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.
AQA provides the Periodic Table/Data Booklet for 7405; formulae are otherwise supplied in questions only where the specification says so. For these values, the skill is 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 flashcardsFree revision tool by Oscar Song Tutoring
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