Chemical shift equivalent hydrogens

Spin-spin splitting is not observed between chemical-shift-equivalent hydrogens. 

Four sets of chemical shift equivalent hydrogens are present in this isomer one for the methyl hydrogens and three for the chemically non-equivalent ring hydrogens. 

There are only three cases possible for each carbon atom. If a line drawn encounters no cross peaks, then the carbon has no attached hydrogens. If the drawn line encounters only one cross peak, then the carbon may have either 1,2, or 3 protons attached if 2 protons are attached, then they are either chemical shift equivalent or they fortuitously overlap. If the horizontal line encounters two cross peaks, then we have the special case of diastereotopic protons attached to a methylene group. Much of this information will already be available to us from DEPT spectra (see Section 4.6) indeed, the HMQC spectrum should, whenever possible, be considered along with the DEPT. 

Homotopic hydrogens have identical environments and will have the same chemical shift. They are said to be chemical shift equivalent. 

Chloroethane, therefore, has two sets of hydrogens that are heterotopic with respect to each other, the CH3 hydrogens and the CH2 hydrogens. The hydrogens of these two sets are not chemical shift equivalent, and chloroethane gives two NMR signals. 

Thus far, we have predicted splitting patterns by grouping hydrogens according to their chemical shift equivalence and then determining the number of nearest neighbors n. There are cases, however, in which nuclei may have equivalent chemical shifts but are not magnetically equivalent this may complicate the appearance and interpretation of the spectrum. This phenomenon is often seen in the NMR spectra of aromatic compounds. 

The six hydrogens of ethane are homotopic and are, therefore, chemical shift equivalent. Ethane, consequently, gives only one signal in its NMR spectrum. [Remember, the barrier to rotation of the carbon-carbon bond of ethane is so low (Section 4.8), the various conformations of chloroethane interconvert rapidly.]... 

The diastereotopic nature of H and H at C3 in 2-butanol can also be appreciated by viewing Newman projections. In the conformations shown below (Fig. 9.16), as is the case for every possible conformation of 2-butanol, H and H experience different environments because of the asymmetry from the chirality center at C2. That is, the molecular landscape of 2-butanol appears different to each of these diastereotopic hydrogens. H and H experience different magnetic environments, and are therefore not chemical shift equivalent. This is true in general diastereotopic hydrogens are not chemical shift equivalent. 

Figure 9.16 and H (on C3, the front carbon in the Newman projection) experience different environments in these three conformations, as well as in every other possible conformation of 2-butanol, because of the chirality center at C2 (the back carbon in the Newman projection). In other words, the molecular landscape as viewed from one diastereotopic hydrogen will always appear different from that viewed by the other. Hence, and experience different magnetic environments and therefore should have different chemical shifts (though the difference may t>e small). They are not chemical shift equivalent. 

In Summary The properties of symmetry, particularly mirror images and rotations, help to establish the chemical-shift equivalence or nonequivalence of the hydrogens in organic molecules. Those structures that undergo rapid conformational changes on the NMR time scale show only averaged spectra at room temperature. In some cases, these processes may be frozen at low temperatures to allow distinct absorptions to be observed. 

Chemically equivalent hydrogens and carbons have the same chemical shift. Equivalence is best estabhshed by the application of symmetry operations, such as those using mirror planes and rotations. 

Thus the methyl hydrogens are chemical shift equivalent. 

Again, flipping and rotating members of each pair demonstrates their equivalence. There are then, three sets of chemical shift equivalent protons—one for the methyl hydrogens and one each for the two different ring hydrogens. 

It s important to know the chemical shift of each chemically equivalent set of hydrogen atoms. A proton adjacent to an atom that has a high electronegativity has a lower electron density than a proton adjacent to an atom with a low electronegativity. Therefore, a proton adjacent to an oxygen atom, for example, comes into resonance at a higher frequency than a proton adjacent to, for example, a carbon atom (lower electronegativity). 

As can be seen in the meso-form of 4, the two hydrogens Ha and Hb at the central carbon are in different chemical environments. Thus, they are diastereotopic and have different chemical shifts. In the d,/-pair, however, a C2-axis exists which converts Ha into Hb and vice versa. The two hydrogens are equivalent in this case and can be differentiated by their appearance in the spectrum. 

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