Compounds with very polar groups often dissolve best in water. NMR spectra are usually run in CDCl₃, but heavy water, D₂O, is an excellent NMR solvent. Here are some results in that medium.

Glycine is expected to exist as a zwitterion (Chapter 8, p. 167). It has a 2H signal (green) for the CH₂ between the two functional groups, which would do for either form. The 3H signal at 4.90 ppm (orange) might suggest the NH₃⁺ group, but wait a moment before making up your mind.

The aminothiol salt has the CMe₂ and CH₂ groups about where we would expect them (brown and green), but the SH and NH₃ ⁺ protons appear as one 4H signal.

The double salt of EDTA has several curious features. The two (green) CH₂ groups in the middle are fine, but the other four CH₂ (brown) groups all appear identical, as do all the pro tons on both the CO₂H and NH₃⁺ groups.

The best clue to why this is so comes from the strange coincidence of the chemical shifts of the OH, NH, and SH protons in these molecules. They are all the same within experimental error: 4.90 ppm for glycine, 4.80 ppm for the aminothiol, and 4.84 ppm for EDTA. In fact, all correspond to the same species: HOD, or monodeuterated water. Exchange between XH (where X=O, N, or S) protons is extremely fast, and the solvent, D2O, supplies a vast excess of exchangeable deuteriums. These immediately replace all the OH, NH, and SH protons in the molecules with D, forming HOD in the process. Recall that we do not see signals for deuterium atoms (that’s why deuterated solvents are used). They have their own spectra at a different frequency.

The same sort of exchange between OH or NH protons with each other or with traces of water in the sample means that the OH and NH peaks in most spectra in CDCl₃ are rather broader than the peaks for CH protons. Two questions remain. First, can we tell whether glycine is a zwitterion in water or not? Not really: the spectra fi t either or an equilibrium between both—other evidence leads us to expect the zwitterion in water. Second, why are all four CH₂CO groups in EDTA the same? This we can answer. As well as the equilibrium exchanging the CO₂H protons with the sol vent, there will be an equally fast equilibrium exchanging protons between CO₂D and CO₂ −. This makes all four ‘arms’ of EDTA the same. You should leave this section with an important chemical principle firmly established in your mind.

●Proton exchange between heteroatoms is fast

Proton exchange between heteroatoms, particularly O, N, and S, is a very fast process in comparison with other chemical reactions, and often leads to averaged peaks in the 1H NMR spectrum.

مواضيع ذات صلة

Some compounds can exist as a pair of mirror-image forms

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Protons on heteroatoms have more variable shifts than protons on carbon

Non-aldehyde protons in the aldehyde region: pyridines

The aldehyde region: unsaturated carbon bonded to oxygen

Structural information from the alkene region

Electron-rich and electron-deficient alkenes

How electron donation and withdrawal change chemical shifts

Uneven electron distribution in aromatic rings

The benzene ring current causes large shifts for aromatic protons

Summary chart of proton NMR shifts

Chemical shifts of CH groups