19.10:
NMR Spectroscopy Of Amines
The proton NMR spectrum of an amine shows the N–H signal as a broad band in the region of 0.5 to 4 ppm.
The peak's position depends on the extent of hydrogen bonding, the solvent's nature, the amine's concentration, and the temperature conditions.
Because the N–H protons are easily exchangeable, no coupling occurs with the neighboring protons—the peak is broad, unsplit, and difficult to identify.
A D2O shake exchanges the N–H protons for the N–D deuterons causing the N–H peak to disappear, confirming the presence of amine protons in the sample.
Since the α carbon protons are deshielded by nitrogen, they have higher chemical shift values than the β carbon protons that are far less deshielded and appear further upfield.
In the 13C NMR spectrum, the α carbon shows the highest chemical shift as the nitrogen atom greatly deshields it.
An increasing distance between carbon and nitrogen decreases the deshielding effect, causing the carbon atom's chemical shift value to decrease.
19.10:
NMR Spectroscopy Of Amines
In proton NMR spectroscopy, primary amines and secondary amines showcase their N–H protons as a broad signal in the chemical shift range between δ 0.5 and 5 ppm. The exact position in this range depends on several factors, including sample concentration, hydrogen bonding, and the type of solvent used. Since amine protons undergo fast proton exchange in solution, the protons are labile and therefore do not participate in any splitting with adjacent protons. Thus, the observed peak is broad and does not provide any information about the adjacent proton environment. This problem is resolved, and N–H protons are made identifiable, by adding D2O to the mixture. The addition causes the N–H protons to exchange with deuterons, leading to the disappearance of N–H proton peaks. This disappearance is indicative of the presence of labile protons in the sample.
In aliphatic amines, the α protons are deshielded by the electron-withdrawing nitrogen atom. Consequently, the α protons show higher chemical shifts (δ 2.2 to 2.9 ppm) than β protons (δ 1 to 1.7 ppm) that are less deshielded due to the increased distance from the nitrogen atom.
In the 13C NMR spectroscopy, the α carbons of aliphatic amines show the highest chemical shift values ranging from δ 30 to 60 ppm because of the deshielding effect of electronegative nitrogen.