CIE A Level Chemistry复习笔记8.1.7 Deuterated Solvents in Proton NMR

• When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent
• Tetramethylsilane (TMS) is a commonly used solvent in NMR
• Despite TMS showing one sharp reference peak on NMR spectra, the proton atoms can still interfere with peaks of a sample compound
• To avoid this interference, solvents containing Deuterium can be used instead
  • For example CDCl₃
  • Deuterium (²H) is an isotope of hydrogen (¹H)
• Deuterium nuclei absorb radio waves in a different region to the protons analysed in organic compounds
• Therefore, the reference solvent peak will not interfere with those of the sample

Identifying -OH & -NH signals on a 1H NMR spectrum

• The proton in -OH is not affected by its neighbouring molecular environments
• As a result, the hydrogen atom of -OH group appears as a singlet
• This is due to the hydrogen atom readily exchanging with hydrogens atoms of water molecules or any acid that may be present

• When interpreting ¹H NMR spectra of amines and amides, the same exchanging phenomenon can be seen
• Protons of these functional groups exchanging leads to changes in their chemical shift ranges
• This table shows the range of chemical shifts for -OH and -NH- protons
• Their surrounding molecular environment has a direct impact on this range

The range of chemical shifts for -OH & -NH- protons table

Using D₂O

• Deuterium oxide (D₂O) can be added to correctly identify -OH and -NH- protons
• Adding a small quantity of this solvent ‘removes’ the peaks from the spectrum

The -OH proton and the -NH proton both undergo the same exchanging process as seen before. This time with a deuterium atom (D) of D₂O