16.2 : 构象柔性分子的 ¹H NMR：变温 NMR

The variable-temperature NMR technique helps distinguish between the two types of protons in cyclohexane - axial and equatorial.

To prevent the spin-spin coupling between the adjacent protons, eleven of the twelve protons are replaced by deuterium — the remaining proton switches between the axial and equatorial positions.

This sample is labeled cyclohexane-d₁₁, and the corresponding proton NMR spectrum is recorded at various temperatures.

At room temperature, the rapid chair-chair interconversion produces a single sharp peak. However, the reduced rate at −60°C results in peak broadening. Further lowering of the temperature continues to broaden the peak — first, to form a saddle, which then splits into two peaks.

Below −89°C, two sharp, well-resolved peaks are formed at δ 1.62 and 1.14, corresponding to the equatorial and axial protons. At this temperature, NMR detects the two sets of protons due to the significantly low rate of chair-chair interconversion.