Anti-shielding

The EFG parameters Vzz and described by (4.42a) and (4.42b) do not represent the actual EFG felt by the Mossbauer nucleus. Instead, the electron shell of the Mossbauer atom will be distorted by electrostatic interaction with the noncubic distribution of the external charges, such that the EFG becomes amplified. This phenomenon has been treated by Stemheimer [54—58], who introduced an anti-shielding factor (1 —y 00) for computation of the so-called lattice contribution to the EFG, which arises from (point) charges located on the atoms surrounding the Mossbauer atom in a crystal lattice (or a molecule). In this approach,the actual lattice contribution is given by... 

From an extensive set of data concerning annelation of cyclopropane rings to cyclopentanes, Christl and coworkers were able to employ y-syn shielding and y-anti shielding correlations to assign structures. Four compounds which contain all four valencies within one hemisphere, inverted carbons , were considered these were 41, 24, 42, 43 and their respective precursors without the cyclopropyl methylene group viz 44, 45, 11, 46. 

For some quadrupolar nuclei with a very small quadrupolar moment, the dipole-dipole mechanism contributes significantly to relaxation, e.g. 7Li. 87Rb has a very large quadrupolar moment and Sternheimer anti-shielding factor, and thus, quadrupolar relaxation dominates completely the same is true of 23Na and 39K. The Sternheimer anti-shielding factor describes how sensitive the NMR relaxation of a nucleus is to its electronic environment.37... 

PbEu (anti-knock agents), radiation and sound shields, in paints, high quality glass. World production 1980 4 0 megatonnes. Lead compounds are toxic and hazardous to health. 

In general, epoxidation of steroids with trans-anti-trans ring fusions leads to exclusive formation of the a-oxirane. Steroid Reactions lists examples of exclusive a-epoxide formation from 2-, 4-, 6-, 7-, 8(9)-, 14-, 16- and 17(20)-unsaturated steroids. Further examples of a-epoxidation of steroid 1-enes, 3-enes, 8-enes, 9(ll)-enes, 8(14)-enes and 16-enes have been reported. The preferred attack by the reagent on the a-side of the steroid nucleus can be attributed to shielding of the -side of the molecules by the two angular methyl groups. 

Plant/process emissions Anti-corrosion coatings heat shields, extractors... 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

When an additional methyl substituent is placed at C(3), there is a strong preference for alkylation anti to the 3-methyl group. This is attributed to the conformation of the enolate, which places the C(3) methyl in a pseudoaxial orientation because of allylic strain (see Part A, Section 2.2.1). The axial C(3) methyl then shields the lower face of the enolate.55... 

An 8-phenylmenthol ester was employed as the chiral auxiliary to achieve enantioselectivity in the synthesis of prostaglandin precursors.83 The crucial features of the TS are the anti disposition of the Lewis acid relative to the alcohol moiety and a tt stacking with the phenyl ring that provides both stabilization and steric shielding of the a-face. 

Lead is also used in organ pipes, of course. Other uses include the lead-acid battery, radiation shielding, ceramic glazes, and in lead glass. It is a toxic element, and its organic derivatives are also toxic. Tetraethyllead was used for many years as an anti-knock agent in petrol. 

Few of these studies (460, 462) dealt with the Michael reaction one study (461) with the Henry reaction. The efficiency, stereoselectivity, and enantiose-lectivity of this process are rather high. The mechanism of the transformations is poorly known. Presumably, the chiral cation should shield the Si surface of nitronate, thus providing the Re approach of the substrate. In addition, the approach of the reagents, resulting in generation of syn isomers, is considered less favorable than the approach yielding anti isomers. 

---