Spectroscopy
IR, mass spec and NMR — the values you're given, the fragments you're not, and how to read a spectrum. One page, with a recognition drill.
These values are in your exam data booklet — so the marks are for recognising and using them fast. Test mode drills that.
Infrared (IR) absorptions
Work top-down: broad 2500–3000 = acid; strong sharp ~1700 = C=O; broad ~3300 = O–H or N–H. The fingerprint region below 1500 cm⁻¹ is unique to each molecule.
| Bond / group | Absorption |
|---|---|
| O–H alcohols, phenols | 3230–3550 cm⁻¹ (broad) |
| O–H carboxylic acids | 2500–3000 cm⁻¹ (very broad) the give-away for a carboxylic acid |
| N–H amines, amides | 3300–3500 cm⁻¹ |
| C–H | 2850–3100 cm⁻¹ |
| C≡N nitriles | 2220–2260 cm⁻¹ |
| C=O aldehydes, ketones, acids, esters, amides | 1680–1750 cm⁻¹ (strong, sharp) strong sharp peak — the carbonyl fingerprint |
| C=C alkenes | 1620–1680 cm⁻¹ |
| C–O alcohols, esters, acids | 1000–1300 cm⁻¹ |
Mass spectrometry fragments
| Peak / loss | Means |
|---|---|
| Molecular ion, M⁺ | = relative molecular mass (Mr) the peak at the highest m/z (ignore the tiny M+1) |
| Loss of 15 | –CH3 (methyl) |
| Loss of 17 | –OH |
| Loss of 18 | –H2O |
| Loss of 29 | –CHO or –C2H5 |
| Loss of 31 | –OCH3 or –CH2OH |
| Loss of 45 | –COOH or –OC2H5 |
| m/z = 15 | CH3⁺ |
| m/z = 29 | CHO⁺ or C2H5⁺ |
| m/z = 43 | CH3CO⁺ (acylium) or C3H7⁺ |
| m/z = 77 | C6H5⁺ (phenyl) — signals a benzene ring |
| M and M+2 (1 : 1) | molecule contains bromine |
| M and M+2 (3 : 1) | molecule contains chlorine |
¹H NMR
Reading a ¹H spectrum
- Number of peaks = number of different proton environments.
- Integration (area) = ratio of protons in each environment.
- Splitting (n + 1 rule) = a peak split into (n + 1) lines has n protons on the adjacent carbon. Singlet 1, doublet 2, triplet 3…
- TMS is the δ = 0 reference; CDCl3 / D2O are solvents. O–H / N–H peaks disappear on a D2O shake.
| Proton environment | Chemical shift δ |
|---|---|
| R–CH3 | δ 0.7–1.2 ppm |
| R–CH2–R | δ 1.2–1.4 ppm |
| R–CO–CH3 (next to C=O) | δ 2.1–2.7 ppm |
| R–O–CH3 / R–O–CH2 (next to O) | δ 3.1–3.9 ppm |
| R–O–H / R–N–H | δ 1–5 ppm (variable, D2O-exchangeable) |
| Aromatic H (benzene ring) | δ 6.5–8.0 ppm |
| R–CHO (aldehyde H) | δ 9.0–10.0 ppm |
| R–COOH (acid H) | δ 10–12 ppm (broad) |
Lock it in with flashcards
The AQA Chemistry deck drills IR, NMR and mass-spec recognition with spaced repetition.
AQA A-level Chemistry flashcardsStruggling with spectroscopy? The first tutoring lesson is free.
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