Lecture Notes: Nuclear Shielding and NMR

Key Concepts

• Electric Current and Magnetic Fields
  • Current is represented by 'i' in physics.
  • Moving charges create magnetic fields.
  • The direction of the magnetic field is determined using the right-hand rule.
  • In a loop of wire, if current flows clockwise, the magnetic field goes into the page (from a top view).
  • Electrons move in the opposite direction to the current due to their negative charge.

Proton NMR

• External Magnetic Field (B₀)

  • External magnetic fields cause electron density around protons to circulate, creating an induced magnetic field.
  • Induced magnetic fields oppose applied magnetic fields, an effect known as diamagnetism.

• Shielding and Deshielding

  • Shielded Proton: Has circulating electron density that creates a magnetic field opposing the applied magnetic field, resulting in a smaller effective magnetic field (B effective).
  • Deshielded Proton: Lacks electron density and experiences the full effect of the applied magnetic field.

Magnetic Field and Energy Differences

• The effective magnetic field experienced by a proton determines the energy difference between its alpha and beta spin states.
• A smaller effective magnetic field (shielded proton) means a smaller energy difference and thus a lower frequency absorption.
• Conversely, a larger effective magnetic field (deshielded proton) means a greater energy difference and higher frequency absorption.

NMR Spectrum

• Deshielded Protons: Absorb at higher frequencies, appear to the left on the NMR spectrum (downfield).
• Shielded Protons: Absorb at lower frequencies, appear to the right on the NMR spectrum (upfield).

FT-NMR (Fourier Transform NMR)

• In FT-NMR, the external magnetic field is held constant while the sample is exposed to short pulses containing a range of frequencies.
• Frequencies correspond to energy differences, generating signals as protons return to lower energy states.

Older NMR Terminology

• Upfield: Refers to regions of the spectrum where higher magnetic fields were needed.
• Downfield: Refers to regions needing lower magnetic fields.

Summary

• Different protons experience different amounts of shielding, resulting in varied magnetic field strengths and NMR signals.
• Two protons in different environments will produce different signals and frequencies on an NMR spectrum.
• If protons are in the same environment, they produce a single signal on the NMR.

Note: These concepts are crucial for understanding how NMR spectroscopy works and how different molecular environments affect NMR readings. Further exploration of these topics will be continued in subsequent lectures.