Use TMS and deuterated solvents in proton NMR
Explain TMS as the chemical-shift reference and deuterated solvent as a way to reduce solvent proton signals.
Section 37 · Analytical techniques
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What chemical-shift value is assigned to TMS?
What you will learn
- Describe the role of TMS
- Explain why deuterated solvents are used
- Distinguish reference from solvent
TMS establishes the zero point
Tetramethylsilane (TMS) is used as a standard for chemical-shift measurement. Its signal is assigned δ = 0 ppm, and sample signals are measured relative to it.
A reference is needed so chemical shifts measured in different spectra can be placed on a consistent scale. TMS is not a reactant or an analyte peak to assign to the unknown structure.
Deuterated solvent reduces background proton signal
Ordinary solvents contain hydrogen-1 atoms that could produce strong solvent signals in a proton NMR spectrum. A deuterated solvent such as CDCl₃ has deuterium in place of most hydrogen-1 atoms, reducing solvent proton interference.
Deuterated solvents can still show small residual proton signals, so do not assume the spectrum contains absolutely no solvent peak. The syllabus asks why deuterated solvents are needed, not how to operate the spectrometer.
Pause and explain
Why use a deuterated solvent for proton NMR?
Worked example
A sample is run in CDCl₃ with TMS. State the purpose of each.
Show the worked solution
- TMS is the shift reference.
- CDCl₃ reduces proton signals from solvent hydrogen.
- Separate the scale marker from the solvent.
Answer TMS sets δ = 0 ppm for chemical shifts; CDCl₃ is a deuterated solvent that reduces interfering solvent ¹H signals.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why an NMR spectrum uses TMS even when a deuterated solvent is present.
Give me a hint
They have different jobs.
Compare my reasoning
- TMS supplies a chemical-shift reference.
- The deuterated solvent reduces the solvent's proton signal.
- State both roles separately.
TMS sets δ = 0 ppm so sample shifts can be measured; the deuterated solvent reduces interfering solvent ¹H signals.
Look for these in your work
- Reference role
- Solvent role
- Distinction clear
A student says CDCl₃ makes all solvent-related signals impossible. Correct the statement.
Give me a hint
Deuteration reduces, but may not eliminate, residual proton signals.
Compare my reasoning
- Most solvent H atoms are replaced by deuterium.
- Residual ¹H signals can remain.
- The purpose is to reduce interference.
CDCl₃ reduces solvent proton signals because most hydrogen is deuterium, but residual ¹H signals can still be present.
Look for these in your work
- Isotope replacement stated
- Residual signal acknowledged
- Claim corrected
A compound signal is measured at 7.2 ppm relative to TMS. What does this number mean?
Give me a hint
Interpret the shift on the reference scale.
Compare my reasoning
- TMS is defined as 0 ppm.
- The sample signal lies 7.2 ppm from that reference on the scale.
- It is not a retention time or peak area.
The proton environment's chemical shift is δ = 7.2 ppm relative to TMS at 0 ppm.
Look for these in your work
- Reference identified
- Shift interpreted
- Other measurements distinguished
Common mistakes
- Treating TMS as the unknown compound
- Using the solvent peak as the shift reference
- Claiming deuterated solvent produces no signal of any kind
- Confusing deuterium with proton environments in the sample
What can you explain now?
What chemical-shift value is assigned to TMS?
Compare with the explanation
0 ppm
TMS is the standard reference at δ = 0.
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Make it stick
How do TMS and a deuterated solvent serve different purposes in a proton NMR measurement?
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