Understanding Proton NMR and Chemical Shifts

Aug 13, 2024

Lecture Notes: Proton NMR and Chemical Shifts

Methane and NMR

  • Methane Overview: Contains four equivalent protons.
  • NMR Signal: Results in one signal at approximately 1 part per million (ppm).
  • Energy Difference: Related to the energy difference between alpha and beta spin states.
    • Equation: Energy difference corresponds to the frequency (E = hν).
    • Magnetic Field Influence: Energy difference influenced by the effective magnetic field experienced by the proton.
  • Frequency and Chemical Shift:
    • Lower frequency = lower chemical shift.
    • Methane protons are shielded compared to chloromethane.

Chloromethane and NMR

  • Chloromethane Overview: Three equivalent protons.
  • NMR Signal: Signal occurs just past 3 ppm (~3.1 ppm).
  • Electronegative Atom Impact:
    • Chlorine, being electronegative, withdraws electron density, deshielding protons.
    • Deshielded protons experience a greater effective magnetic field.
  • Frequency and Chemical Shift:
    • Greater energy difference = higher frequency = higher chemical shift.

Summary of Shielding Effects

  • Shielded Protons:
    • Lower frequency signal → Lower chemical shift.
  • Deshielded Protons:
    • Higher frequency signal → Higher chemical shift.
  • Electronegativity:
    • Electronegative atoms deshield protons, resulting in higher chemical shifts.

Chemical Shifts and Functional Groups

  • NMR Spectrum Overview:
    • Left (downfield): More deshielded protons.
    • Right (upfield): More shielded protons.
  • Alkane Environment:
    • Shielded proton; chemical shift ~0.5 - 2 ppm.
  • Chloromethane Environment:
    • Deshielded by electronegative atoms (e.g., halogens, oxygen); chemical shift ~2.5 - 4.5 ppm.
  • Proton Next to Carbonyl:
    • Chemical shift ~2 - 2.5 ppm.
  • Alcohol Proton:
    • Affected by hydrogen bonding; broad range ~2 - 5 ppm.
  • Proton on Double Bond:
    • Shift ~4.5 - 6.5 ppm.
  • Proton on Triple Bond:
    • Shift ~2 - 2.5 ppm. Not just due to electronegativity.
  • Aldehyde Proton:
    • Shift ~9 - 10 ppm.
  • Carboxylic Acid Proton:
    • Shift ~10 - 12 ppm. Influenced by electronegativity, resonance, and hydrogen bonding effects.

Special Considerations

  • Triple bonds and benzene rings involve different effects on chemical shifts to be discussed in the next video.
  • Understanding these shifts helps in identifying functional groups and molecules.