Chemical Shift Equivalent and Nonequivalent Protons

For example, the chemical shift for benzene protons is 2181 Hz when the instrument is operating at 300 MHz. Therefore, 

the chemical shift expressed in ppm is the same whether measured with an instrument operating at 300 or 60 MHz (or any other field strength). 

Two or more protons that are in identical environments have the same chemical shift and, therefore, give only one H NMR signal. How do we know when protons are in the same environment For most compounds, protons that are in the same environment are also equivalent in chemical reactions. That is, chemically equivalent protons are chemical shift equivalent in H NMR spectra. 

How do we decide whether two or more protons in a molecule are in identical environments  

If replacing the hydrogens by a different atom gives the same compound, the hydrogens are said to be homotopic. 

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One possible point of confusion should be cleared up It is often stated that chemical-shift-equivalent protons do couple with one another, but peak splitting is not observed in the spectrum. This statement is insufficient and holds only for first-order systems. But if magnetic nonequivalence is involved, the system is not first order and splitting is observed. 

Chemical shift nonequivalence of the methyl groups of an isopropyl moiety near a chiral center is frequently observed the effect has been measured through as many as seven bonds between the chiral center and the methyl protons. The methyl groups in the terpene alcohol, 2-methyl-6-methylen-7-octen-4-ol (ipsenol) are not chemical shift equivalent (Figure 3.55). They are diastereotopic, so a strong magnetic field is usually necessary to avoid superposition. 

AH of the protons found in chemically identical environments within a molecule are chemically equivalent, and they often exhibit the same chemical shift. Thus, all the protons in tetramethylsilane (TMS), or aU the protons in benzene, cyclopentane, or acetone—which are molecules that have protons whieh are equivalent by symmetry considerations— have resonance at a single value of 8 (but a different value from that of each of the other molecules in the same group). Each such compound gives rise to a single absorption peak in its NMR spectrum. The protons are said to be chemically equivalent. On the other hand, a moleeule that has sets of protons that are chemically distinct from one another may give rise to a different absorption peak from each set, in which case the sets of protons are chemically nonequivalent. The following examples should help to clarify these relationships. 

The C line assignments were made from the combination of DEPT and 2D C- H correlated spectroscopy despite the complexity of the conventional C spectrum. DEPT spectroscopy allowed the multiplicity of each resonance to be determined unambiguously. Hence, C assignments were made easily from the 2D C- H correlated spectrum even in situations where overlap of methine and methylene signals occurs in the proton spectrum. Furthermore, equivalent and nonequivalent methylenes were distinguished in the 2D C- H correlated spectrum, and this allowed assignments to be made despite spectral overlap of proton resonances. Proton chemical shifts were determined more accurately from the correlated... 

By addition of one equivalent of base to solutions of 7-(2-hydroxyethoxy)-4-nitrobenzofurazan and 7-(2-hydroxyethoxy)-4-nitrobenzofuroxan in either water or a dipolar aprotic solvent, the spiro adducts 169 and 170 are completely formed212 and can be characterized by their UV-visible and H-NMR spectra. They display substantial upfield chemical shifts relative to the ring signals for the starting substrate and a complex multiplet centered at 8 4.16 for the nonequivalent dioxolane methylene protons. Isolation of the adducts as the potassium salts from acetonitrile solution was accomplished by removal of the solvent. 

Methylene protons adjacent to a chiral centre will be non-equivalent, despite the fact that there is free rotation about the carbon-carbon bond. Such protons are described as diastereotopic, since replacement of either of the two hydrogens in turn by a group X produces a pair of diastereoisomers. Such is the case of the two methylene protons (Ha and Hb) in l-phenylpropan-2-ol, which are nonequivalent and therefore have different chemical shifts in the p.m.r. spectrum. 

Given a structure, explain which protons are equivalent and which are nonequivalent. Predict the number of signals in the proton NMR and their approximate chemical shifts. Problems 13-34,35, 50, and 54... 

The H-nmr spectrum of ion [100] shows two doublets from the nonequivalent methylene hydrogens. In [106] only the upheld signal of these two experienced an isotopic perturbation and shifted to higher held. The down-held methylene proton peak was unaffected. Since an isotopic perturbation obviously occurs, the frozen chemical shift difference (A) must be very different within these two non-equivalent sets of methylene hydrogens. 

The AA XX notation may be interpreted as follows. The chemical shifts of the A and X nuclei are very far from each other (at opposite ends of the alphabet). The A and A nuclei are chemically equivalent, but magnetically nonequivalent, as are the X and X nuclei. Figure 4-2 illustrates the proton AA part of the spectrum of 1,1-difluoroethene, in which 10 peaks are visible. This appearance is quite different from the simple 1 2 1 triplet expected in the first-order case. The multiplicity of peaks in Figure 4-2 in fact permits the measurement of 7aa Ihe coupling between the equivalent protons. 

For example, all the protons of ethane (CH3-CH3) are in identical electronic environments and are therefore equivalent. Hence there will be no spin-spin splitting of the protons the nmr spectrum will contain only singlets (signals with one peak). Furthermore, since all the protons in ethane are equivalent they will have identical chemical shifts, and will occur as one signal. Hence the nmr spectrum of ethane will have one singlet. An example where spin-spin splitting occurs is propane. The methylene protons are nonequivalent to the methyl protons. As a result, the six nonequivalent adjacent methyl protons will split the methylene proton s signal into a heptet (seven peak multiplet). 

When the protons attached to a single carbon are chemically equivalent (have the same chemical shift), the n -I-1 Rule successfully predicts the splitting patterns. In contrast, when the protons attached to a single carbon are chemically nonequivalent (have different chemical shifts), the n -t-1 Rule no longer applies. We shall examine two cases, one in which the n + 1 Rule applies (1,1,2-trichloroethane) and one in which it fails (styrene oxide). 

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