IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2HL only~13 min read
Proton NMR Spectroscopy
Mass spectrometry weighs the molecule. Infrared finds the bonds. NMR does something better than either: it counts the hydrogen atoms and tells you what each one is sitting next to. Give a chemist an NMR spectrum and they can usually draw the whole structure.
📘 What you need to know
Only nuclei with an odd mass number have nuclear spin and give an NMR signal. 1H does; 12C does not.
In a strong magnetic field, these nuclei align either with the field (lower energy) or against it (higher energy).
Radio frequency energy makes them flip between the two states. This is resonance.
Protons in different chemical environments are shielded by different amounts, so they resonate at slightly different frequencies.
Shifts are measured against TMS (tetramethylsilane), which is defined as 0 ppm.
Number of signals = number of different hydrogen environments.
Area under each signal (shown by the integration trace) is proportional to the number of hydrogens in that environment.
Chemical shift values are given in section 21 of the data booklet.
Counting hydrogen environments
This is the single most important skill on the page, and it comes before any spectrum. Two hydrogens are in the same environment if they are chemically equivalent — if you could swap them and the molecule would be identical. In practice that means: hydrogens on the same carbon are equivalent, and so are hydrogens on carbons that are mirror images of each other.
Propanone is the cleanest example of why symmetry matters. Its structure is symmetrical about the carbonyl, so the six hydrogens are indistinguishable and the spectrum has exactly one line.
Symmetry is your friend here. Before counting, look for a mirror line through the molecule. Anything reflected onto itself is one environment, not two. That is why propanone gives one signal, and why 1,4-dimethylbenzene gives two rather than four.
Where the signals come from
Put a proton in a strong magnetic field and it behaves like a tiny magnet, lining up either with the field or against it. Lining up with the field is slightly lower in energy. Supply a radio-frequency photon of exactly the right energy and the proton flips to the higher state — it resonates — and the instrument records that absorption.
The clever part is that the frequency needed is not the same for every proton. The electrons around a proton create their own small magnetic field that opposes the applied one, so the proton feels a slightly weaker field than the instrument is producing. This is shielding.
The rule that explains every shift
more electron density around the proton → more shielding → smaller chemical shift
Put an electronegative atom nearby and it pulls electron density away from the proton. The proton is then deshielded, feels more of the applied field, and appears further to the left of the spectrum at a higher chemical shift. That is the whole logic: a CH3 on the end of a chain sits near 1 ppm, but a CH2 next to an oxygen is dragged out to 3–4 ppm.
TMS is used as the zero point for three reasons worth remembering. Its twelve hydrogens are all equivalent, so it gives one sharp peak. Silicon is less electronegative than carbon, so those hydrogens are unusually well shielded and sit further right than almost anything else. And it is inert and volatile, so it does not react with your sample and is easy to remove afterwards.
Reading a low-resolution spectrum
Here is ethanol. Three environments means three signals, and the heights are in the ratio 3 : 2 : 1 because that is how many hydrogens each environment contains.
The integration trace is a running total of peak area. Its step heights, not the peak heights, are what you measure — and you only ever read them as a ratio. A 3 : 2 : 1 trace could mean 3, 2 and 1 hydrogens, or 6, 4 and 2.
The OH signal moves. The data booklet gives alcohol O–H as anything from 1.0 to 6.0 ppm, which is an enormous range. It shifts because it depends on how much hydrogen bonding is happening, which changes with concentration, temperature and solvent. So never identify an OH by its position — identify it by its integration of 1H and by the fact that nothing else fits.
Chemical shift ranges
You are given these in section 21 of the data booklet, so the job is reading the table rather than memorising it. Even so, knowing roughly where things sit lets you sanity-check an answer.
The pattern to notice is that everything near an oxygen or a double bond is pushed to the left. An aldehyde proton at 9–10 ppm is the most easily recognised signal in the whole of NMR, because almost nothing else appears there.
🧩 Reading a low-resolution spectrum
Ignore the peak at 0 ppm. That is TMS, the reference, and it is not part of your compound.
Count the remaining signals. That is the number of different hydrogen environments.
Read the integration as a whole-number ratio. Divide through by the smallest step.
