Chemical shift steric effects

No Vg effect has to be considered for the CH3 carbon atoms. So it is reasonable to explain the splitting solely by the y-gauche effect. As can be seen from Fig. 14 the steric arrangement of the two CH3 carbon atoms within the segment differs by one y-gauche position of the carbons opposite of the CH—CH bond. With respect to the other carbons the steric situation is equivalent. Hence, the chemical shift... 

They are applicable to compounds in the common NMR solvents - but not in D6-benzene (or D5-pyridine). The substituent effects are additive, but don t place too much reliance on chemical shifts predicted using the table, in compounds where more than two groups are substituted next to each other, as steric interactions between them can cause large deviations from expected values. Note that Table 5.4, like all others, does not cater for solvent shifts, etc ... 

For the methyl-substituted ethylenes, i.e. in the absence of any steric effects, there is a roughly linear relationship between the chemoselectivity and the 13C nmr chemical shift of the most substituted carbon atom of the bromonium ions (Dubois and Chretien, 1978). This selectivity is therefore discussed in terms of the magnitude of the charge on the carbon atom and the relative hardness of the competing nucleophiles, according to Pearson s theory (Ho, 1977). However, this interpretation does not take into account the substituent dependence of the nucleophilic solvent assistance, which must play a role in determining this chemoselectivity. 

A substantial divergence from the behaviour described up to now occurs with the 2,6-(V,(V-tetramethyl anilines (Table 7). The effect of the para substituent is no longer systematic, and no good correlation with substituent electronic properties is displayed. Evidently, the extent of nitrogen lone-pair delocalization in these compounds reflects a balance between electronic demand for, and steric inhibition of, delocalization that varies with the individual compounds. Nitrogen chemical shifts are a sensitive probe for these effects. 

The effect on 15N and 13C chemical shifts in anilines75-77 and nitrobenzenes74 of a reduced resonance interaction caused by ortho substituents which sterically twist the amino and nitro groups from coplanarity with the aryl ring has been extensively investigated and is fairly well documented. 

Further doubt about the validity of the original Grant-Cheney model was expressed by Seidman and Maciel (185), whose INDO calculations of proximity effects in hydrocarbons revealed that there is no simple correlation between carbon chemical shifts and calculated electron-density increases caused by steric C-H bond polarization they report the conformational relation of interacting bonds and groups to be at least equally important, if not more so (185). 

Olah and Watkins (187) correlated l3C chemical shifts in crowded phenyl-ethanes with bond-electron polarizations brought about by van der Waals interactions. They found that these effects cannot be confined to one single C7-H bond but operate throughout the whole molecule and produce shielding of ortho and deshielding of a- and meta carbon atoms. The para carbon atoms are unaffected, which is taken as evidence that only the o-electron systems of the phenyl groups are involved in these steric interactions (187). 

C NMR spectroscopy of 1,7-dioxa- (314), l-oxa-7-thia- (315), and 1,7-dithiaspiro[5.5]undecane (316) and many of their derivatives, with special emphasis on stereoelectronic and steric effects governing the configurational and conformational properties of these compounds (484-486). This work showed that chemical-shift parameters developed for six-membered carbocyclic compounds in chair conformations (149) could be applied successfully only to sulfur and not to oxygen heterocycles (486). 

Such a restriction does not exist for a readily accessible experimental parameter such as the chemical shift of the carbonyl C-atom (A<5). This parameter, as measured by 13C-NMR, expresses a complex mixture of electronic and steric effects, some of which may not be relevant to the mechanism of hydrolysis. Its correlation with log k is not as good as that of Taft s polarity parameter, yet it is of sufficient quality and includes enough variation between substituents to have fair predictive value (Eqn. 8.2) ... 

Three parameters are readily obtainable from FiMR spectra which may be useful in studying binding interactions the chemical shift [jS], the linewidth (Av) or the apparent or effective spin-spin relaxation time (T2 ), and the spin-lattice relaxation time (Ti). C chemical shifts can reflect steric strain and change in the electronic environment within a molecule when it hinds to another species. Spin-lattice and spin-spin relaxation times can yield information on the lifetimes, sizes and conformations of molecular complexes. 

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