The Nature of NMR Absorptions

From the description thus far, you might expect all H nuclei in a molecule to absorb energy at the same frequency and all nuclei to absorb at the same frequency. If so, we would observe only a single NMR absorption band in the H or spectmm of a molecule, a situation that would be of little use. In fact, 

All nuclei in molecules are surrounded by electrons. When an external magnetic field is applied to a molecule, the electrons moving around nuclei set up tiny local magnetic fields of their own. These local magnetic fields act in opposition to the applied field so that the effective field actually felt by the nucleus is a bit weaker than the applied field. 

In describing this effect of local fields, we say that nuclei are shielded from the full effect of the applied field by the surrounding electrons. Because each chemically distinct nucleus in a molecule is in a slightly different electronic environment, each nucleus is shielded to a slightly different extent and the effective magnetic field felt by each is slightly different. These tiny differences in the effective magnetic fields experienced by different nuclei can be detected. 

Because of this blurring effect, NMR spectroscopy can be used to measure the rates and activation energies of very fast processes. In cyclohexane, for example, a ring-flip (Section 4.6) occurs so rapidly at room temperamre that axial 

A thin glass tube containing the sample solution is placed between the poles of a strong magnet and irradiated with rf energy. 

In describing the effect of local fields, we say that nuclei are shielded from the full effect of the applied field by the surrounding electrons. Because each specific nucleus in a molecule is in a slightly different electronic 

FIGURE 11.3 (a) The H NMR spectrum and (b) the 3c NMR spectrum of methyl acetate, CH3CO2CH3. The small peak labeled TMS at the far right of each spectrum is a calibration peak, as explained in the next section. 

CHAPTER 11 STRUCTURE DETERMINATION NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 

CH3CO2CH3. The horizontal axis shows the effective field strength felt by the nuclei, and the vertical axis indicates intensity of absorption of rf energy. Each peak in the NMR spectrum corresponds to a chemically distinct nucleus in the molecule. [Note that NMR spectra are formatted with the zero absorption line at the bottom, whereas IR spectra are formatted with the zero absorption line (100% transmittance) at the top Section 12.5.] Note also that and spectra can t both be observed at the same time on the same spectrometer because different amounts of energy are required to spin-flip the different kinds of nuclei. The two spectra must be recorded separately. 

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