NMR Spectroscopy and Spin-Spin Splitting

Introduction to NMR Signals

• NMR Spectrum: Predicts signals based on the environment of protons in a molecule.
• Chemical Shifts:
  • Proton 1: 7.25 ppm
  • Proton 2: 6.7 ppm
  • Three equivalent protons: 3.9 ppm (one signal for all three)

Expected vs. Actual NMR Spectrum

• Expected Spectrum: Signals at predicted chemical shifts.
  • Proton 1 (blue): 7.25 ppm
  • Proton 2 (red): 6.7 ppm
  • Equivalent protons: 3.9 ppm
• Actual Spectrum: Shows splitting of signals.
  • Red proton: Split into two peaks
  • Blue proton: Split into two peaks

Spin-Spin Splitting (Coupling)

• Concept: Interaction between magnetic fields of protons causes splitting.
• Mechanism:
  • Proton magnetic moments can align with or against the external magnetic field.
  • Interaction affects the effective magnetic field felt by neighboring protons.

Detailed Example

• Without Interaction:
  • Red proton: Single peak at 6.7 ppm
  • Blue proton: Single peak at 7.25 ppm
• With Interaction:
  • Spinning magnetic moments cause splitting into multiple peaks.

Effects of Magnetic Moment Alignment

• Field Aligned with External:
  • Increases effective magnetic field
  • Increases energy difference, frequency, and chemical shift
• Field Against External:
  • Decreases effective magnetic field
  • Decreases energy difference, frequency, and chemical shift

Resulting Spectrum

• Blue Proton Signal:
  • Split into two peaks (doublet) due to interaction with the red proton
• Red Proton Signal:
  • Experiences similar splitting (doublet)

More Complex Example

• Protons in Red:
  • Chemical shift: 3.95 ppm (equivalent protons)
• Proton in Blue:
  • Chemical shift: 5.77 ppm

Spin Combinations

• Possible Combinations:
  • Both red protons up, both down, one up one down (twice)
• Effect on Blue Proton:
  • Higher Shift: Both red protons magnetic fields up
  • Correct Shift: Opposing fields cancel out
  • Lower Shift: Both fields down

Triplet and Doublet Formation

• Blue Proton:
  • Experiences a triplet due to three possible field combinations from red protons
• Red Protons:
  • Form a doublet due to the blue proton's two possible magnetic alignments

Conclusion

• Spin-Spin Coupling: Causes splitting into doublets and triplets in NMR spectra.
• Next Steps: Further exploration of more complex spin-spin splitting examples.