S3.2.11 H NMR spectra interpretation (Higher Level Only) Notes

Splitting Patterns in Proton Nuclear Magnetic Resonance ('H NMR) Spectroscopy

What Causes Splitting in 'H NMR?

1. When you observe an 'H NMR spectrum, you’ll notice that some signals are not just single peaks: they are split into clusters of peaks (e.g., doublets, triplets, quartets).
2. This splitting provides crucial information about the number of neighboring hydrogen atoms (protons) near the hydrogen responsible for that signal.
3. The splitting occurs because of a phenomenon called spin-spin coupling.
4. Each hydrogen nucleus has a magnetic moment (a tiny magnetic field) due to its nuclear spin.
5. When hydrogen atoms are close to each other (on neighboring carbon atoms), their magnetic fields interact.
6. This interaction causes the signal of one hydrogen to split into multiple peaks.

The $n+1$ Rule for Splitting Patterns

1. The splitting pattern of a signal is determined by the n+1 rule. This rule states:
2. The number of peaks in a signal = n + 1, where n is the number of equivalent neighboring hydrogen atoms.

Key Points:

1. Neighboring hydrogens are those attached to adjacent carbon atoms.
2. Hydrogens that are chemically equivalent (in the same chemical environment) do not split each other’s signals.
3. The number of peaks in the splitting pattern corresponds to n + 1.

Common Splitting Patterns

Let’s explore the most common splitting patterns and what they tell you:

Singlet

1. Number of neighboring hydrogens (n): 0
2. Number of peaks: 1
3. Appearance: A single peak.
4. Interpretation: The hydrogen responsible for this signal has no neighboring hydrogens.

Doublet

1. Number of neighboring hydrogens (n): 1
2. Number of peaks: 2
3. Appearance: Two peaks of equal intensity.
4. Interpretation: The hydrogen responsible for this signal is adjacent to one hydrogen.

Triplet

1. Number of neighboring hydrogens (n): 2
2. Number of peaks: 3
3. Appearance: Three peaks with an intensity ratio of 1:2:1.
4. Interpretation: The hydrogen responsible for this signal is adjacent to two hydrogens.

Quartet

1. Number of neighboring hydrogens (n): 3
2. Number of peaks: 4
3. Appearance: Four peaks with an intensity ratio of 1:3:3:1.
4. Interpretation: The hydrogen responsible for this signal is adjacent to three hydrogens.

Multiplets and Complex Patterns

1. When a hydrogen is influenced by multiple groups of non-equivalent neighboring hydrogens, more complex splitting patterns can arise.
2. These are often referred to as multiplets and require more advanced analysis.

Interpreting a Splitting Pattern

Let’s analyze the splitting patterns in the 'H NMR spectrum of $CH_3CHBr_2$ (1,1-dibromoethane).

1. Step 1: Identify the hydrogen environments.
   1. The molecule has two distinct hydrogen environments: $CH_3$ and CH.
2. Step 2: Analyze the $CH_3$ group.
   1. The $CH_3$ group is adjacent to the CH group, which has one hydrogen.
   2. According to the n+1 rule, the $CH_3$ group’s signal will be split into n+1 = 1+1 = 2 peaks (a doublet).
3. Step 3: Analyze the CH group.
   1. The CH group is adjacent to the $CH_3$ group, which has three hydrogens.
   2. According to the n+1 rule, the CH group’s signal will be split into n+1 = 3+1 = 4 peaks (a quartet).
4. Result:
   1. The 'H NMR spectrum will show a doublet for the $CH_3$ group and a quartet for the CH group.