures (A) and (B) can be eliminated immediately because they are not electrostatically feasible. These two structures are cis and trans forms of the same compound which juxtaposes positive charge - an unlikely occurrence. The peroxide linkage (0-0) has low thermodynamic stability and makes structure (C) improbable as the arrangement of the dimer. Structure (D) seems to be the correct form for the dimer in that it has resonance stabilization and is electrostatically feasible. The electron arrangement, however, does not make the geometry of the molecule clear. 

For the two-electron process considered, the wave should have two distinct transition times if A" > 10 (for a soluble product) or/T/4C >10 (for an insoluble product). The curves should have characteristic distortions at 10" 2 < Z < 10 or 10 < K/AC <10 accordingly. Only at lower constant values the process should look as the undistorted single two-electron step. That means, at sufficiently low thermodynamic stability of the intermediate and no kinetic limitation of the reaction (2.56), the stepwise character of the process cannot be revealed experimentally. 

Fig. 3.8, or higher. The Si(III) intermediates are not traced in the experiment because of low thermodynamic stability in cryolite media. 

Varying kinetic dependences on hydroxide ion concentration have been identified in the oxidation of mandelic acid by osmium(vm). Although extensive data are available on osmium-catalysed systems, relatively little is known on the nature of the ion as an independent oxidant. The reaction mechanism may be written as in Scheme 6. The kinetic order is unity with respect to oxidant and organic substrate, indicating an ester intermediate of low thermodynamic stability. The change in order of [OH ] from two to zero on increasing hydroxide ion is not, however, satisfactorily explained. 

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