Proton NMR splitting patterns and aromatic proton signals help identify structural features in organic molecules by revealing neighbouring proton numbers and characteristic aromatic chemical-shift ranges.

Proton NMR Splitting: Core Principles

Proton NMR spectroscopy provides detailed information about proton environments, neighbouring groups, and structural patterns in organic molecules. A key aspect of interpreting spectra is understanding spin–spin splitting, which shows how many protons are adjacent to a particular hydrogen.

The Origin of Spin–Spin Coupling

Spin–spin coupling occurs when non-equivalent neighbouring protons influence one another’s magnetic environment, causing a signal to appear as multiple peaks rather than a single resonance.

This interaction is transmitted through the σ-bonds connecting the atoms, generally across three bonds, meaning only protons on adjacent carbon atoms usually cause splitting.

The n + 1 Rule

The n + 1 rule stated in the OCR specification allows prediction of proton signal splitting based on the number of adjacent protons (n). The observed signal becomes a multiplet of n + 1 peaks.

This ¹H NMR spectrum of ethanol shows how neighbouring protons split signals into multiplets. The CH₃ group appears as a triplet and the CH₂ group as a quartet, directly illustrating the n + 1 rule. Source

In practice, this allows rapid structural deduction by linking peak patterns to likely bonding arrangements. After considering splitting, students should also evaluate relative peak areas and chemical shifts to confirm assignments.

Typical Multiplets and Their Structural Meaning

Splitting patterns follow predictable forms:

• Singlet (1 peak)

  • No adjacent non-equivalent protons.

• Doublet (2 peaks)

  • One neighbouring proton.

• Triplet (3 peaks)

  • Two neighbouring protons.

• Quartet (4 peaks)

  • Three neighbouring protons.

• Multiplet (complex splitting)

  • Several neighbouring sets or asymmetrical coupling.

Coupling patterns for simple alkyl groups are particularly characteristic:

• CH₃–CH₂– fragments often show a triplet for the CH₃ protons and a quartet for the CH₂ protons.

• Isolated CH₃ groups bonded to quaternary carbons show singlets.

• CH groups attached to different proton environments may appear as multiplets.

Coupling Constants (J-values)

Spin–spin interactions give rise to measurable separation between peaks called coupling constants. These are expressed in hertz (Hz) and reflect the strength of interaction between the coupled protons.

Coupling constants help distinguish overlapping patterns and confirm whether peaks arise from the same coupling system. For example, a doublet and triplet sharing the same J-value often come from neighbouring CH and CH₂ groups.

Aromatic systems show characteristic J-values that can indicate substitution patterns, but OCR A-Level primarily requires recognition of aromatic chemical-shift regions rather than advanced coupling analysis.

Aromatic Protons in Proton NMR

Aromatic rings create distinctive deshielded proton environments due to their circulating π-electron system, which induces an external magnetic field effect known as the ring-current effect. As a result, aromatic protons appear downfield relative to aliphatic protons.

This diagram shows how circulating π electrons in an aromatic ring generate an induced magnetic field that deshields aromatic protons, shifting their signals downfield. Source

Aromatic Proton Chemical-Shift Range

According to the OCR specification, students must be able to recognise aromatic proton chemical shifts. Aromatic hydrogens typically resonate in a characteristic region:

This ¹H NMR spectrum shows aromatic proton signals clustered around 7 ppm, a diagnostic feature of aromatic rings, alongside non-aromatic signals elsewhere in the spectrum. Source

• Chemical shift range: δ 6.0–9.0 ppm

These values vary slightly depending on ring substituents:

• Electron-withdrawing groups (e.g., NO₂, COOH) shift aromatic protons further downfield (closer to 8.5–9.0 ppm).

• Electron-donating groups (e.g., OH, NH₂, alkyl groups) can shift some aromatic protons upfield (around 6.0–7.0 ppm).

• Substitution pattern affects equivalence:

  • Ortho, meta, and para positions may produce distinct chemical shifts if they make the protons non-equivalent.

Why Aromatic Protons Are Deshielded

The ring-current effect plays a central role:

This phenomenon creates a predictable spectral signature, allowing rapid recognition of aromatic signals even when they form complex multiplets.

In-plane substituents and ring symmetry can also influence the degree of deshielding, but the fundamental requirement for OCR is the ability to identify aromatic resonances within their diagnostic ppm region.

Multi-Peak Patterns in Aromatic Regions

Although OCR does not require detailed aromatic coupling analysis, students should be aware that aromatic regions often contain overlapping multiplets due to the number of interacting protons around the ring. Aromatic protons can couple with several neighbours, often producing:

• Multiplets with unclear symmetry

• Clusters of peaks between 6–9 ppm

• Characteristic downfield shifts marking aromatic presence

Key Points for Structural Identification Using Splitting and Aromatic Signals

When interpreting a proton NMR spectrum:

• Check for splitting patterns using the n + 1 rule to determine the number of neighbouring protons.

• Note J-values when available to confirm which peaks belong to the same coupling system.

• Identify aromatic proton signals by their distinctive 6–9 ppm region.

• Evaluate the number of aromatic protons to infer substitution patterns:

  • Five aromatic hydrogens often indicate a monosubstituted benzene ring.

  • Four or fewer hydrogens suggest di- or tri-substitution.

• Combine splitting, chemical shift, and relative proton numbers to map out both aromatic and aliphatic structural features.

Understanding these principles enables construction of accurate structural proposals consistent with OCR exam expectations.