Working out molecular structure
Structure determination is a process of determining what is and what isn’t present in a molecule. Each technique below answers a different question: which bonds are present (or not), how heavy the molecule is, how many hydrogens sit in each environment, and how many distinct carbons make up the skeleton? Work through each, then try spectra problems to become comfortable with the ideas.
InfraredMass spectrometryProton NMRCarbon NMR
01Infrared spectroscopy
Infrared light makes bonds stretch and bend. A bond absorbs only when the light matches its natural vibrational frequency, so an IR spectrum is a list of the bonds a molecule contains. Read the region above 1500 cm⁻¹ for functional groups; below it, the fingerprint region is best used for comparison against a known sample.
What to look for
- A strong, sharp peak near 1710 cm⁻¹ — C=O.
- A broad hump from 3200–3600 cm⁻¹ — O–H, hydrogen bonded.
- Weak absorptions at 2100–2260 cm⁻¹ — C≡C or C≡N.
- C–H just above 3000 cm⁻¹ means sp² carbon; just below, sp³.
More detail on IR spectroscopy →Practice problems on IR spectroscopy →
02Mass spectrometry
A mass spectrometer ionises the molecule, then sorts the resulting positively charged species by mass-to-charge ratio. The molecular ion gives you the molecular weight; the fragments tell you how the skeleton comes apart, which is often more informative than the mass itself.
What to look for
- The highest significant peak is usually M⁺, the molecular ion.
- An M+2 peak of equal height means bromine; one third the height means chlorine.
- An odd molecular weight suggests an odd number of nitrogens.
- Losses of 15, 29, or 43 point to methyl, ethyl, or propyl fragments.
More on mass spectrometry →Practice problems on mass spectrometry →
03Proton NMR spectroscopy
A ¹H NMR spectrum carries four pieces of information: chemical shift tells you the electronic environment, integration counts the hydrogens, multiplicity counts the neighbours, and the coupling constant describes the geometry between them. Used together they usually settle the structure.
What to look for
- Count signals first — that is the number of distinct hydrogen environments.
- Integration ratios, not absolute heights, give the hydrogen count.
- n equivalent neighbours split a signal into n+1 lines.
- 6.5–8.5 ppm is aromatic; 9–10 ppm is an aldehyde; past 10 ppm, a carboxylic acid.
More on proton NMR →Practice problems on proton NMR →
04Carbon NMR spectroscopy
Only about 1% of carbon is the NMR-active ¹³C isotope, so signals are weak and carbon–carbon coupling is not observed. Proton decoupling collapses each carbon to a single line, which makes ¹³C NMR a clean count of the unique carbons in the skeleton — and a direct read on molecular symmetry.
What to look for
- Fewer signals than carbons means the molecule has symmetry.
- 0–50 ppm is sp³ carbon; 50–90 ppm means an attached oxygen or halogen.
- 100–150 ppm covers alkenes and aromatic rings.
- 160–185 ppm is an ester or acid; 190–220 ppm, a ketone or aldehyde.