Structure 3.2.11 – Signal Splitting in ¹H NMR

Signal Splitting

In proton nuclear magnetic resonance (¹H NMR), signal splitting refers to the phenomenon where a single proton signal is split into multiple smaller peaks due to interactions with non-equivalent neighboring hydrogen atoms. 

Reason for Splitting

Nuclei with spin, such as hydrogen-1, create their own tiny magnetic fields. When protons are on adjacent carbon atoms, they can influence each other’s magnetic environments, causing the signals to split.

Only non-equivalent neighboring protons (on adjacent carbon atoms called vicinal protons) cause splitting. Equivalent protons do not split each other’s signals unless they are diastereotopic.

The (n + 1) Rule

Each set of equivalent protons in a molecule gives rise to a signal. However, if these protons are adjacent to non-equivalent hydrogen atoms, the signal may be split into multiple peaks. This splitting follows the well-known (n + 1) rule:

Number of peaks = n + 1

Where n = number of hydrogen atoms on adjacent carbon atoms.

Example:

• If a proton has 0 neighbors → 1 peak (singlet)
• If a proton has 1 neighbor → 2 peaks (doublet)
• If a proton has 2 neighbors → 3 peaks (triplet)
• If a proton has 3 neighbors → 4 peaks (quartet)

Appearance of Multiplets

The peaks in a multiplet follow predictable intensity patterns based on Pascal’s triangle:

Interpreting ¹H NMR Splitting Patterns

Signal splitting in ¹H NMR spectra provides valuable information about how hydrogen atoms are connected in a molecule. By interpreting the number and pattern of peaks in a signal, we can deduce how many neighboring hydrogen atoms (protons) are present. This phenomenon is known as spin-spin coupling.

Let’s explore the four basic types of splitting patterns commonly observed in simple organic compounds:

1. Singlet

A singlet appears as a single peak. This indicates that the proton giving rise to the signal has no neighboring hydrogen atoms. There are no adjacent protons to couple with it.

Example:

In \( CH_3OH \), the methyl protons appear as a singlet because the adjacent oxygen atom does not contain any hydrogen atoms capable of coupling.

2. Doublet

A doublet appears as two peaks of equal intensity, meaning the proton is coupled to one neighboring hydrogen atom (n = 1).

Example:

In \( CH_3CHCl \), the methyl protons are adjacent to a single proton on the methine carbon, resulting in a doublet.

3. Triplet

A triplet is a signal split into three peaks, with intensity ratio 1:2:1. This occurs when the proton is adjacent to two equivalent neighboring protons (n = 2).

Example:

In \( CH_3CH_2Br \), the methyl group is adjacent to a \( CH_2 \) group, so its protons are split by the two neighboring hydrogens, producing a triplet.

4. Quartet

A quartet appears as four peaks with an intensity ratio of 1:3:3:1. This splitting occurs when a proton is adjacent to three equivalent neighboring protons (n = 3).

Example:

Again in \( CH_3CH_2Br \), the \( CH_2 \) protons are adjacent to the methyl group (\( CH_3 \)), so the signal for \( CH_2 \) is split into a quartet.

Key Interpretation Tips

• Coupling only occurs between non-equivalent neighboring protons (typically on adjacent carbons).
• Protons on oxygen or nitrogen (such as OH or NH) often appear as broad singlets because they undergo rapid exchange and do not participate in spin-spin coupling under normal conditions.
• The intensity of the peaks within a multiplet follows Pascal’s triangle (1:1 for doublet, 1:2:1 for triplet, etc.).

Combining Splitting with Other Data

Splitting patterns must be interpreted together with:

• Integration: tells you how many protons are in that environment
• Chemical shift (δ): indicates the chemical environment (e.g., alkyl, aromatic, near oxygen, etc.)

Together, these features allow you to reconstruct the possible structure of the molecule.