Oxygen-18, shielding effect

Access of air and water will also affect the corrosion rate. Metal inserts in corrosive plastics are most actively attacked at the plastic/metal/air interfaces with certain metals, notably aluminium titaniumand stainless steel, crevice effects (oxygen shielding and entrapment of water) frequently accelerate attack. Acceleration of corrosion by bimetallic couples between carbon-fibre-reinforced plastics and metals presents a problem in the use of these composites. 

Size, shape, and density The shielding effects of dendritic shells can likewise be caused by steric factors. Thus, the access of foreign molecules to the central functional unit can be hindered or prevented according to size and density of the dendritic shell. Sometimes, even a certain size selectivity is observed. These effects are especially interesting for electrochemically, catalytically active, redox-and photo-active functional units, since interactions with foreign molecules, such as oxygen quenching of the luminescence (photo-active units) or the access of substrates (catalytically active units) can be influenced.14 11 17,221... 

The isolation of vincadioline (31) from C. roseus has been reported (96) only in the patent literature. The C-NMR data of 31 (76) (Table X) support the 3 -hydroxyvinblastine structure. Both C-2 and C-4 absorptions shifted downfield compared to the corresponding values of vinblastine (1). In addition, upfield shifts of C-1 and C-19 were observed as a result of the y-effect of the new oxygen atom at C-3. The resonance of C-19 shifted 4.8 ppm, a magnitude consistent with an axial hydroxyl at C-3. Such circumstances should lead to a nearly equal shielding effect at C-5, as is indeed observed. These data allow the assignment of the a relative steric position of the C-3 hydroxyl group. 

In contrast, the mannosyl radical 8 does not undergo such a conformational change, and the observed a-attack results from the shielding effect of the axial C-2 substituent in the chair conformation and the stereoelectronic effects mentioned earlier. In radicals 7b and 8 the C-O bonds adjacent to the radical center are coplanar with the singly occupied orbital. This reminds us of the anomeric effect in which an interaction between the nonbonding electron pair of the ring oxygen and the LUMO of the C-O bond stabilizes the conformation. 

In addition to heat-transfer problems, scales can exhibit shielding effects, thus increasing the risk of corrosion through differential oxygen concentration cells. A buildup of scale deposits will also tend to directly impede the flow of cooling water, as well as acting as a key for the accumulation of muds, silt, and biomass. Thus the buildup of a deposit acts as an indirect foulant. 

It is also interesting that a parallel trend is observed for 33S, 31P and 77Se chemical shifts when lone pairs are used to form hypervalent species. When one lone pair is used to coordinate one oxygen atom, S, P and Se chemical shifts increase significantly, which means that the nuclei are more deshielded (as expected, since the electron density around the nucleus decreases). However, in coordinating another oxygen atom, the loss of the second lone pair produces a shielding effect. 

The isolation of the isomeric axial and equatorial sulfoxides (94) and (95), from the preparation of a series of hexahydrothiopyrano[4,3-c]pyrazoles (93), prompted a study (79JOC2513) to determine what effect the introduction of an S—O bond had on the carbon shifts of the methylene carbons a to sulfur. When the oxygen atom is situated in an axial position as in isomer (94), it has a deshielding effect of +17 ppm on the / carbon atoms and a shielding effect on the carbons y (—10 ppm and — 9 ppm) and 5 (— 3 ppm) relative to the carbon atoms in the unsubstituted derivative (93). The trends are similar for the equatorial derivative (94). The magnitude and trends are comparable to reported data on monocyclic thiopyran derivatives. The chemical shifts of the carbon atoms of the sulfone derivative are comparable to the signals in both sulfoxides (94) and (95). 

In the nmr spectrum of oxirane, the protons 3 to the oxygen display a small chemical shift compared to that of the j3 protons of larger cyclic ethers this can only be explained by the shielding effect of the abnormal electron density, which assumes a low electron density around the oxygen, as suggested earlier on theoretical grounds. The proton chemical shifts for various substituted oxiranes are given in a number of reviews and handbooks. ... 

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