NMR Spectroscopy: Understanding Proton Chemical Shifts

Methane and Proton NMR

• Methane Protons
  • Four equivalent protons result in one signal on an NMR spectrum.
  • Signal occurs at approximately 1 part per million (ppm).
  • The signal corresponds to the energy difference between alpha and beta spin states.
  • Energy difference is related to frequency (E = hν) and effective magnetic field.
  • Low frequency signal corresponds to low chemical shift (shielded protons).

Chloromethane and Proton NMR

• Chloromethane Protons
  • Three equivalent protons, one signal at approximately 3.1 ppm.
  • Presence of electronegative chlorine atom withdraws electron density.
  • Deshielded protons experience a greater effective magnetic field.
  • Greater energy difference in spin states results in higher frequency signal.
  • Higher frequency signal corresponds to higher chemical shift.

Chemical Shift and Electronegativity

• Shielded vs. Deshielded Protons
  • Shielded protons: Lower frequency, lower chemical shift.
  • Deshielded protons: Higher frequency, higher chemical shift.

Chemical Shift Ranges for Different Environments

• Alkane Environment

  • Shielded proton on carbon: Chemical shift ~0.5 to 2 ppm.

• Electronegative Atom Bonded

  • Proton on carbon bonded to electronegative atom: Chemical shift ~2.5 to 4.5 ppm.

• Proton Next to Carbonyl

  • Proton on carbon adjacent to carbonyl group: Chemical shift ~2 to 2.5 ppm.

• Proton on Alcohol

  • Hydrogen bonding affects deshielding, broad range ~2 to 5 ppm.

Hybridization and Proton Shifts

• Double Bond Protons

  • Proton on sp2 carbon: Chemical shift ~4.5 to 6.5 ppm.

• Triple Bond Protons

  • Proton on sp carbon: Chemical shift ~2 to 2.5 ppm (lower than expected).

Special Cases

• Aldehydes

  • Proton on aldehyde: Chemical shift ~9 to 10 ppm.

• Carboxylic Acids

  • Proton on carboxylic acid: Chemical shift ~10 to 12 ppm.
  • Effects include electronegativity, resonance, and hydrogen bonding.

Summary

• Electronegativity and hydrogen bonding play significant roles in determining chemical shifts in NMR spectroscopy.
• Understanding the environment and hybridization of protons helps predict their location on the NMR spectrum.