¹H NMR — Spectroscopy | Organic Chemistry

Proton NMR is the single most informative technique in organic chemistry. Where IR identifies functional groups, ¹H NMR maps the carbon skeleton — how many distinct hydrogen environments there are, how many hydrogens are in each, and which environments sit next to which. Read together, those three pieces of information usually determine a structure outright.

What NMR measures

A hydrogen nucleus is a tiny magnet. Placed in a strong external field it can align with or against that field, and the energy gap between those two states depends on the field the nucleus actually experiences — which is the external field minus whatever its surrounding electrons shield it from. Radio waves at the matching frequency flip the nucleus, and that absorption is the signal.

The whole technique hangs on that shielding, because electron density around a nucleus varies with chemical environment. More electron density means more shielding, a smaller effective field, and a signal further upfield. Less means deshielding, and a signal further downfield.

Chemical shift: where a signal appears

The shift axis, and the one thing it is measuring. A hydrogen surrounded by plenty of electron density is shielded — the electrons set up a small opposing field, so the magnet has to work harder and the signal lands on the right. Put an oxygen or a halogen nearby and that density is pulled away, leaving the hydrogen exposed and moving it left. The ranges are worth knowing roughly rather than exactly: what you are usually asked is which of three signals sits furthest downfield, and that only needs the ordering.One idea sets this whole axis: a hydrogen sitting in more electron density is SHIELDED from the magnet and appears on the right. Put an electronegative atom next to it and that shielding is stripped away, so the signal moves left. Everything else — aromatic rings, aldehydes, acids — is that same effect, turned up.

Position is reported as chemical shift in parts per million, referenced against tetramethylsilane at 0. Using ppm rather than frequency makes the numbers independent of the instrument's field strength, so a shift measured on a 300 MHz machine matches one from a 600 MHz machine.

Electronegative neighbours deshield. So does an adjacent carbonyl, an adjacent pi system, and — most dramatically — an aromatic ring.

Environment	ppm
alkyl C–H	0.9–1.5
C–H next to C=C or aryl	1.6–2.5
C–H alpha to a carbonyl	2.0–2.5
C–H next to N	2.3–3.0
C–H next to O or halogen	3.3–4.5
vinyl C–H	4.5–6.5
aromatic C–H	6.5–8.5
aldehyde C–H	9–10
carboxylic acid O–H	10–13

Two entries deserve a note. Aromatic C–H sits unusually far downfield because the ring's pi electrons circulate in the applied field and generate a small field of their own that reinforces the deshielding on the outside of the ring — the ring current effect, which is specific to aromatic rings and is itself a test for aromaticity. And a carboxylic acid O–H is the most deshielded proton on the list, which is the same acidity you worked out in Module 10 showing up as a chemical shift.

Integration: how many hydrogens

The area under a signal is proportional to the number of equivalent hydrogens producing it. Only relative areas are meaningful, so integration gives you ratios — 3:2:1 — which you then scale against the molecular formula to get absolute counts.

Chemically equivalent hydrogens give one signal. The three hydrogens of a freely rotating methyl group are exchanged by that rotation faster than NMR can resolve, so they always appear together. Symmetry does the same job: the four aromatic hydrogens of p-xylene appear as a single peak. This means a symmetric molecule gives a much simpler spectrum than its hydrogen count suggests — which is information, not a loss.

Splitting: which environments are adjacent

A hydrogen's signal is split by non-equivalent hydrogens on adjacent carbons, because each neighbour's spin adds or subtracts a little from the local field. The result is the n + 1 rule: a hydrogen with n equivalent neighbours appears as n + 1 peaks, with relative intensities following Pascal's triangle.

No neighbours gives a singlet; one gives a doublet at 1:1; two give a triplet at 1:2:1; three give a quartet at 1:3:3:1. The spacing between the peaks of a multiplet is the coupling constant J, measured in Hz, and it is identical for both partners in a coupled pair — which is how you confirm that two multiplets are talking to each other rather than coincidentally similar.

J values carry their own structural information. In an alkene, trans coupling runs 12–18 Hz and cis coupling 6–12, which distinguishes E from Z directly. On an aromatic ring, ortho coupling is 7–10 Hz and meta coupling 2–3, which locates substituents.

Worked example — ethanol, signal by signal

Every ¹H NMR question is these three readings made together. How many signals counts distinct environments — ethanol has three. Integration gives the ratio of hydrogens, 3:2:1 here. Splitting reports on the neighbours, not on the signal itself: the methyl is a triplet because the CH₂ next door has two hydrogens, and the CH₂ is a quartet because the methyl has three. The OH stays a singlet because it swaps places with other OH protons faster than the instrument can watch.Splitting does not tell you about the hydrogens giving the signal. It tells you about their NEIGHBOURS: n equivalent hydrogens on the adjacent carbon split a signal into n + 1 lines. The methyl sees two, so it is a triplet; the CH₂ sees three, so it is a quartet. Each one is counting the other.

1.2 ppm, 3H, triplet. Three hydrogens means a methyl; a triplet means two neighbours; 1.2 ppm means nothing electronegative nearby. The CH₃.

3.6 ppm, 2H, quartet. Two hydrogens, three neighbours, and shifted well downfield by the adjacent oxygen. The CH₂.

2–5 ppm, 1H, broad singlet. The OH — a singlet despite having neighbours, because it exchanges with other OH protons faster than coupling can register, and broad and variable in position for the same reason.

Three independent lines of evidence — shift, integration, splitting — all pointing at the same structure. That mutual agreement is what makes an assignment convincing.

O–H and N–H protons usually do not follow the n + 1 rule. They exchange rapidly with each other and with traces of water, which averages away the coupling and leaves a broad singlet at an unpredictable shift. If a spectrum has one broad, un-split, oddly placed peak, suspect an exchangeable proton — and confirm it by adding D₂O, which swaps it for deuterium and makes the peak vanish entirely.

Working a spectrum

A reliable order of operations. Count the signals — that is the number of distinct hydrogen environments, and a small number relative to the formula means symmetry. Read the integrations as a ratio and scale to the formula. Use the shifts to say what each environment is attached to. Use the splitting to work out what is adjacent to what. Then assemble the fragments into a structure consistent with the molecular formula and the degrees of unsaturation.

Certain patterns become recognizable with practice: a 3H triplet with a 2H quartet is an ethyl group; a 6H doublet with a 1H septet is an isopropyl; a 9H singlet is tert-butyl; and two 2H doublets in the aromatic region is a para-disubstituted ring.

Worked example — C₄H₈O₂ from three signals

Data: 1.3 ppm (3H, t), 2.0 ppm (3H, s), 4.1 ppm (2H, q). IR shows a strong peak at 1735.

DoU = (8 + 2 − 8)/2 = 1, and the 1735 IR band says that one degree is an ester carbonyl.

The triplet/quartet pair is an ethyl group, and the quartet at 4.1 puts that CH₂ on an oxygen — so it is OCH₂CH₃. The 3H singlet at 2.0 is a methyl with no neighbours, at a shift typical of alpha to a carbonyl.

Assemble: CH₃–CO–O–CH₂CH₃, ethyl acetate. Every piece of data accounted for.

What carries forward

¹H NMR is the centrepiece of every structure-determination problem, and it works best alongside the others: IR for functional groups, mass spectrometry for the formula, ¹³C for the carbon count. The next section covers the carbon experiment, which is simpler to read and answers a different question.