Oxide model systems

The now classic Farmer-type hydrogen-abstraction Initiation of free radical autoxldatlon accounts for a large portion of the nonenzymlc oxidations of n-3 fatty acids (45). Because fish lipids contain substantial concentrations of EPA and DHA (47-48), they provide many allowed sites (18, 22, 45, 46, 49) of hydroperoxide formations, and thus can account for a large array of decomposition products. Oxidizing model systems of unsaturated methyl esters of fatty acids yielded monohydroperoxides, but also produce dlhydroperoxldes that are formed by cycllzatlon of Intermediate hydroperoxy radicals when suitable H-donatlng antioxidants are not present to quench the free radical reaction (45, 50, 51). Decomposition of monohydroperoxides of fatty acids In model systems yields a very different profile of lower molecular weight products than observed for similar decompositions of dlhydroperoxldes of the same fatty acids (45, 46). 

In order to prepare oxide model systems well-suited for characterization by high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), AFM or X-ray photoemission spectroscopy (XPS), as well as for kinetic studies by gas chromatography (GC), oxide films and oxide nanoparticles were vacuum-grown on a crystalline soluble substrate (e.g., NaCl(OOl)) via oxide (or metal) evaporation in a low background pressure ( 10" Pa) of oxygen. 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other paHadium-cataly2ed oxidations of olefins ia various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Chemical Antioxidant Systems. The antioxidant activity of tea extracts and tea polyphenols have been determined using in vitro model systems which are based on hydroxyl-, peroxyl-, superoxide-, hydrogen peroxide-, and oxygen-induced oxidation reactions (109—113). The effectiveness of purified tea polyphenols and cmde tea extracts as antioxidants against the autoxidation of fats has been studied using the standard Rancimat system, an assay based on air oxidation of fats or oils. A direct correlation between the antioxidant index of a tea extract and the concentration of epigallocatechin gallate in the extract was found (107). 

Biological Antioxidant Models. Tea extracts, tea polyphenol fractions, and purified catechins have all been shown to be effective antioxidants in biologically-based model systems. A balance between oxidants and antioxidants is critical for maintenance of homeostasis. Imbalances between free radicals and antioxidants may be caused by an increased production of free radicals or decreased effectiveness of the antioxidants within the reaction system. These imbalances can be caused by the radicals overwhelming the antioxidants within the system, or by an excess of antioxidants leading to a prooxidant functionaHty (105—118). When antioxidant defense systems are consistently overwhelmed by oxidative reactions, significant damage can... 

In this work the development of mathematical model is done assuming simplifications of physico-chemical model of peroxide oxidation of the model system with the chemiluminesce intensity as the analytical signal. The mathematical model allows to describe basic stages of chemiluminescence process in vitro, namely spontaneous luminescence, slow and fast flashes due to initiating by chemical substances e.g. Fe +ions, chemiluminescent reaction at different stages of chain reactions evolution. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

This chapter presents detailed and thorough studies of chemical synthesis in three quite different chemical systems zinc ferrite, intermetallic, and metal oxide. In addition to different reaction types (oxide-oxide, metal-metal, and metal oxide), the systems have quite different heats of reaction. The oxide-oxide system has no heat of reaction, while the intermetallic has a significant, but modest, heat of reaction. The metal oxide system has a very large heat of reaction. The various observations appear to be consistent with the proposed conceptual models involving configuration, activation, mixing, and heating required to describe the mechanisms of shock-induced solid state chemistry. 

To return to a more historical development the mercuric acetate oxidation of substituted piperidines (77) should be discussed next. This study established that the normal order of hydrogen removal from the aW-carbon is tertiary —C—H > secondary —C—H > primary —C—H, an observation mentioned earlier in this section. The effect of substitution variations in the piperidine series can be summarized as follow s l-mcthyl-2,6-dialkyl and 1-methyl-2,2,6-trialkyl piperidines, as model systems, are oxidized to the corresponding enamines the 1,2-dialkyl and l-methyl-2,5-dialkyl piperidines are oxidized preferentially at the tertiary a-carbon the 1-methyl-2,3-dialkyl piperidines gave not only the enamines formed by oxidation at the tertiary a-carbon but also hydroxylated enamines as found for 1-methyl-decahydroquinoline (48) (62) l-methyl-2,2,6,6-tctraalkyl piperidines and piperidine are resistant to oxidation by aqueous mercuric acetate and... 

