Chemical-shift criterion

Magnetically equivalent nuclei must be chemically equivalent and hence must have the same chemical shift that is, they must be isochronous. Nonequivalence resulting from unequal couplings to a single partner nucleus sometimes is referred to as magnetic nonequivalence by the coupling-constant criterion. The term is intended to provide a distinction from chemically nonequivalent nuclei, which have different chemical shifts and illustrate magnetic nonequivalence by the chemical-shift criterion. 

Hg and Hb in this system are, therefore, chemically distinct and diastereotopic and would be expected to be anisochronous in either achiral or racemic media. Since they constitute an AB or AX system and are magnetically nonequivalent by the chemical-shift criterion, any considerations of magnetic equivalence by the coupling-constant criterion are not applicable. 

H-nmr chemical shifts of N-1—H and N-3—H signals have been used as a criterion for distinguishing between N-l-substituted and N-3-substituted hydantoin derivatives (22). They can often be related to electronic properties, and thus good linear correlations have been found between the shifts of N—H and Hammett parameters of the substituents attached to the aryl group of 5-arylmethylenehydantoins (23). 

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 ... 

Garcia-Viloca, M., Gelabert, R., Gonzalez-Lafont, A., Moreno, M., Lluch, J. M., 1998, Temperature Dependence of Proton NMR Chemical Shift as a Criterion to Identify Low-Barrier Hydrogen Bonds , J. Am. Chem. Soc., 120, 10203. 

The shielded chemical shift is in turn due to the strong magnetic anisotropy of the sp -hybridized carbons. All the above dications are definitively classical, nonbridged cations, as also shown by the chemical shift additivity criterion of Schleyer and coworkers. 

Up to this point, the criterion used to ascertain the presence of an equilibrium, i.e. the simultaneous presence of two well defined entities, was mainly based on 1H, 13C or 31P NMR determinations at different temperatures or in various solvents. However, it turns out that the chemical shift in 31P NMR spectra (usually negative with respect to the H3P04 reference) is often inadequate to permit a choice among various possibilities, namely a true phosphorane, a phosphonium salt or an equilibrium between these two forms. 

The reader may gain better appreciation of the many basic differences responsible for the division into different classes of heteronin by comparing certain representative members, directly or through appropriate models, in terms of the information presented in Table II. First, one notes that the classification of oxonin (24a) as atropic, jV-methylazonine (27a) as nondescript, and 1 //-azonine or its anion as diatropic, originally proposed on the basis of NMR chemical shifts (data shown in first three rows), was confirmed by the determination of solvent shift character (S values)38 39 that revealed 1//-azonine to possess significant diatropic influence (comparable to that of naphthalene +1.3538), the V-methyl counterpart to exhibit a far weaker effect in the same direction, and oxonin to be atropic or mildly paratropic under this criterion, its S value being closely similar to that of the family s 8 --electron polyenic model, all-cis-cyclononatetraene (24 X = CH2). Major differences between oxonin and parent azonine are also seen to exist in terms of thermal stability and 13C NMR and UV spectroscopy, all of which serve further to emphasize the close structural similarity of oxonin with n-... 

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