Reversible Intramolecular Processes Involving Rotation Around Bonds

2 REVERSIBLE INTRAMOLECULAR PROCESSES INVOLVING ROTATION AROUND BONDS 

However, there are many molecules where rotation around a particular single bond is restricted, that is, prevented from rotating freely. If rotation is much slower than the NMR time scale, the molecule is essentially locked in one conformation. One example of such a compound is N./V-dimethylformamide (DMF, 10-1), whose NMR data are given in Table 8.1  

As the temperature of a collection of molecules increases, the molecules acquire more thermal energy, and the rates of physical processes such as translation, rotation, and torsion increase. At sufficiently high temperature, the rate of rotation of the C-N bond in DMF becomes significant, bringing about reversible exchange of the two methyl groups The cis methyl exchanges position with the trans  

By the time DMF reaches 130°C, rotation becomes fast on the NMR time scale (the fast-exchange limit) and the two methyl signals are averaged to give one sharp signal midway between the two slow-exchange signals ( H 8 2.825, l3C 8 32.65).1 

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