Tetramethylsilane (TMS) & Deuterated Solvents (Cambridge International A-Level Chemistry)

Use of Tetramethylsilane (TMS)

Spec point — 37.4.3

• In NMR spectroscopy, tetramethylsilane (TMS) is used as a reference compound

• The organic compound is dissolved in TMS before being introduced to the magnetic field of the spectrometer

• It is an ideal chemical to use as a reference

  • TMS is inert and volatile
  • This reduces undesirable chemical reactions with the compound to be analysed
  • It also mixes well with most organic compounds

• TMS gives a single sharp peak on the NMR spectrum and is given a value of zero

• The molecular formula of TMS is Si(CH₃)₄

  • There are 12 hydrogens in this molecule
  • All of the protons are in the same molecular environment. Therefore gives rise to just one peak
  • This peak has a very high intensity as it accounts for the absorption of energy from 12 ¹H nuclei

The structure of tetramethylsilane

Tetramethylsilane (TMS) – Si(CH₃)₄

• When peaks are recorded from the sample compound, they are measured and recorded by their shift away from the sharp TMS peak
• This gives rise to the chemical shift values for different ¹H environments in a molecule

The ¹H NMR spectrum for tetramethylsilane

¹H NMR spectrum for TMS showing its signal at 0 ppm

Deuterated Solvents in Proton NMR

Spec point — 37.4.4

• When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent

• Tetramethylsilane (TMS) is a commonly used solvent in NMR as it gives one sharp reference peak on NMR spectra

• However, the proton atoms can still interfere with peaks of a sample compound

• To avoid this interference, solvents containing deuterium can be used instead, e.g. 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 the -OH or -NH signal in an NMR spectrum

• In ¹H NMR, samples are dissolved in a solvent to help separate molecules and prevent them from interacting

• The solvent must:

  • Be a good solvent for organic molecules
  • Not contain any hydrogen (¹H) atoms, so it does not interfere with the NMR signals

Deuterated and non-deuterated solvents

• Carbon tetrachloride, CCl₄:

  • This solvent does not contain hydrogen, so it does not produce ¹H NMR signals.
  • It is suitable for ¹H NMR but does not dissolve all molecules well.

• Deuterated solvents are often used in ¹H NMR spectroscopy because deuterium (²H) is an isotope of hydrogen with no nuclear spin, which does not affect NMR results

• Deuterochloroform / Chloroform-d, CDCl₃:

  • This is often preferred because it contains deuterium (²H) instead of hydrogen, so it does not interfere with the proton NMR spectrum

• Deuterium oxide / Heavy water, D₂O:

  • The deuterium atoms exchange reversibly with the protons in the -OH and -NH groups, allowing these signals to be identified in the NMR spectrum

Identifying -OH and -NH signals

• Protons in -OH (hydroxyl) and -NH (amine) groups give singlet peaks in ¹H NMR, but these signals can be tricky:

  • They are broad or sometimes fall outside normal chemical shift ranges
  • The proton in these groups exchanges quickly with protons from water or acids, so only one peak appears
  • Their chemical shift ranges may overlap with other types of protons, making them difficult to interpret

• To identify these groups more clearly, proton exchange with deuterium oxide (D₂O) is used:

  • The deuterium atoms in D₂O exchange reversibly with the protons in the -OH or -NH groups

-OH + D₂O ⇌ -OD + HOD

-NH-CO- + D₂O ⇌ -ND-CO- + HOD

• Since deuterium does not absorb in the same region as protons in the NMR spectrum, the signal for -OH or -NH disappears after D₂O is added

• This confirms the presence of -OH or -NH groups in the molecule

  • If a peak disappears after adding D₂O, it must have been due to the exchange of a proton from an -OH or -NH group

• This technique is particularly useful because -OH and -NH peaks can be broad and difficult to assign confidently without D₂O exchange