3.2.10Additional lessons
Proton NMR Spectroscopy (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 67 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The Basics
Principle
- Principle Hydrogen nuclei behave like tiny magnets and absorb radio-frequency radiation in a magnetic field,
- and the frequency depends on the chemical environment.
Number of signals
- Number of signals Each group of chemically equivalent hydrogen atoms gives one signal, so the number of signals equals the number of environments.
Chemical shift
- Chemical shift The position of each signal in ppm (δ) reflects how shielded the hydrogens are, and it is measured relative to TMS at 0.
Integration
- Integration The area under each signal is proportional to the number of hydrogens in that environment.
TMS standard
- TMS standard Tetramethylsilane is used as a reference because it gives one sharp signal from 12 equivalent hydrogens and is inert and volatile.
Part 2 of 5Interpreting a Spectrum
1
Count the signals to find the number of different hydrogen environments in the molecule.
2
Use the integration trace or ratios to find the relative number of hydrogens in each environment.
3
Compare each chemical shift with the data booklet to suggest a group, such as 0.9 for R–CH₃ or 9.4 to 10.0 for an aldehyde.
4
Combine signal information with formula and IR data to propose a structure that fits every signal.
5
Check the final structure by counting its environments and the hydrogen ratio against the spectrum.
| Environment | Approximate δ / ppm |
|---|---|
| R–CH₃ | 0.9–1.0 |
| CH adjacent to C=O | 2.0–2.5 |
| Aldehyde –CHO | 9.4–10.0 |
| Carboxylic acid –COOH | 10.0–12.0 |
Part 3 of 5Practical Points
Practical Points
Solvents
- Solvents Samples are dissolved in deuterated solvents such as CDCl₃ so that the solvent does not give its own hydrogen signals.
Symmetry
- Symmetry Symmetric molecules have fewer signals than the number of carbon atoms would suggest, as in propanone with one signal.
Shielding
- Shielding Electronegative atoms such as O or Cl pull electron density away, deshielding nearby hydrogens and moving shifts to higher ppm.
Medical use
- Medical use The same principle underlies MRI scanning, which maps water and fat hydrogens in the body.
Part 4 of 5Worked Example
Question Predict the number of signals and relative areas in the ¹H NMR spectrum of ethanol, CH₃CH₂OH.
- The three CH₃ hydrogens are equivalent and give one signal.
- The two CH₂ hydrogens are in a different environment and give a second signal.
- The single OH hydrogen is in a third environment.
- There are three signals, and their relative areas are 3 : 2 : 1.
Answer 3 signals, ratio 3 : 2 : 1
Part 5 of 5Exam Checklist & Tips
3.2.10 – Key Points for the Exam
- Chemical shift The position of an NMR signal relative to TMS, measured in ppm.
- Integration The area under a signal, proportional to the number of hydrogens.
- TMS Tetramethylsilane, the standard reference compound with δ = 0.
- Deshielding Reduced electron density around a nucleus, increasing the chemical shift.
Common Mistakes
- Counting signals by the number of hydrogens rather than the number of environments.
- Using the peak height rather than the area to find the proton ratio.
- Forgetting that deuterated solvents are used to avoid signals from the solvent.
- Assuming OH signals are always at the same position, when hydrogen bonding makes them vary.
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