16.1 : 构象柔性分子的 NMR：时间分辨率

The stable chair conformation of cyclohexane has a staggered arrangement of six pairs of diastereotopic protons, with a total of six axial and six equatorial protons.

At room temperature, the ring of the chair conformer flips to an equivalent chair conformer, where the two conformers are in equilibrium.

The rapid ring flipping - about 10⁵ times per second - results in the interconversion of axial to equatorial protons and equatorial to axial protons.

So, at room temperature, the NMR spectrometer reads all the protons as equivalent, exhibiting a single peak.

As with cyclohexane at room temperature, NMR averages all the conformations of any molecule with rapid conformational equilibria.