Lecture Notes: Nuclear Shielding and NMR

Basics of Magnetic Fields and Current Loops

• Current in a Loop of Wire

  • Current, represented as I, creates a magnetic field.
  • In physics, current is considered as moving positive charges.
  • Right-Hand Rule: Thumb points in direction of current; fingers curl in direction of magnetic field.

• Magnetic Field (B)

  • Created by current in a wire loop.
  • From a top view, magnetic field goes into the page.
  • Electrons, which are negatively charged, move in the opposite direction of the current.

Proton NMR and Magnetic Fields

• External Magnetic Field (B₀)

  • Applied in proton NMR.
  • Causes electron density to circulate around the proton.

• Induced Magnetic Field

  • Created by circulating electron density.
  • Opposes the applied magnetic field (B₀).
  • This opposition is due to diamagnetism.

• Shielding in NMR

  • Proton Shielding: Electrons create a field opposing the external field, reducing the effective magnetic field (B_effective).
  • Deshielded Proton: No electrons, experiences full effect of the external field.
  • Shielded Proton: Electron density reduces the magnetic field felt by the proton.

Energy Differences and NMR Spectra

• Energy Difference

  • Difference between alpha and beta spin states.
  • Related to the strength of the magnetic field.

• Deshielded vs. Shielded Protons

  • Deshielded: Larger energy difference, higher frequency absorbed.
  • Shielded: Smaller energy difference, lower frequency absorbed.

• NMR Spectrum

  • Move left: Higher frequency signal, more deshielded.
  • Move right: Lower frequency signal, more shielded.

FT NMR and Frequency

• Fourier Transform NMR (FT NMR)

  • External magnetic field is constant.
  • Short pulse with a range of frequencies applied.
  • Frequencies correspond to energy differences detected by NMR.

• Older NMR Methods

  • Frequency held constant, magnetic field strength varied.
  • Terminology: "Upfield" (higher magnetic field strength, right on spectrum), "Downfield" (lower strength, left on spectrum).

Conclusion

• Discussed two protons with different shielding environments leading to different signals in NMR spectra.
• Upcoming discussion: Protons in the same environment and corresponding single signals.