Other Steric Effects

The possibility of forming a six-membered ring does not insure that a strong hydrogen bond will be formed, for other steric effects may... 

Similar to the y-effect, other steric effects such as (3- and 8-effects are also dominated by the a, component (3). In particular, the P-effect is characterized as a... 

Chung, C,-S. J. Chem. EJiic. 1984, 61. 1062-1064. Meyers, R, T, lnor)>. Chem. 1978, 17. 952-958, Ethylenediamine and ammonia are almost identical in field strengths (their / factors differ by less than 3%, Chapter II). and so the LFSE for complexes with the same metal ion will be almost identical. However, ethylenediamine is also a stronger base than ammonia from the inductive effect of the methylene groups. In addition there may be enthalpy differences in the form of ring strain or other steric effects. See Table 12.6 and the following di.scussion. 

The facility and accuracy of empirical calculations for carbon have been exploited through commercial computer programs for general predictions of C chemical shifts. As with calculations (Section 3-2e), the results are only as good as the data set used in their creation. The programs assume that the effects of multiple substitution are additive, unless specific corrections have been incorporated. Unconsidered or nonadditive phenomena, such as conformational and other steric effects, can cause unexpected deviations between observed and calculated chemical shifts. 

The steric effects related to the coplanarity of an activating cen-ter7i 7 i.ioi-i23 will not generally be applicable to azine activating centers. Other steric effects are directly applicable. 

An alternative approach to the definition of localized effect parameters is due to Roberts and Moreland (17) who proposed the use of pA s of 4-substi-tuted bicyclo[2.2.2]octane-l-carboxylic acids, I, to define a constants. In this system the substituent, X is bonded to an sp hybridized carbon atom thus X exerts only the localized effect. As I is a rigid system free from conformational effects, and the substituent X and the active site CO2H are not in proximity to each other, steric effects are not observed. Structures such as I are therefore ideal for the definition of localized effect parameters. The a x parameters were defined from the equation... 

The other steric effect has to do with intramolecular van der Waals repulsions... 

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