The results observed in the oxidation of alkaloids which indicated something of the stereochemistry required for oxidation and prompted studies on model systems can now be interpreted more confidently. However, care must be used when basing steric differentiation on mercuric acetate oxidation studies since conditions must be employed which avoid epimerization at carbons alpha to the nitrogen. 

The oxidation of unsymmetrical tertiary amines with mercuric acetate may also lead to isomeric enamines. In such cases, structures can often be established by NMR and IR spectra of the enamines and their corresponding imonium salts, through comparison with model systems (202-205). 

With the objective of producing model systems to mimic the metal oxide surfaces of catalysts, a great deal of effort has been devoted to the preparation of large polymetallate structures. 

Nicolaou in his model system for an approach to the thiopeptide antibiotic thiostrepton, in particular, the elaboration of the quinaldic acid moiety. The tetrahydroquinoline 21 was converted to the A-oxide by /n-CPBA oxidation. Subsequent treatment with TFAA, to carry out the Boekelheide reaction, was followed by hydrolysis of the resultant ester to produce 22 as a mixture of alcohols. 

The important commercial feature of these juices, especially significant with blackcurrant and tomato juices, is their ascorbic acid (or vitamin C) content, of which loss by oxidation is known to be accelerated both by heat and by metal (particularly copper) contamination. The effect of copper has been carefully investigated for pure ascorbic acid", and more recently ascorbic acid in blackcurrant juice and model systems. There are, however, oxidation inhibitors of different kinds (which may themselves be heat-sensitive) present in various fruits, which give differing results. The presence of metals will also affect flavours", may cause discoloration, and may give rise to clouding effects, as in apple juice. ... 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Parallel ketonization of acetic acid and propionic acid was one of the transformations of this type studied in our Laboratory. Ryba6ek and Setinek (94) investigated the kinetics of these reactions in the gaseous phase at 316°C using thorium oxide on activated carbon (p. 27) as the catalyst. This model system allowed the study of each reaction separately as well as of the simultaneous conversion of both acids. 

Addresses of suppliers of catalyst pastes included in Table B.l are presented below. Other companies (e.g. Johnson-Matthey) may also supply similar products. The suitability of these products for preparing catalyst films for electrochemical promotion studies should be tested on the basis of the requirements already mentioned. A useful approach before proceeding with the study of new systems is to try to reproduce results of electrochemical promotion studies in model systems, such as ethylene oxidation on Pt, which has been thoroughly studied. It has to be pointed out that in general suppliers do not provide calcination procedures or the provided calcination procedures aim to the production of very dense and non-porous films not necessarily suitable for electrochemical promotion studies. 

Park, E. Y. Murakami, H. Matsumura, Y. (2005). Effects of the addition of amino acids and peptides on lipid oxidation in a powdery model system. Journal of Agricultural and Food Chemistry, Vol. 53, No. 21, (September 2005), pp. 8334-8341 7, ISSN 0021-8561. 

In 1990, Schroder and Schwarz reported that gas-phase FeO" " directly converts methane to methanol under thermal conditions [21]. The reaction is efficient, occuring at 20% of the collision rate, and is quite selective, producing methanol 40% of the time (FeOH+ + CH3 is the other major product). More recent experiments have shown that NiO and PtO also convert methane to methanol with good efficiency and selectivity [134]. Reactions of gas-phase transition metal oxides with methane thus provide a simple model system for the direct conversion of methane to methanol. These systems capture the essential chemistry, but do not have complicating contributions from solvent molecules, ligands, or multiple metal sites that are present in condensed-phase systems. 

NICOLI M c, CALLIGARIS s and MANZOCCO L (2000) Effect of enzymatic and chemical oxidation on the antioxidant capacity of catechin model systems and apple derivatives , JAgric Food Chem, 48 (10) 4576-80. 

SEVERINI c and lerici c r (1995) Interaction between Maillard reaction and lipid oxidation in model systems during high temperature treatment , Ital J Food Sci, 1 (2) 189-96. 

Hu, C. et al., Black rice (Oryza sativa L. indica) pigmented fraction suppresses both reactive oxygen species and nitric oxide in chemical and biological model systems, J. Agric. Food Chem., 51, 5271, 2003. 

Figure 2 shows the results of heating a model system consisting of a 30 X platinum film on oxidized titanium. A linear temperature ramp was applied until the foil reached 760 K, after which the temp-... 

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