Combined Spectroscopy Problems: A Working Order
Given four spectra of an unknown, the order in which you read them determines whether the problem takes five minutes or thirty.
The order, and why it matters
- Mass spectrum → molecular formula and any halogen present.
- Degrees of unsaturation from that formula.
- Infrared → which functional groups are present and, equally useful, which are absent.
- Carbon NMR → how many distinct carbon environments, which reveals symmetry.
- Proton NMR → the detailed connectivity.
- Check that the proposed structure predicts every observation.
Step by step
Degrees of unsaturation
Four almost always means a benzene ring. One means a double bond or a ring. Zero means the molecule is saturated and acyclic, which immediately excludes a large number of possibilities.
Infrared — what to look for and in what order
| Region (cm−¹) | Indicates | Notes |
|---|---|---|
| 3200 – 3600 broad | O–H | Very broad suggests a carboxylic acid |
| 3300 – 3500 sharp, one or two bands | N–H | Two bands means a primary amine |
| ~2250 sharp | Nitrile | Distinctive and hard to confuse |
| 1650 – 1750 strong | Carbonyl | Exact position distinguishes the type |
| 1600 and 1500 | Aromatic ring | Confirms the DoU of four |
Absences are as informative as presences. No band near 1700 rules out every carbonyl compound at once, which eliminates a great many candidate structures in one observation.
Carbon NMR for symmetry
The number of signals is the number of distinct carbon environments. Comparing it with the carbon count from the formula reveals symmetry directly: fewer signals than carbons means equivalent carbons, and how many fewer tells you how much symmetry.
This is often the fastest way to distinguish between isomers that would otherwise require careful proton analysis.
Proton NMR last
By this point the functional groups and the degree of symmetry are known, so the proton spectrum is being used to settle connectivity rather than to identify the compound from scratch. Integration gives hydrogen counts, shifts give environments, multiplicity gives neighbours.
The final check
Having proposed a structure, predict what each spectrum should look like and compare. Specifically:
- Does the molecular formula match the mass spectrum, including isotope pattern?
- Does every strong infrared band have an explanation?
- Does the carbon signal count match the number of distinct environments in the structure?
- Does the proton integration total match the formula, and does every multiplicity make sense?
A structure that explains most but not all of the data is usually wrong. The commonest failure is a structure that fits the proton NMR but predicts the wrong number of carbon signals.
Frequently asked questions
Which spectrum should I read first?
The mass spectrum, for the molecular formula. Everything afterwards is constrained by it, so obtaining it first saves the most time.
What if the molecular ion is absent?
Work from the largest fragments and the other spectra. Carbon NMR gives a minimum carbon count, and the infrared identifies functional groups, so a formula can often be reconstructed.
How do I distinguish an aldehyde from a ketone?
The proton NMR settles it: an aldehyde has a characteristic signal near 9.5 to 10 ppm that a ketone lacks. The infrared carbonyl positions are close enough to be ambiguous on their own.
What does a very broad infrared band around 3000 mean?
Almost certainly a carboxylic acid O–H, which is far broader than an alcohol O–H because of strong hydrogen-bonded dimerisation.
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