Special Topic Dynamic NMR

The classic example of this phenomenon involves the spectrum of cyclohexane. As you know, there are two kinds of hydrogen in cyclohexane, axial and equatorial, and these are interconverted by a ring flip, with an activation energy of about 11 kcal/mol (p. 198). Despite the existence of two kinds of hydrogen, the room-temperature spectrum of cyclohexane shows only a single line at 81.4 ppm (Fig. 15.58). 

FIGURE 15.59 At low temperature, ring flipping is slowed and the axial and equatorial hydrogens of cyclohexane can be resolved in the NMR spectrum. As the temperature is raised, the — 89 °C spectrum approaches the 25 °C spectrum. 

In practice, for a signal to be detectable for a given species, it must have a lifetime of about 1 s. As the temperature is raised to room temperature, the lifetime of individual cyclohexanes decreases as the rate of ring flipping increases, and an averaged spectrum appears. 

FIGURE 15.60 In static ethyl alcohol, there are three kinds of hydrogen attached to carbon. However, rapid rotation around the carbon-carbon bond interconverts the methyl hydrogens and makes them equivalent. The two methylene hydrogens are enantiotopic. 

as long as rotation is fast, neither the indi tidual methyl hydrogens nor the individual methylene hydrogens of ethyl alcohol are distinguishable by NMR spectroscopy. 

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