This chemistry grades 11-12 lesson on spectroscopy notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on spectroscopy and check yourself with the quiz.
Quick answer: Mass spectrometry gives the relative molecular mass from the molecular ion and the structure from fragments, infrared spectroscopy identifies functional groups from bond absorptions, and proton and carbon-13 NMR show how many hydrogen and carbon environments exist and how they are joined. Together the three methods determine an unknown structure.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S3.2
Spectroscopy notes: the key ideas
What does a mass spectrum tell you?
In mass spectrometry a sample is ionised and the ions are sorted by their mass-to-charge ratio, m/z. The peak with the largest m/z is usually the molecular ion, M⁺, formed when the molecule loses one electron without breaking.
The m/z of the molecular ion equals the relative molecular mass, Mr. Smaller peaks come from fragments. For propanone the molecular ion is at 58 and the tallest peak, the base peak, is at 43 from CH₃CO⁺. Fragment peaks reveal how the molecule is built, because the bonds next to a C=O or a branch break most easily.
Carbon-13 makes a small peak one unit above M⁺. About 1.1 % of carbon atoms are ¹³C, so each carbon adds about 1.1 % to the M+1 peak. Chlorine and bromine give distinctive pairs: chlorine shows M and M+2 in a 3 : 1 ratio and bromine in a 1 : 1 ratio.
| m/z | Typical fragment |
|---|---|
| 15 | CH₃⁺ |
| 29 | C₂H₅⁺ or CHO⁺ |
| 43 | C₃H₇⁺ or CH₃CO⁺ |
| 57 | C₄H₉⁺ or C₂H₅CO⁺ |
An ester shows M⁺ at m/z 88 with height 20.0 mm and an M+1 peak of height 0.9 mm. Find the number of carbon atoms.
Ratio = 0.9 ÷ 20.0 = 4.5 %. Number of C = 4.5 ÷ 1.1 = 4.1, so there are 4 carbon atoms. With Mr = 88: 88 − 48 (C₄) − 32 (O₂) = 8, so the formula is C₄H₈O₂.
How does infrared spectroscopy identify bonds?
Infrared (IR) spectroscopy measures the wavenumbers, in cm⁻¹, at which bonds absorb radiation and vibrate more strongly. A bond absorbs at a characteristic wavenumber, so an absorption shows which functional groups are present.
Wavenumber is 1 divided by wavelength in centimetres. Stronger and lighter bonds absorb at higher wavenumbers, which is why O–H and C=O lie far apart. Compare your spectrum with the chart, and use the region below 1500 cm⁻¹ only to match a compound with a reference.
Do not assign the fingerprint region bond by bond. It is a complex pattern that only matches a known sample, whereas groups such as C=O and O–H are read above 1500 cm⁻¹.
What does proton NMR show?
Hydrogen nuclei in different chemical environments absorb at different chemical shifts, δ, measured in ppm from tetramethylsilane, TMS (δ = 0). Four pieces of information appear: the number of signals (the number of environments), the chemical shift (the surroundings), the integration (the ratio of hydrogens) and the splitting.
| Environment | Approximate δ / ppm |
|---|---|
| CH₃–C or –CH₂–C | 0.9–1.5 |
| CH₃–C=O | 2.0–2.5 |
| –O–CH₂ and –O–CH₃ | 3.3–4.3 |
| Aromatic H | 6.5–8.0 |
| Aldehyde –CHO | 9.4–10.0 |
A hydrogen with n equivalent neighbours on adjacent carbons splits its signal into n + 1 lines.
Ethyl ethanoate, CH₃COOCH₂CH₃, has three environments. The CH₃ of ethyl (δ 1.26) has two neighbours, so a triplet of 3H. The CH₂ (δ 4.12) is next to oxygen, which pulls electron density away, and has three neighbours, so a quartet of 2H. The CH₃CO group (δ 2.04) has none, so a singlet of 3H.
What does carbon-13 NMR add?
Each carbon in a different environment gives one signal, normally a single line. Ethyl ethanoate shows four signals, at about 14, 21, 60 and 171 ppm. Carbons in C=O absorb at 160–220 ppm, those bonded to oxygen at 50–90 ppm, alkene and aromatic carbons at 110–150 ppm, and simple alkyl carbons below 55 ppm.
How do you solve a structure from several spectra?
Take M⁺ for the formula, use IR to identify groups, count 13C signals for symmetry, then use proton NMR to join the pieces. Check each fragment of your structure against the data.
Write the signal list in a table first: shift, integration, splitting. Then draw the fragment for each row, and join the fragments so that every neighbour count works.
- Molecular ion M⁺ has m/z equal to Mr; each carbon adds about 1.1 % to the M+1 peak.
- IR: C=O at 1680–1750 cm⁻¹, O–H alcohol at 3230–3550 cm⁻¹, acid O–H very broad at 2500–3300 cm⁻¹.
- Proton NMR gives number of signals, shift, integration and n + 1 splitting; TMS is at δ = 0.
- 13C NMR shows one signal per carbon environment; C=O appears at 160–220 ppm.
Solving structures with mass, IR and NMR spectra: frequently asked questions
What does the molecular ion peak show?
The molecular ion peak shows the relative molecular mass of the compound, because M⁺ is the molecule with one electron removed. It is normally the peak with the highest m/z, apart from a small M+1 peak.
Why is TMS used as a standard in NMR?
TMS gives one sharp signal because its twelve hydrogens are equivalent, and this appears at the right-hand end of the scale at δ = 0. It is chemically inert and volatile, so it is easy to remove.
How do you recognise a carboxylic acid in IR?
A carboxylic acid shows a very broad O–H absorption from about 2500 to 3300 cm⁻¹ together with a strong C=O absorption near 1710 cm⁻¹. An ester shows C=O but no broad O–H.
What does the n + 1 rule predict?
A signal for hydrogens with n equivalent neighbours on adjacent carbons splits into n + 1 lines. Two neighbours give a triplet and three neighbours give a quartet.
How many signals does 13C NMR show?
It shows one signal for each different carbon environment. Symmetrical molecules give fewer signals than the number of carbon atoms.
