Learn & Review: Integration of H NMR Signals

Summary of H NMR Spectroscopy and Integration

This content explains how to interpret H NMR spectra using the concept of integration to identify which signals correspond to which protons in a molecule.

Main Idea: Using Integration to Correlate Signals with Protons

• Integration in H NMR spectroscopy measures the area under a signal's curve.
• This area is directly proportional to the number of hydrogen atoms (protons) that produce that signal.
• Therefore, by comparing the relative areas (or heights) of signals, we can determine the ratio of protons responsible for each signal.

Key Concepts and Applications

1. Signal Intensity and Proton Count:

   • Signals corresponding to a larger number of protons will appear taller (have a larger integral area).
   • Signals corresponding to fewer protons will appear shorter (have a smaller integral area).

2. Determining Proton Ratios:

   • The ratio of the integral traces of different signals is proportional to the ratio of the number of protons they represent.
   • For example, if the integral ratio is 3:1, it means one signal represents three times as many protons as the other.

Example 1: A Ketone Molecule

• Molecule: A ketone with two distinct types of protons.
  • Nine protons from three identical methyl groups.
  • Three protons from a single methyl group.
• Spectrum: Two signals are observed.
  • Signal A: Corresponds to the nine protons. It appears taller and is located at approximately 1 ppm.
  • Signal B: Corresponds to the three protons. It appears shorter and is located at approximately 2.3 ppm.
• Integration Rationale: The ratio of protons (9:3) is 3:1. The ratio of integral traces (e.g., 3.6:1.2) also yields 3:1, confirming the assignment.

Example 2: Another Ketone Molecule

• Molecule: A ketone with three distinct types of protons.
  • Set A: Nine protons from three identical methyl groups.
  • Set B: Six protons from two identical methyl groups attached to the same carbon.
  • Set C: Three protons from a single methyl group.
• Spectrum: Three signals are observed at approximately 1 ppm, 2.2 ppm, and 2.3 ppm.
• Integration Assignment:
  • Signal A (tallest): Corresponds to the nine protons (Set A).
  • Signal B (second tallest): Corresponds to the six protons (Set B).
  • Signal C (shortest): Corresponds to the three protons (Set C).
• Chemical Equivalence:
  • Methyl groups in Set A are identical due to free rotation around the single bond, making them chemically equivalent.
  • Methyl groups in Set B are also identical due to free rotation.
  • The methyl group in Set C is in a different chemical environment.

Key Takeaway

Integration is a crucial tool in H NMR spectroscopy that allows chemists to determine the relative number of protons contributing to each signal, thereby helping to elucidate the structure of organic molecules. The tallest signal corresponds to the most protons, and the shortest signal corresponds to the least protons.