Chemical shifts differences

Chemical shift difference much larger than coupling constant 

NMR Chemical-shift differences in their H NMR spectra aid the structure deter- 

NMR Chemical shift differences m their H NMR spectra aid the structure deter mmation of esters Consider the two isomeric esters ethyl acetate and methyl propanoate As Figure 20 9 shows the number of signals and their multiplicities are the same for both esters Both have a methyl singlet and a triplet-quartet pattern for their ethyl group 

Calculations show that only 60% of the chemical shift difference between and H is the result of the aromatic ring current, and that even Ha is shielded it would appear at 5 = w5.5 without the ring current Childs, R.F. McGlinchey, M.J. Varadarajan, A. J. Am. Chem. Soc., 1984, 106, 5974. 

Figure 1.4. Two-spin system of type AX with a chemical shift difference which is large compared with the coupling constants (schematic)

Figure 1.6. Two-spin system of type AB with a small chemical shift difference compared to the coupling constant (schematic)

Problem 13.3 in the preceding section was based on the chemical shift difference between the proton in CHCI3 and the proton in CHBr3 and its relation to shielding. 

In some crystalline polymers chemical shift differences between crystalline and amorphous phases have been observed and interpreted and for several crystalline forms the signals to be attributed to nuclei in different conformational environments have been identified [111, 112]. 

FIGURE 13.18 The appearance of the splitting pattern of two coupled protons depends on their coupling constant J and the chemical shift difference Av between them. As the ratio Av/J decreases, the doublets become increasingly distorted. When the two protons have the same chemical shift, no splitting is observed. 

The rehability of these analytical methods may be questionable when chemical shift differences of derivatives are of the same magnitude as variations encountered from solvent, concentration, and temperature influences. Reported fluorine chemical shift ranges for tnfluoroacetylated alcohols (1 ppm), p-fluorobenzoylated sterols (1 ppm), and p-fluorobenzoylated ammo acids (0.5 ppm) are quite narrow, and correct interpretation of the fluonne NMR spectra of these denvatized mixmres requires strict adherence to standardized sampling procedure and NMR parameters. 

The Kt = [N3(H)]/[N1(H)] values were calculated with use of 83 ppm chemical shift difference for N1 and N3. The values Kt based on calculations for averaged N1 shifts are given. For all the structures numbering the ring atoms is as given in formula 15. 

The sensitivity of 13C solid-state chemical shifts to small conformational changes is well illustrated by the case of i-PB. Table 1 summarizes some chemical shifts differences for the forms, which have been interpreted in terms of variations of the y-shielding parameter corrected for the deviations, with respect to the exactly G conformations, in the slightly different nearly gauche — nearly trans sequences, characterizing the three crystalline forms of i-PB [116]. 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds  

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

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