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

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

What Causes Splitting in 'H NMR?

  • 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).
This splitting provides crucial information about the number of neighboring hydrogen atoms (protons) near the hydrogen responsible for that signal.
  • The splitting occurs because of a phenomenon called spin-spin coupling.
  • Each hydrogen nucleus has a magnetic moment (a tiny magnetic field) due to its nuclear spin.
  • When hydrogen atoms are close to each other (on neighboring carbon atoms), their magnetic fields interact.
This interaction causes the signal of one hydrogen to split into multiple peaks.

Analogy

Think of spin-spin coupling like two people in a conversation. Just as one person’s speech influences the other’s response, neighboring hydrogens influence each other’s signals in the NMR spectrum.

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.

Tip

When counting neighboring hydrogens, focus on hydrogens attached to carbons directly adjacent to the carbon of the hydrogen being analyzed. Ignore hydrogens that are further away or part of equivalent groups.

n+1 rule for splitting patters.
n+1 rule for splitting patters.

Common Splitting Patterns

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

1.Singlet

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

Example

The methyl group in CH3−O−CH3 (dimethyl ether) produces a singlet because the hydrogens on each CH3 group have no adjacent hydrogens.

2.Doublet

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

Example

In CH3CHCl2 (1,1-dichloroethane), the CH3 group produces a doublet because it is adjacent to a single CH proton.

3.Triplet

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

Example

In CH3CH2OH (ethanol), the CH3 group produces a triplet because it is adjacent to the CH2 group with two hydrogens.

4.Quartet

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

Example

In CH3CH2OH (ethanol), the CH2 group produces a quartet because it is adjacent to the CH3 group with 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.

Note

Splitting patterns beyond a quartet (e.g., quintets, sextets) are less common but follow the same principles of the n+1 rule.

Illustrating splitting patterns.
Illustrating splitting patterns.

Interpreting a Splitting Pattern

Let’s analyze the splitting patterns in the 'H NMR spectrum of CH3CHBr2 (1,1-dibromoethane).

1H NMR for 1,1-dibromoethane.
1H NMR for 1,1-dibromoethane.
  1. Step 1: Identify the hydrogen environments.
    • The molecule has two distinct hydrogen environments: CH3 and CH.
  2. Step 2: Analyze the CH3 group.
    • The CH3 group is adjacent to the CH group, which has one hydrogen.
    • According to the n+1 rule, the CH3 group’s signal will be split into n+1 = 1+1 = 2 peaks (a doublet).
  3. Step 3: Analyze the CH group.
    • The CH group is adjacent to the CH3 group, which has three hydrogens.
    • 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:
    • The 'H NMR spectrum will show a doublet for the CH3 group and a quartet for the CH group.

Self review

Based on the n+1 rule, can you predict the splitting pattern for the hydrogens in CH3CH2CH2OH? Which groups would produce triplets, doublets, or singlets?

Applications of Splitting Patterns

Splitting patterns are a powerful tool for deducing molecular structures. By combining splitting information with:

  • Chemical shifts (where the peaks appear on the spectrum),
  • Integration traces (the relative number of hydrogens in each environment), and you can piece together the arrangement of hydrogens and their neighbors in a molecule.

Hint

  • A triplet-quartet pattern suggests an ethyl group (CH3CH2-).
  • A doublet-doublet pattern might indicate two hydrogens on a double bond (cis/trans isomers).

Theory of Knowledge

How does the ability to interpret 'H NMR spectra demonstrate the interplay between observation and inference in scientific knowledge? What are the limitations of relying solely on NMR data to deduce molecular structures?

Common Mistakes to Avoid

Common Mistake

Many students forget that equivalent hydrogens (in the same chemical environment) do not split each other’s signals. Always count onlynon-equivalentneighboring hydrogens when using the n+1 rule.

Common Mistake

Another frequent error is misinterpreting overlapping peaks. Use the integration trace and J values to distinguish between overlapping signals and true splitting patterns.

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Question 1

What are the common splitting patterns observed in 'H NMR spectra, and how can they be used to deduce the structure of organic compounds?

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What is spin-spin coupling in 'H NMR?

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Note

Introduction to H NMR Spectra Interpretation

  • Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful analytical technique used to determine the structure of organic compounds.
  • In H NMR spectroscopy, we focus on the behavior of hydrogen nuclei in a magnetic field.
  • The chemical environment of hydrogen atoms affects their NMR signals, providing valuable structural information.

Analogy

Think of H NMR spectroscopy like listening to a symphony where each instrument represents a different hydrogen environment. The unique sound of each instrument helps you identify its role in the orchestra.

Definition

Chemical Shift

The position of an NMR signal on the spectrum, measured in parts per million (ppm), indicating the electronic environment of a hydrogen atom.

Definition

Integration

The area under an NMR signal, proportional to the number of hydrogen atoms contributing to that signal.