Oxygen caveats

This caveat could be circumvented with the use of triisopropylsilyl (TIPS) protecting groups (Scheme 2a,b),12 16 or when 6-deoxy glycals (devoid of the C-6 oxygen substituent) are used as substrates (Scheme 2c,d,e).13,17,18... 

Amine nucleophiles appear generally to add by attack at the allyl ligand. Similar to oxygen nucleophiles, however, a caveat must be noted in evaluating the observed regioselectivities in that the allyl amine products are capable of rearrangement by reentry into the ir-allyl manifold. 

The reaction of amino acids with the hydrated electron occurs at a range of different rates depending upon the side chain. Aliphatic amino acids tend to react very slowly with e q whereas aromatic amino acids react at a somewhat more rapid rate and cysteine has a very facile reaction to lose HS" see Table I. The caveat in these systems is that reaction (5), the reaction of e with oxygen, is diffusion controlled and oxygen is omnipresent in most systems at a relatively high concentration (250 pM in air and 1.2 mM in Oj-saturated water). 

Despite these caveats, oxygen isotopic ratios are probably the most widely used climate proxy in ocean history research. Reasons for this widespread use relate to the history of oxygen isotopes in geological research (see Section 6.14.2), the fact that they can be measured quite precisely by mass spectrometry and are relatively immune, at least in younger deposits, to secondary effects, the fact that records tend to be quite reproducible and clearly record climate variability, and finally, because records have proven so useful for stratigraphic and chronological purposes. 

With the caveat that release of toxic ozone to the ambient atmosphere must be avoided (see Section 5.11.7), disinfection of water by ozonation is a virtually ideal example of green chemical practice. The only raw material is universally available air, which is free. Ozone is produced only where it is needed as it is needed, without by-products. The ozone does not persist in water, where it decomposes to elemental oxygen, and there is very little likelihood of producing harmful disinfection by-products with ozone. 

The reduction of oxygen in aqueous solutions is irreversible in nature in terms of both the chemical and electrochemical processes involved, and hence it is not reoxidised on the reverse pulse, such that the voltammetric wave as measured is not enhanced. Thus in a solution containing a reversible species of interest and oxygen (where both react electrochemicaUy at similar potentials), the use of SWV may allow analytically useftil voltammetric data to be obtained without degassing of the solution. As a caveat, it should be noted that although it is possible to measure an analytically useftil voltammetric response in the presence of oxygen, the chemistry occurring within the solution may be altered by its presence. 

Because the local structure and the three-dimensional water-hydrogen-bond network in gas hydrates is similar to ice Ih, a number of properties of gas hydrates, especially those dominated by the host lattices, are expected to be similar to those of ice, with the caveat that allowance is made for differences between the densities of the hydrates and ice. Thus, the heat capacities, sound velocities, enrichment of heavy isotopes of hydrogen and oxygen in the water framework, and, some spectroscopic, electrical and mechanical properties... 

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