Generalized shields

It can be seen that electron-donating groups generally shield, and electron-withdrawing substituents generally deshield all of the fluorine... 

The iminotriphenylphosphorane series (1) gave a plot with a positive slope (3.19) showing that the groups which increase electron availability at the ylidic nitrogen cause shielding of the phosphorus nucleus. The statistical parent chemical shift 6P° was 3.30. This parameter relates to the general shielding of the specified series and is usually close to the chemical shift of the unsubstituted compound. 

Figure 1. Calculated absorption lineshapes and representative C-13 spectra for polycrystalline samples (a) an axially symmetric chemical shielding tensor (o. and

The 33S substituent chemical shift can therefore be rationalized in the same way as for the carbonyl carbon in carboxylate groups. When R is unsaturated (CH=CH2 C6H5, etc.) the 33S resonance is generally shielded relative to R = CH3. This was rationalized104 in terms of electron release from the conjugated 7c-system to the sulphur atom. There is still much work to be done in 33S NMR. 

Prior to 1985, Mo(III) complexes had not been studied by Mo NMR but Enemark and coworkers have since reported data for several triply bonded dinuclear Mo(III) species (see Table 17). The general shielding trends follow the orders ... 

Concrete, as discussed previously, is also a good attenuator of gamma rays and is superior to water. This is mainly a result of the presence of moderately high mass number elements, such as calcium and silicon. As a general shield material, there is much to recommend about concrete it is strong, inexpensive, and adaptable to both block and monolithic types of... 

Many sunscreens contain more than one component to filter out different regions of the UV spectrum. Conjugated compounds generally shield the skin from UV-B radiation, but often have little effect on longer-wavelength UV-A radiation, which does not bum the skin, but can still cause long-term damage to skin cells. 

Generally, shielding gas is not supplied to the welding torch when welding titanium in a metal chamber, and excellent welds can be made if the chamber atmosphere is maintained properly. In some applications, however, where heavy or long welds are required, gas is supphed to the torch to improve shielding. 

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