Under normal conditions for NMR spectra acquisition, no coupling is observed between the hydroxyl hydrogen and hydrogens on the carbon atom to which the hydroxyl group is attached. A typical alcohol undergoes intermolecular proton exchange at a rate of 105 protons per second. This means that the average time of residence of a proton on oxygen is 10-5 seconds. About 10-2 to 10-3 second is required for an NMR transition event to occur and be recorded. As far as the NMR spectrometer is concerned, the hydroxyl proton is unattached more frequently than it is attached to oxygen, and the spin interaction between the hydroxyl proton and any other proton in the molecule is effectively decoupled.
Rapid chemical exchange decouples spin interactions, and the NMR spectrometer records only the average environment it senses for the exchanging proton.
To observe coupling between hydroxyl protons and other protons in the molecule, the temperature of the experiment can be reduced. This will slow down the exchange rate of the hydroxyl protons, and coupling with other protons in the molecule can then be observed.
Figure 32 contains the NMR spectra for methanol at temperatures ranging from -40 oC to 25 oC. At lower temperatures, coupling between the hydroxyl proton and the methyl protons is clearly indicated. As temperature is increased, the peaks broaden, and at sufficiently high temperatures, the peaks collapse into uncoupled singlets.
Figure 32. NMR spectra of methanol at various temperatures.
When compounds with acidic hydrogens are placed in D2O, the acidic hydrogens exchange with deuterium. The exchange may require addition of acid or base catalyst, but often occurs spontaneously. The net result of deuterium exchange is that the signal due to the acidic hydrogen "disappears" from the spectrum. This can be useful in assigning peaks in the NMR spectrum.
Another method to remove acidic hydrogens is the "D2O shake". The compound is dissolved in a typical NMR solvent like d-chloroform, and then a small amount of D2O is added to the NMR tube. Shaking the tube promotes mixing and exchange of acidic hydrogens.
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