¹H NMR and Combined Techniques

Where ¹³C NMR gives you a map of the carbon skeleton, ¹H NMR (proton NMR) gives you a map of the hydrogens. Because there are usually more hydrogens than carbons in an organic molecule, and because each hydrogen "feels" its neighbours via spin-spin coupling, ¹H NMR gives more structural information per spectrum than any other technique. Combined with ¹³C, IR and MS, it usually lets you pin down a structure uniquely.

This lesson covers the OCR A-Level Chemistry A (H432) specification points 6.3.2 (b)–(c): interpretation of ¹H NMR spectra including integration, multiplicity (n+1 rule), D₂O exchange, and combined use of NMR with IR and MS.

1. What ¹H NMR Tells You

A ¹H NMR spectrum contains four separate pieces of information for each set of peaks:

Chemical shift (δ, ppm) — tells you the kind of environment (alkyl, near O, aromatic, etc.).
Integration (area under peak) — tells you the relative number of H nuclei in each environment.
Splitting pattern (multiplicity) — tells you the number of neighbouring H nuclei (n+1 rule).
Number of peak groups — tells you the number of different H environments.

Combined, these four give you the full picture of the hydrogen skeleton.

2. Chemical Shifts in ¹H NMR

The x-axis of a ¹H spectrum runs from ~0 to ~12 ppm, with TMS at 0 ppm as the reference (same as ¹³C). OCR provides a shift table in the data booklet.

2.1 Typical chemical shifts (OCR data booklet)

Proton type	Typical δ (ppm)
R–CH₃ (alkyl methyl)	0.7 – 1.3
R–CH₂–R (alkyl methylene)	1.2 – 1.4
R₃C–H (alkyl methine)	1.5 – 2.0
CH₃ adjacent to C=O	2.0 – 2.5
CH₂ adjacent to C=O	2.2 – 2.7
CH₃ adjacent to O (ester, ether)	3.3 – 3.9
CH₂ adjacent to O	3.3 – 4.5
R–OH (variable — broad, D₂O exchangeable)	0.5 – 5.0
R–NH₂ (variable — broad, D₂O exchangeable)	1.0 – 4.5
Alkene =CH	4.5 – 6.0
Aromatic Ar–H	6.5 – 8.0
R–CHO (aldehyde H)	9.0 – 10.0
R–COOH (acid OH)	10.0 – 13.0 (broad)

Pattern to remember: Alkyl H around 0–2 ppm, H next to O or N around 3–5 ppm, aromatic around 7 ppm, aldehyde around 9–10 ppm, acid OH around 11–13 ppm.

2.2 Working out environments

Use the same equivalence rules as for ¹³C. Equivalent hydrogens (related by symmetry) all appear as a single group with a single chemical shift.

Example: Ethanol, CH₃CH₂OH

3 × CH₃ hydrogens → 1 environment
2 × CH₂ hydrogens → 1 environment
1 × OH hydrogen → 1 environment
Total: 3 environments, 3 groups of peaks.

Example: 1,4-dimethylbenzene, (CH₃)₂C₆H₄

6 × CH₃ hydrogens (both methyls equivalent) → 1 environment
4 × aromatic hydrogens (all equivalent by the C2 symmetry of the ring) → 1 environment
Total: 2 environments, 2 groups of peaks.

3. Integration — Counting the Hydrogens

Modern ¹H NMR spectra are displayed with a step curve or numerical integration values above each peak. The integral represents the area under the peak — not the height — and is proportional to the number of hydrogens giving rise to that peak.

3.1 How to read integration

The integrals are usually given as relative numbers (e.g. 3:2:1) — you need to deduce the actual number of Hs from the molecular formula.

Example — ethanol (C₂H₆O): Spectrum shows three peaks with integrals in ratio 3:2:1.

Total H in molecule = 6.
Ratio 3:2:1 adds to 6.
Peak with integral 3 = 3 H (the CH₃).
Peak with integral 2 = 2 H (the CH₂).
Peak with integral 1 = 1 H (the OH).

Exam Tip: Integration gives the relative, not absolute, number of hydrogens. Always cross-check against the molecular formula from the mass spectrum or elemental analysis.

4. Multiplicity — The n+1 Rule

This is the crown jewel of ¹H NMR interpretation. Each peak is not a single line but a multiplet split by neighbouring hydrogens.

4.1 The n+1 rule

Key Rule — n+1 Rule: If a hydrogen environment has n equivalent hydrogens on directly bonded neighbouring carbons, its peak is split into (n+1) lines (a multiplet of n+1 peaks).

Number of neighbours (n)	Number of peaks (n+1)	Name
0	1	Singlet (s)
1	2	Doublet (d)
2	3	Triplet (t)
3	4	Quartet (q)
4	5	Quintet
5	6	Sextet
6	7	Septet

The lines within a multiplet have characteristic relative intensities given by Pascal's triangle: doublet 1:1, triplet 1:2:1, quartet 1:3:3:1.

4.2 Which neighbours count?

Hydrogens on adjacent carbons (one bond away from the carbon bearing the hydrogen you are looking at).
Hydrogens on the same carbon as the observed H do not count (they are equivalent to the observed H, and equivalent H do not split each other).
Hydrogens bonded to O or N are usually not coupled (or appear as a broad singlet) — they exchange rapidly with solvent (see D₂O section below).

4.3 Worked example — ethanol

CH₃CH₂OH has three H environments. Let's predict each multiplicity:

CH₃ peak (~1.2 ppm): The neighbouring carbon (CH₂) has 2 H. So n = 2 → splits into (2+1) = 3 lines → triplet.
CH₂ peak (~3.7 ppm): Neighbour 1 is CH₃ (3 H). Neighbour 2 is O–H (1 H — but this is typically an exchange singlet, see below, so treat as 0 neighbours from OH). Effective n = 3 → (3+1) = 4 lines → quartet.
OH peak (~2.5–5 ppm, variable): Usually a broad singlet because of fast exchange with trace H⁺ — the coupling to CH₂ is averaged out.

So ethanol gives a triplet + quartet + broad singlet pattern — a diagnostic fingerprint for an ethyl group attached to oxygen.

4.4 Worked example — ethyl ethanoate

CH₃COOCH₂CH₃ has:

Lesson 14: ¹H NMR and Combined Techniques