Differential shielding

From differential shielding effects in 13C-NMR spectra Wright et al. [225] concluded that the most likely conformations of the glycosidic bonds of a series of gentamycin derivatives are in accordance with the exo anomeric effect. Furthermore, the C-H vectors on both sides of the glycosidic bond are supposed to be in an almost parallel orientation, which is in agreement with the expectation. 

In Table 2 are summarized the shift differences (both H and 13C) between the diastereotopic methyl groups of the compounds46) shown in Fig. 28. (Arguments are adduced in the paper 461 that the conformation shown is by far the preferred one, at least for R = COX.) It is immediately obvious that these differences in shift between diastereotopic protons are much larger for the phenyl than for the cyclohexyl compound presumably because of the much larger differential shielding of the... 

Hj 4 protons are more highly shielded than the H2,j protons and that the efficiency of this differential shielding passes through a minimum in the progression from the methyl to the ferf-butyl substituent. This shielding parallels the hyperconjugative (resonance) contribution (both C—H and C—C) to the 4 shift in each of the members of this series. 

The whole of these results indicate that the polymeric matrix does not act as an inert support but it has an important influence on the steric course of the reaction. This influence may be due to a differential shielding of the enantiotopic faces of the double bond and to a modification of the conformational preferences of the catalyst-dienophile complex. 

Similarly, the monohomocyclooctatetraene di-anion, 74, (see Table 15) 212,218) deshielded peripheral protons, and a differential shielding of the methylene protons, whereby that proton situated above the ring (H(a), t 10.0) is more shielded than the "outer" proton (H(b), t 8.0). The chemical shifts of the peripheral protons (centered at ca. t 4.5 to 4.8) are shifted upfield from those of the parent hydrocarbon by only 0.4 to 0.7 ppm. As the upfield shift expected from charge density considerations alone is 2.5 ppm, a deshielding effect associated with a diamagnetic current is clearly implicated 218>. 

Differential shielding effects on the proton chemical shifts of ethyl (3R)-(/ -nitrophenyl)-3-(10)5-dihydroartemininoxy)propionate and its (35) diastereomer are correlated with crystal structure. In the two isomers, the phenyl substituent adopts markedly different conformations with respect to the nearby methine protons on the dihydropyran moiety thereby producing sizable differences in chemical shift <95AX(B)1063). 

Sulfoxides form hydrogen-bonded complexes with a-methoxyphenylacetic acid, which results in differential shielding of the two substituents. ... 

Such an arrangement constitutes an external reference and internal look configuration and now introduces the problem of differential shielding, since the field experienced by the reference and the sample depends on a shape factor (Sf) and on the difference in magnetic susceptibility (Ax) between the reference and the sample (M 31b). Equations have been theoretically derived which enable the chemical shift to be corrected for the susceptibility effect. The correction factor depends on the disposition of the sample with respect to the magnetic field direction. 

In Scheme 3.5 the relatively acidic hydroxyl and methine protons are involved in hydrogen-bonding with the primary and secondary basic sites in the solvate, B and B, to give complexes I and II. The solvate substituents andR experience differential shielding due to the CSA aryl substituent. The opposite sense of non-equivalence for substituents on opposing sides of the chelate plane is a hallmark of the CSA technique. 

Additional data in ref cs at S, 35, 50, 65, 80 °C cs vs. temperature profiles of all protons differential shieldings (with respect to the corresponding monomers, data for Cmp from theoretical projections). 

Additional data in [SOLI] cs vs. temperature of HI and base protons for the A residues in r(AAA), r(AAQ, r(AUU) the U2 residues in r(AUU), r(CUG) for the G3 residue in r(CUG) differential shieldings of HI and base protons (with respect to the constituent monomers) in [S3L1] differential shieldings of ribose protons (with respect to the constituent monomers). 

Additional data in ref. differential shieldings of HI and base protons (with respect to the constituent mononucleotides) at20 C, lOmM. 

Additional data in ref. cs vs. temperature profiles H8, H2, H5, HI differential shieldings of base and sugar protons ... 

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