Debating exact nature

From the synthetic investigations that have been described in the previous schemes, an appreciation of the mechanism for these reactions (methods A-H) has emerged in our group. However, the characteristics and exact nature of the o-quinone methide intermediate are still debated. Our past observations clearly indicate the cascade leading to the reactive species that behaves as an o-quinone methide should behave is... 

This is not the only subject of debate the nature of the adsorbed forms of oxygen and reaction products on the electrode surface has been widely discussed, as well as the various steps of ORR. Very often, for conditions apparently similar, for example, the same electrode and solution, observations made by two research groups are different and so are, of course, the deductions and the pathways for the electrode reaction. There are so many possible steps, reactions and species that various combinations can be envisaged moreover, as other methods for observing the species or intermediates involved in the electrode reaction are rare, electrochemistry is often the only source of experimental facts. As for other multiparametric problems, when a plausible explanation is found, there is no certainty that this is the only possibility and the correct solution. Moreover, experimental conditions that look identical are not really exactly similar the problem... 

The principal routes of penetration are thus transcellular and intercellular. Currently there is considerable debate as to which of these predominates. Work with esters of nicotinic acid has shown that the intercellular channels are significant [5.] and considerable effort is being conducted to identify their exact nature and role. Microscopic examination shows that they contain structured lipids the chemical nature of which is complex [6J. Cholesterol esters, cerebrosides and sphingomyelins are present in association with other lipids in smaller concentrations. It is likely that the main barrier to skin penetration resides in the channels and that a diffusing drug molecule experiences a lipid environment which has considerable structure. Penetration enhancers may act by temporarily altering the nature of the structured lipids, perhaps by lowering their normal phase transition temperature which occurs around 38°C. 

This mechanism, although understandable in a conceptual sense, is not fully understood in a mechanistic sense. For instance, the exact nature of active species and the role of the activator and/or counterion is a subject of debate this concerns methylaluminoxane-activated group 4 metallocene systems in particular. Methylaluminoxane may act to generate the active species and remove impurities from the polymerisation system as well as playing a more fundamental role such as assisting in the insertion of each monomer unit or reactivating dormant sites [358]. 

Extraframework aluminum species created by mild steaming were shown to increase the catalytic activity of zeolites. This increase in activity was ascribed to the creation of sites exhibiting enhanced acidity through interaction of bridging hydroxyl groups (BrOnsted sites) and neighboring small extraframework aluminum species (Lewis acid sites) (16). Their exact nature is still a matter of debate, but is schematically represented as ... 

The exact nature of the environmental hazards generated by the release of various synthetic chemicals into the environment is under constant debate. There is little doubt that this debate will continue until science unequivocally resolves the uncertainties in toxicological data (exposure, fate, and transport) and risk analyses. 

Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state NMR spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by CP/MAS solid-state NMR spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-<(>, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). 

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