Stabilizer cooperative effects

When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

Zinc hydroxide and alkoxide species are particularly relevant to catalytic processes, often forming the active species. The cooperative effects of more than one zinc ion and bridged hydroxides are exploited in some enzymatic systems. Zinc alkyl phosphate and carboxylate materials have been important in the formation of framework compounds, often containing large amounts of free space for the inclusion of guest molecules. Aldehyde and ketone compounds are of low stability due to the poor donor capabilities of the ligands however, a number of examples have recently been characterized. 

Guo, H., and M. Karplus. 1994. Solvent Influence on the Stability of the Peptide Hydrogen Bond A Supramolecular Cooperative Effect. J. Phys. Chem. 98, 7104-7105. 

The bisanthraceno-crown ether E-l (Figure 10.26) exhibits a fluorescence spectrum composed of the characteristic monomer and excimer bands. Gradual addition of sodium perchlorate to a solution in methanol induces a decrease in the monomer band and an increase in the excimer band. Complexation is indeed expected to bring closer together the two anthracene units, which favors excimer formation. A 2 1 (metabligand) complex is formed with Na+ in methanol and acetonitrile with a positive cooperative effect (see Appendix B). Interestingly, the overall stability constant obtained from absorption data was found to be lower than that... 

A recent report on bimetallic ytterbium bis(amides) 116 and 117 stabilized by a flexible bridged bis(phenolato) ligand claimed a straightforward polymerization of L-lactide. The conversion is characterized by short reaction times (30 min to 2 h) and PLAs displaying broad PDls (1.71-2.24). The reason for the apparent enhancement of the activity found in the bimetallic lanthanide amides is tentatively explained by cooperative effects within these bimetallic systems [114]. 

Other approaches to the use of these complexes in practical systems for the utilization of solar energy include (i) the creation of appropriate chemically linked systems (for example, Ru(bpy)2(py)2+ units bound to a poly-vinyl-pyridine resin) with the aim of obtaining cooperative effects in the electron transfer reactions197 and (ii) the stabilization of an excited state or a primary redox product for a time long enough to allow the absorption of a second photon. 

Dialkyl-substituted anthracene-bridged bis-crown ethers 92 formed 1 1 and 1 2 (crown to metal) complexes with alkali metal ions <1999J(P2)1193>. The stability constants of the latter were suggestive of a negative cooperation effect between the two crown ether units < 0.25). The derivative with R = Et showed a decrease in... 

Stability of the Peptide Hydrogen Bond A Supramolecular Cooperative Effect. 

In principle, two (possibly cooperative) effects could be responsible for the observed pronounced substituent effect on the automerization rate different stabilization (or destabilization) of the metallacyclic topomeriza-... 

Guo, H. and Karplus, M., Solvent influence on the stability of the peptide hydrogen bond A supramolecular cooperative effect, J. Phys. Chem. 98, 7104—7105 (1994). 

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