- Define and explain Infrared and NMR Spectroscopy in your own words
- Use key terms such as fingerprint region accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Infrared and NMR Spectroscopy: identify bonds and hydrogen environments in molecules.
Identify bonds and hydrogen environments in molecules.
Key ideas
IR and NMR see inside molecules
Infrared spectroscopy detects bond vibrations: an O–H stretch appears as a broad peak around 3300 cm⁻¹ and a C=O stretch as a sharp peak near 1700 cm⁻¹. Proton NMR goes further, showing how many hydrogen environments a molecule has, how many hydrogens are in each (integration), and how they split each other (the n+1 rule). Together they can identify an unknown organic compound completely.
Mass spectrometry weighs molecules
Molecules are ionised, usually by electron impact, and the molecular ion's m/z value gives the relative molecular mass. The molecular ion then fragments, and the pattern of fragment peaks identifies structural features — a peak at m/z 29, for example, suggests a C₂H₅⁺ fragment. Isotope abundances in the spectrum also give relative atomic masses: chlorine's 3:1 ratio of ³⁵Cl to ³⁷Cl produces the famous twin peaks.
Key term — fingerprint region: The complex region below 1500 cm⁻¹ in an infrared spectrum, unique to each molecule.
The ¹H NMR spectrum of ethanol (CH₃CH₂OH) shows three signals. Explain why, and state the splitting of the CH₃ signal.
Three distinct environments: CH₃, CH₂ and OH. The CH₃ signal is split into a triplet by the two neighbouring CH₂ hydrogens (n+1 rule).
Answer: Three distinct environments: CH₃, CH₂ and OH. The CH₃ signal is split into a triplet by the two neighbouring CH₂ hydrogens (n+1 rule).
- Forgetting that NMR needs non-equivalent environments Chemically equivalent hydrogens give one shared signal — the three hydrogens of a CH₃ group appear as a single peak (split by neighbours), not three separate peaks.
- Confusing the fingerprint region with functional-group peaks Useful functional-group absorptions appear above 1500 cm⁻¹; the fingerprint region below 1500 cm⁻¹ identifies a molecule by matching its whole pattern, not single peaks.
Practice
Oxygen.
A carboxylic acid — the broad O–H stretch and the C=O stretch together are characteristic.
Chloride — silver chloride is white and dissolves in dilute ammonia (bromide needs concentrated ammonia).
C₃H₆O₂: (3 × 12) + (6 × 1) + (2 × 16) = 36 + 6 + 32 = 74 (e.g. propanoic acid or methyl ethanoate).
Quick check
Which of these best defines "fingerprint region"?
Distinguish between propan-1-ol and propan-2-one using a chemical test.
- Infrared and NMR Spectroscopy: identify bonds and hydrogen environments in molecules.
- IR and NMR see inside molecules: Infrared spectroscopy detects bond vibrations: an O–H stretch appears as a broad peak around 3300 cm⁻¹ and a C=O stretch as a sharp peak near 1700 cm⁻¹.
- molecular ion: The ion formed when a molecule loses one electron in a mass spectrometer; its m/z gives the relative molecular mass.
- Watch out for: forgetting that NMR needs non-equivalent environments