Factors Influencing Shielding

It is now generally agreed that variation of the paramagnetic term, a , [Chapter 3, equation (10)] is the dominating factor for all four elements, and discussion usually centers on the effects of changing the radial expansion terms, the electron 

In molecules in which the metal atom is part of a ring system the silicon, tin, or lead chemical shifts are often found to be anomalous when compared with those of their acyclic counterparts. This behavior is particularly well documented for tin and lead for species such as and it appears that a reduction of the cyclic inter- 

Incorporation of the silicon atom in a four- or five-membered ring decreases its nuclear shielding by 15-20 ppm and it has been suggested that comparison of the behavior in four- and five-membered rings can provide an indication of the amount of angle strain in the former. In contrast with this, however, incorporation of silicon in a three-membered ring increases the shielding by some 40-60 ppm.  

It is well established that an increase in coordination number from four to five or six will lead to significant increases in the shielding of tetravalent silicon, tin, and lead. The limited available evidence indicates that this is also true for germanium in that the germatranes IX with five-coordinate germanium have ( Ge) some 20-40 ppm 

Exceptionally high-frequency shifts are found for tin bound to one or more first row transition metals, and these have been attributed to a reduction of JE [equation (10), Chapters] resulting from dn-dn overlap between tin and the transition metal. Such interaction is less favorable for silicon, and silicon chemical shifts apparently do not show such large effects, although XIV provides an exception to this with 5( Si)= -1-173 ppm. Similarly, dn-dn overlap is less efficient between tin and a second or third row transition metal, with the result that shifts to relatively low frequency are found.  

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There is substantial overlap between discussing intermolecular effects and shielding surfaces in the solid state. Identification of the nature of these interactions, however, can remain separate. Hydrogen bonding appears to be the major factor influencing and shielding tensors in the two... 

In lH,6H-pyrrolo[2,3-e]indole (1), the reactivity of the P-positions in the two different pyrrole rings are not the same. Substitution in the 8-position is difficult owing to shielding by the neighbouring pyrrole ring. It was interesting to elucidate how these factors influence the electrophylic substitution reaction for an equimolar reactant ratio. It was established that three monosubstituted products are formed 3-, 8- and 2-formylpyrroloindoles 34-36 (Scheme 8) [16]. 

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... 

As already stated, theoretical calculations are as yet too involved and not accurate enough for routine 13C NMR spectral interpretation, particularly where more complicated molecules are concerned probably this situation will not change in the near future. Therefore, different approaches have been developed to identify certain influences of structural modifications, for example substituent introduction, on 13C shieldings. The partition of oA into two local (o A and o A) and several nonlocal contributions (30,32) is reasonable on physical grounds. On that basis, the factors that play a role in organic molecules were divided into two classes (56) ... 

Gentile had been stabbed in the back on his left side, possibly a misdirected attempt to find his heart in near darkness. An assassin in a crowd will try to position his own body to shield his actions from other people, which in this case suggested a left-handed killer getting directly behind his victim. Zorzi was left-handed, and that might well have been another factor that influenced the Ten in reaching their verdict. 

The kinetics of these reactions in comparison with those for methylenecyclo-propane analogs of compounds 160 have been studied by following the progress at pressures up to 3 kbar by on-line FT-IR spectroscopy [129]. The rate-enhancing influence of the additional strain in 160 overcompensates the expected retarding effect of the increased steric shielding by the second cyclopropane unit in 1 compared to methylenecyclopropane, and the cyclization rates for compounds 160 were faster by a factor of 6.8 to 8.1 in comparison with the corresponding methylenecyclopropane derivatives. 

A variety of concave pyridines 3 (Table 1) and open-chain analogues have been tested in the addition of ethanol to diphenylketene (59a). Pseudo-first-order rate constants in dichloromethane have been determined photometrically at 25 °C by recording the disappearance of the ketene absorption [47]. In comparison to the uncatalyzed addition of ethanol to the ketene 59a, accelerations of 3 to 25(X) were found under the reaction conditions chosen. Two factors determine the effectiveness of a catalyst basicity and sterical shielding. Using a Bronsted plot, these two influences could be separated from one another. Figure 4 shows a Bronsted plot for some selected concave pyridines 3 and pyridine itself (50). 

The same arguments should hold well for C and H solutes in the case of CHCI3, because their individual masses are considerably different (C = 12 g/ mol and H = 1.008 g/mol), which means that H is more free to move than C and therefore the solvent influence on H is likely to have a dominant role in the determination of the C-H bond friction. Furthermore, C is actually shielded by the presence of three Cl atoms, a factor that has not been considered here. However, this is not expected to be serious because the dephasing is more sensitive to the friction dynamics of the H atom. The time-dependent friction profiles for H and C show similar strong bimodal behavior as in the case of CH3 and I systems. 

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