Lecture on NMR Spectroscopy and Coupling

Understanding Proton Environments

• Protons attached to the same carbon can exhibit different environments.
• Double Bonds:
  • The rigidity of double bonds prevents rotation, locking protons into distinct environments.
  • Red and blue protons on a carbon involved in a double bond are chemically non-equivalent.

Geminal Coupling

• Occurs when protons attached to the same carbon affect each other.
• Key concept: Geminal coupling is the interaction of protons on the same carbon.

NMR Spectrum Without Coupling

• Expect separate signals for each non-equivalent proton (e.g., one for red, one for blue).

NMR Spectrum With Coupling

• Protons can align with or against the external magnetic field, causing splitting:
  • Doublet Formation:
    • Red proton splits blue proton’s signal into a doublet.
    • Blue proton splits red proton’s signal into a doublet.

Coupling Constant

• Definition: Distance between peaks of a split signal, measured in Hertz (Hz).
• Properties:
  • Same for coupled protons.
  • Consistent regardless of NMR spectrometer frequency.
  • Example given: 1.4 Hz for both red and blue protons in the example discussed.

Actual NMR Spectrum Observations

• Signal heights may vary slightly in actual spectra.
• Roof Effect:
  • Peaks may form a 'roof' pointing towards the coupled proton.
  • Useful for interpreting NMR spectra.

Example of Coupling in Ethyl Groups

• Protons on Different Carbons:
  • Blue protons (CH2) have 3 neighboring protons (CH3), expecting a quartet.
  • Red protons (CH3) have 2 neighboring protons (CH2), expecting a triplet.
• N+1 Rule: Used to predict peaks:
  • Blue protons: n+1 = 4 (quartet)
  • Red protons: n+1 = 3 (triplet)
• Coupling Constant: 7 Hz in this example for both sets of protons.

Conclusion

• Understanding coupling constants is crucial for interpreting more complex splitting patterns, to be discussed further in subsequent lessons.