Lecture 9-5: Proton NMR and Signal Splitting

Introduction to Signal Splitting

• Signal splitting is unique to NMR.
• Helps deduce molecular structure by organizing parts of a molecule like a jigsaw puzzle.
• Observed through interaction between magnetic fields of hydrogen nuclei.

Concept of Signal Splitting

• Magnetic fields from adjacent hydrogen nuclei influence the observed magnetic field of a hydrogen signal.
• This influence results in splitting of the signal into multiple lines (doublets, triplets, etc.).
• N + 1 Rule: Helps predict the number of lines in the splitting:
  • Adjacent hydrogens (n) + 1 = Number of lines in the signal

Example of Signal Splitting

• Observing a signal for a red hydrogen adjacent to a methyl group:
  • Methyl group has 3 hydrogens.
  • Signal for red hydrogen is split into four lines (quartet) following the N + 1 rule (3+1=4).
• Methyl hydrogens give a signal that appears as a doublet (1 adjacent hydrogen, 1+1=2).

Overview of Signal Splitting Patterns

• Table of Adjacent Hydrogens and Resulting Signal
  • 0 adjacent hydrogens → Singlet
  • 1 adjacent hydrogen → Doublet
  • 2 adjacent hydrogens → Triplet
  • 3 adjacent hydrogens → Quartet
  • 4+ adjacent hydrogens → Multiplet (lines may be difficult to distinguish, call it a multiplet)

Application: Isopropyl Methyl Ether

• Predicting NMR spectrum signals:
  • Equivalents: Identify equivalent hydrogens.
    • Two methyl groups on the right labeled as A.
    • Unique CH labeled as B.
    • CH3 on the left labeled as C.
  • Signals and Splitting:
    • A: Adjacent to 1 hydrogen → Doublet (1+1=2)
    • B: Adjacent to 6 hydrogens → Multiplet (6+1=7, hard to resolve)
    • C: No adjacent hydrogens → Singlet (0+1=1)

Observation in NMR Spectrum

• Signal A: Appears as a doublet around 1 ppm.
• Signal B: Appears as a multiplet around 4 ppm (can resolve 7 lines here).
• Signal C: Appears as a singlet due to no adjacent hydrogens.
• Area under the signals is proportional to the number of protons.

Conclusion

• Signal splitting provides a way to analyze and predict molecular structures using NMR.
• Upcoming videos will continue to explore examples and applications.

End of Recording

• A question is posed for understanding check; answer to be determined by the student.