Equivalence, chemical-shift

The concept of chemical shift equivalence is central to NMR spectrometry. Chemical-shift equivalent (isochronous) nuclei comprise a set within a spin system (Pople notation, Section 3.5). 

The immediate question is are selected nuclei in a molecule chemical shift equivalent, or are they not If they are, they are placed in the same set. The answer can be framed as succinctly as the question Nuclei are chemical shift equivalent if they are interchangeable through any symmetry operation or by a rapid process. This broad definition assumes an achiral environment (solvent or reagent) in the NMR experiment the common solvents are achiral (Section 3.17). 

We deal first with symmetry operations and later with rapid processes (Section 3.8.3). 

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Chemical shifts for aromatic azoles are recorded in Tables 14-17. As for the proton spectra, fast tautomerism renders two of the chemical shifts equivalent for the NH derivatives (Table 14). However, data for the AT-methyl derivatives (Table 15) clearly indicate that the... 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

On the timescale of the NMR experiment, isochrony (chemical shift equivalence) arises from symmetry equivalence of homotopic and enantiotopic nuclei5 17, while anisochrony (chemical shift nonequivalence, JK s <5) arises from symmetry nonequivalence of diastereotopic nuclei. 

Interpretation of NMR spectra depends on the concept of chemical-shift equivalence, an understanding of which depends on stereochemical concepts these are reviewed with special emphasis on interchange through symmetry operations within the molecule, and through rapid structural changes. 

Chemical shifts for aromatic azoles are recorded in Tables 17-20. Fast tautomerism renders two of the C-13 chemical shifts equivalent for the NH derivatives just as in the proton spectra (Table 17). However, data for the A-methyl derivatives (Table 18) clearly indicate that the carbon adjacent to a pyridine-like nitrogen shows a chemical shift at lower field than that adjacent to a pyrrole-like N-methyl group (in contrast to the H chemical shift behavior). In azoles containing oxygen (Table 19) and sulfur (Table 20), the chemical shifts are generally at lower field than those for the wholly nitrogenous analogues, but the precise positions vary. 

SAMPLE SOLUTION (a) To test for chemical-shift equivalence, replace the protons at C-1, C-2, C-3, and C-4 of 1-bromobutane by some test group such as chlorine. Four constitutional isomers result ... 

This term was coined by G. Binsch following A. Abragam s use of isochronous for chemical-shift equivalent cf. Ref. 5, p. 23. 

The ring system, which has an axis of symmetry, consists of two interchangeable ortho, two interchangeable meta protons, and one para proton, all in the characteristic region for ring protons. The alert student, having absorbed the concept of chemical-shift equivalence, (Section 3.8.3) and the description of the... 

Determination of Chemical Shift Equivalence by Interchange Through Symmetry Operations... 

Chemical Shift Equivalence by Rapid Interconversion of Structures... 

An axial proton becomes an equatorial proton and vice versa in the interconverting structures, and the spectrum consists of a single averaged peak. As the temperature is lowered, the peak broadens and at a sufficiently low temperature two peaks appear-one for the axial protons, one for the equatorial protons. In other words, at room temperature, the axial and equatorial protons are chemical-shift equivalent by rapid interchange. At very low temperatures, they are not chemical-shift equivalent in fact, in each frozen chair form, the protons of each CH2 group are diastereotopic... 

In a fused cyclohexane ring, such as those of steroids, the rings are frozen at room temperature and the axial and equatorial protons of each CH2 group are not chemical-shift equivalent. 

In the same way the three rotating methyl groups of a f-butyl group are chemical shift equivalent except for rare steric hindrance. Both the methyl group and the f-butyl group are described as symmetry tops. ... 

In addition to the above requirements for a first-order spin system, we now consider the concept of magnetic equivalence (or spin-coupling equivalence) by comparison with the concept of chemical-shift equivalence. 

If two protons in the same set (i.e., chemical-shift-equivalent protons in the same multiplet) couple equally to every other proton in the spin system, they are also magnetically equivalent, and the usual Pople notations apply A2, B2, X2 etc. However, if two protons in a set are not magnetically equivalent, the following notations apply AA, BB, XX, etc. To rephrase Two chemical-shift-equivalent protons are magnetically equivalent if they are symmetrically disposed with respect to each proton in the spin system. Obviously magnetic equivalence presupposes chemical-shift equivalence. In other words, do not test for magnetic equivalence unless the two protons in question are chemical-shift equivalent. 

Three isomeric difluoroethylenes furnish additional examples of chemical-shift-equivalent nuclei that are not magnetically equivalent. The systems are AA XX. ... 

In each system, the protons comprise a set and the fluorine nuclei comprise a set (of chemical-shift-equivalent nuclei), but since the nuclei in each set are not magnetically equivalent, the spectra are not first order. Both proton and fluorine spectra are readily available (see Chapter 6). 

With some mastery of chemical-shift equivalence and magnetic equivalence, we return to structure j in Figure 3.43 whose spectrum is given in Figure 3.49. Structure... 

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