Look up each chemical shift in section 21 and write down what kind of proton it could be.
Check the total. Your hydrogen ratio must be consistent with the molecular formula — if the formula has 6 H and your ratio is 3 : 2 : 1, that works exactly.
Worked examples
WORKED EXAMPLE
Predict the number of signals and the integration ratio in the proton NMR spectrum of methyl ethanoate, CH3COOCH3.
Step 1: write out the structure and look for symmetryThere are two methyl groups, but they are not equivalent. One is bonded to the carbonyl carbon; the other is bonded to an oxygen.Step 2: group the hydrogens by environmentCH3 next to C=O: 3 HCH3 next to O: 3 HStep 3: check the total against the formulaC3H6O2, so 6 H altogether. 3 + 3 = 6 ✓2 signals, in the ratio 3 : 3, which is 1 : 1Both signals integrate the same, so integration alone cannot tell them apart. The chemical shift can: the O–CH3 sits near 3.7 ppm and the CO–CH3 near 2.0 ppm.
WORKED EXAMPLE
A compound is either propan-1-ol or propan-2-ol. Its proton NMR spectrum shows three signals in the ratio 6 : 1 : 1. Which is it?
Step 1: work out the environments in propan-1-olCH3CH2CH2OH has four environments: the end CH3, the middle CH2, the CH2 next to the oxygen, and the OH.4 signals, ratio 3 : 2 : 2 : 1Step 2: work out the environments in propan-2-ol(CH3)2CHOH is symmetrical about the middle carbon, so both methyl groups are equivalent.3 signals, ratio 6 : 1 : 1Step 3: compare with the dataThe spectrum has three signals in exactly 6 : 1 : 1, which matches the second structure and not the first.propan-2-olThe number of signals was already enough here. The 6H signal is the giveaway — only equivalent methyl groups can integrate that high.
WORKED EXAMPLE
Explain why the CH2 signal in chloroethane appears at about 3.5 ppm while the CH3 signal appears at about 1.5 ppm.
Step 1: identify what is different about the two carbonsIn CH3CH2Cl the CH2 is bonded directly to the chlorine. The CH3 is one carbon further away.Step 2: think about electron densityChlorine is much more electronegative than carbon, so it pulls electron density towards itself and away from the CH2 hydrogens.Step 3: link electron density to chemical shiftless electron density → less shielding → larger shiftThe CH2 protons are deshielded, so they resonate further to the left. The CH3 protons are two bonds away and barely affected.the CH2 is deshielded by the electronegative chlorine, so its shift is largerThe effect drops off fast with distance. In 1-chlorobutane the carbon furthest from the chlorine is back down near 1 ppm.
💡 Exam tip
Count environments before you look at the spectrum. Predicting the number of signals from the structure is faster and less error-prone than working backwards.
Look for symmetry first. Equivalent groups collapse into one signal, and missing that is the commonest way to over-count.
Read integration as a ratio, never as an absolute number. Divide by the smallest value, then scale to fit the molecular formula.
Use section 21 of the data booklet. The shift values are given, so quoting one from memory risks losing an easy mark.
Ignore the TMS peak when counting signals. It is the reference and belongs to no environment in your compound.
Explain shifts with shielding and electronegativity, not with “it is closer to the oxygen”. Name the mechanism.
⚠ Common mix-up
Counting hydrogens instead of environments. Propanone has six hydrogens but only one signal.
Treating two methyl groups as different when they are equivalent. Check for a mirror line before you commit.
Reading peak height instead of peak area. Area is what is proportional to hydrogen count, and area is what the integration trace measures.
Taking the integration values literally. A 3 : 2 : 1 trace could be 6 : 4 : 2 in a bigger molecule.
Identifying an OH by its chemical shift. It moves anywhere between 1 and 6 ppm depending on conditions.
Including the TMS peak as a signal from the compound. It is always there and always at exactly 0 ppm.
Saying 12C gives a signal. It has an even mass number and no nuclear spin, so it is invisible. Only 13C is active.
Up next: Peak Splitting in Proton NMR — so far each environment gave one line. At higher resolution those lines split apart, and the way they split tells you what is on the neighbouring carbon.
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